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Ultraviolet imaging of globular clusters and galaxies
Adv. SpaceRec. Vol. 11. No. 11, pp. (11)7 l—(1l)80, 1991 Printed in G,~atBritain. Allrigins re.sexved.
0273-4177/91 $0.00 +.50 Copyflght © 1991 COSPAR
ULTRAVIOLET IMAGThIG OF GLOBULAR CLUSTERS AND GALAXIES Robert W. O’Connell Astronomy Department, Universily of Virginia, Charlottesville, VA 22903-0818, U.S.A. ABSTRACT Ultraviolet imaging of star clusters and galaxies is a subject still in its infancy. Here we review the techniques of UV imaging and its scientific potential for studies of advanced stellar evolution, the star formation history and interstellar medium of galaxies, active nuclei, and the evolutionary state of systems observed at high redshift. Early Hubble Space Telescope imaging is expected to emphasize such programs since its image quality is excellent by earlier standards for the UV, despite the presence of spherical aberration. INTRODUCTION Ultraviolet imaging is roughly in the position now of UV spectroscopy in 1978 just prior to the launch of the International Ultraviolet Explorer (IUE). That is to say, enough preliminary work has been done to demonstrate the great promise of the technique, but astronomers still await facilities capable of producing large numbers of observations on faint sources. If things go well over the next few years, there should be a tremendous increase in the availability of high quality UV images of the universe. Deep imaging will move UV astronomy in new directions. Some of the more obvious of these are explored in this review, but the most important probably cannot be anticipated. This article is being written in the wake of the realization that the first generation Hubble Space Telescope (liST) instruments will not achieve their intended image quality. Accordingly, the emphasis in early liST imaging programs is expected to shift from the visible to the ultraviolet. As an introduction, it might therefore be useful to discuss imaging in the context both of UV and ground-based astronomy. THE NEW CAPABILITIES OF ULTRAVIOLET IMAGING UV astronomy to date has been overwhelmingly based on spectroscopy. Over 100,000 UV observations have been made (mostly by OAO-2, Copernicus, TD-1, ANS, and IUE), the vast majority of them being spectroscopy or photometry of stars. Only 1-2% are of star clusters or galaxies, and fewer still are direct images of the UV sky. (Imaging is distinguished from photometry in this discussion by having more than a few resolution elements across extended sources of interest.) Considering the enormous contributions made to optical astronomy by imaging (first visual, later photographic) and that special techniques are not required to form good images in the mid- to far-UV, unlike X-ray astronomy, it is surprising that UV imaging has not received more attention. Astronomers appear to have assumed that interstellar extinction might be a serious handicap and that optical imaging would suffice because of the proximity of the optical band to the UV. Neither is a good assumption, as we shall see. Spectroscopy has had such a strong influence on the course of UV astronomy that it is worth reviewing briefly the special advantages which imaging brings to the UV: Large fields of view: UV imaging experiments have offered field areas ranging up to thousands of square-degrees. This means that many targets can be studied simultaneously. Structural or morphological studies which would be impractical or impossible with small field spectroscopic experiments become straightforward. Better background (sky or source) sampling is possible, increasing S/N. Flux calibration is usually more difficult than for spectrographs, but precisions of order 1% are anticipated for modern UV imagers.
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Improved spatial re.soIntlon: In-iagers generally produce higher spatial resolution than spectrographs or photometers, allowing one to work in crowded or complex fields. Early platforms for imaging experiments, such as sounding rockets, did not yield image diameters better than about 15 arc-sec. However, the ASTRO Ultraviolet Imaging Telescope (UIT) on Spacelab should produce 1.8 arc-sec diameter images, and the liST, even with its degraded optics, produces images with cores of about 0.1 arc-sec. Widely adjustable bandwidths: Using filters or special detector/optics combinations, imaging experiments can employ very broad bands (up to ~A 2000 A) to detect very faint objects in the UV. Alternatively, interference filters can provide relatively narrow bands in the range 50-300 A centered on features of special interest such as C IV .X1550 or redshifted Ly-c~. One of the powerful applications of imaging is full field speciroscopy, where one uses gratings or prisms to simultaneously generate low resolution (R 50—100) spectra of many sources across the field /26/. Such devices are carried on the liST Wide Field/Planetary Camera (WF/PC) and the Faint Object Camera (FOC) and also on UIT. ‘-~
One of the obvious consequences of these features is that UV imaging will reach much fainter thresholds than spectroscopy. The working limit* of TUE is mA (2000 A) 14, and we know almost nothing about the UV sky at significantly fainter levels. With FOCA, UJT and liST one can reach to rn>, 18—25 or fainter, corresponding to a spatial volume as much as 4 x 106 times larger for a given class of UV source. Indeed, this leap in UV source threshold is far larger than that at visible wavelengths between ground-based and liST instruments. Let us now turn to the relationship of UV to ground-based imaging. The particular astrophysical advantages of the ultraviolet for imaging are based on five special considerations: (i) Hot stars (T~> 10000) have their flux maxima in the far-UV. UV—V colors will be about 10 times more sensitive than optical colors in determining the temperatures of hot sources. Further, one can isolate such sources since their contrast against a cool star background is much larger in the UV than the optical. For instance, a hot horizontal branch star in a globular cluster will be brighter than a cool giant of equal V magnitude by a factor of 106 at 1500 A. Smaller, though still very considerable, advantages apply to detecting nonthermal nuclei against a cool star background. (ii) There is a deep minimum in the night sky background for 1600—2400 A, where the sky brightness at high galactic latitudes reaches p~ 26 mag arcsec2 This is the faintest in the entire UV-optical-IR spectrum, and it permits identification and study of ultra low surface brightness objects, perhaps up to 100,000 times fainter than the V-band brightness of the ground-based night sky /1/. Other applications, especially full-field spectroscopy, benefit from the dark sky as well. .
(iii) The dust extinction function is largest in the UV. This, together with the local maximum in the function at .X2175, makes the UV of central importance in understanding dust in our own and other galaxies. Absorption studies can be efficiently pursued with broad or intermediate band imaging. The UV albedo of dust is high enough that scattering from grains can also be easily observed with imagers, providing independent information on grain properties as well as a method for detecting cool regions which do not contain illuminating stars /2,3/. (iv) The UV has a rich emission/absorption line spectrum, including a number of astrophysically unique features. Important cool-star absorption lines in the midUV (MUV), 2000—3200 A, include Fe I, Fe II, Mg I, and Mg II. The confluence of cool metallic features makes intermediate-band colors such as (2600 V) about 10 times as sensitive to metal abundance as the familiar optical ~(U—B)index /4/. Chromospheric emission in Mg II and several far-UV (FUV), 912—2000 A, features can be used as an age indicator for cool dwarf stars even at low spectral resolution / 5,6/. There are many hot-star or ISM lines in the FUV, including the H I Lyman series, C —
IV, Si IV, 0 VI, N V, and the IT *
2 fluorescence features. In the extreme-UV (EUV), We quote magnitudes in the monochromatic system, defined as mA = —2.5 log FA 21.1, where 2 A’. [FA}= erg ~ cm —
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100—912 A, there are other important features of highly ionized species /7/ as well as the resonance lines (228—584 A) of He I and He II. The highly ionized lines are unique tracers of interstellar gas at temperatures of i0~~K. (v) The EUV contains the hydrogen ionizing continuum, which is, of course, of fundamental importance for the physical state of the interstellar medium. Little is known yet about the EUV since the local ISM is opaque at these wavelengths for pathlengths ~ 1 kpc. It is possible that a few percent of the sky has sufficiently low opacity that globular clusters or galaxies could be observed in the EUV. However, the rest-frame EUV of extragalactic objects with high redshifts (z ~ 0.2) can be observed with FUV instruments, and the )t912 Lyman discontinuity can be used in intermediate-band or grism imagery to identify such objects. Prospects for a rich scientific return, even with small aperture instruments, are therefore excellent. UV imaging not only permits exploration of an uncharted domain of the electromagnetic spectrum, but it also promises fundamentally new scientific capabilities which are not available at other wavelengths. HISTORY Small UV instruments (mostly 15-in diameter) have been used to attack a wide variety of imaging problems. One of the earliest orbiting UV instruments was the Celescope experiment on OAO-2 /8/, which employed Uvicons in four 12-in Schwarzschild telescopes to image 2°fields in four broad MUV-FUV bands. Celescope had relatively low spatial resolution and was limited to photometric measurements of some 5000 bright stars over about 10% of the sky. Another important early imaging experiment was the Naval Research Laboratory S201 camera, which was operated from the lunar surface by the crew of Apollo 16 /9/. S201 was an electrographic Schmidt with a 20° field of view and 3 arc-mm spatial resolution. Exposures up to 30 mm were made in the )~A1250—1600band; scattered Ly-x radiation (1215.7 A) contaminated exposures in the .A.\1050—1600 band. On the longest exposures the limiting stellar magnitude was mA -~ 11. S201 provided information on the relative numbers of UV-bright stars at high and low galactic latitudes and showed that H II regions are dominated by dust scattering in the UV. It also provided the first UV images of another galaxy, the Large Magellanic Cloud /10/, and demonstrated the remarkable morphological change caused by the suppression of the cool stars in the bar, which dominate the LMC’s appearance at visible wavelengths. Later versions of S201 were flown on sounding rockets by Carruthers and his colleagues and produced, for example, FUV imagery of M31 /11/. The photometric quality of these early experiments suffered because of the difficulty of calibrating their nonlinear detectors. A number of other wide field experiments have been constructed. Smith and his colleagues used a 12-in Schwarzschild telescope with an 11.4° field of view on sounding rockets to observe the Virgo cluster of galaxies and the LMC /12,13/. The detector was a microchannel plate with Cs-Te photocathode coupled to film. The experiment yielded 50 arc-sec FWHM resolution and a limiting magnitude for the broad-band (1100 A) Virgo exposures of m>,(2420A) 16.3, with an uncertainty in the absolute photometry for the brighter sources of 0.3 mag. In exposures ranging up to 200 sec, they detected some 200 galaxies in the center of Virgo and discovered a strong negative correlation between the UV—V colors and V for the integrated light of SO galaxies (consistent with the metallicity-luminosity effect seen in the visible). The luminous E galaxies in the sample clearly show the effects of the “UVX” component, which is responsible for the FUV upturn (see below), in that their MUV fluxes are higher than expected. This work was followed up by .~\1560imaging of Virgo by a Japanese sounding rocket package including two 17-cm telescopes with 4° fields of view /57/. This yielded spatial resolutions of 6 arc-mm and a threshold of mA 13. Comparison with TD-1 fluxes for field stars in the 7— 10 mag range suggests the presence of significant UV variability in some cases; this possibility emphasizes how little is known about the UV sky. The “UVX” component was detected in two bright ellipticals. Two wide field experiments, FAUST (8°)and VWFC (66°),were carried on Spacelab 1 /14,15/. Both employed image intensifiers coupled to film. Unfortunately, owing to the low altitude (240 km) and high inclination (57°)of the Shuttle orbit, both of these small f-number experiments were seriously compromised by twilight airgiow, notably the 0 I )~1304,1356lines in the tropical JASR 11:U—5~
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TJV arcs /16,25/. Nonetheless, each experiment returned useful images of faint sources, including the Cygnus Loop and the intergalactic “wing” of the Small Magellanic Cloud /17/. The Skylab space station carried a 15-cm objective-prism imaging telescope /18/ which obtained low dispersion UV spectra for nearly 6000 stars in 188 4°x 5°fields. This experiment demonstrated the efficiency of full-field spectroscopy for low resolution work and helped to establish FUV spectral classification criteria for hot stars. It also identified a number of cool stars with previously unrecognized hot subluminous companions /19/. An interesting special purpose FUV imaging experiment with a 95-cm aperture and a 1.6°field was flown by Martin and Bowyer /20/ on a sounding rocket to study the extragalactic background light. Imaging is important for two reasons in this application: to remove the effects of foreground stars and to detect potential background fluctuations associated with distant galaxies. In this case, the experiment provided a strong constraint on the evolution of the FUV light of galaxies over the last third of a Bubble time and may have detected individual distant star-forming systems. CURRENT INSTRUMENTATION Currently in orbit is the Glazar UV imaging telescope in the Kvant astrophysics module of the Mir space station /21/. This is a 40-cm telescope with a 1.3°field and a microchannel plate detector coupled to film. The system is intended for extragalactic surveys in selected fields (extending the well-known Markarian surveys) using an intermediate-width band at )d64O. Most of the published UV imaging studies of globular clusters and galaxies have come from two instrument series: the SCAP/FOCA balloon-borne telescopes developed by the Marseille Laboratoire d’Astronomie Spatiale and the Geneva Observatory and the sounding rocket prototypes for the UIT developed by Stecher and colleagues at NASA—Goddard Space Flight Center. Scientific results from these instruments will be discussed in later sections. The SCAP/FOCA program takes advantage of a minimum in the opacity of the Earth’s atmosphere near 2000 A which permits UV observations to be made from balloon altitudes (.— 40 km). Several versions of the telescopes have been flown with apertures between 13 and 40-cm and fields of view between 1.6°and 6°.The detector is a microchannel plate coupled to film. The bandpass (z~.\-~ 150 A centered at -.~2O20A) is defined on the short-wave side by 02 absorption and on the long-wave side by a multilayer coating on the mirrors. Additional long-wave rejection is provided by the Cs-Te photocathode. Spatial resolution, determined by the star tracking system, is 15 arc-sec. The current FOCA system /22/ has a limiting magnitude of mA —. 18. Further details are given by Milliard, Donas, and Laget (this conference). The UIT, which is one of the three UV instruments on the ASTRO Spacelab payload, is a 38-cm telescope with a 40 arc-mm diameter field of view. There are two selectable cameras, consisting of two-stage magnetically focussed image intensifiers coupled to film transports which carry about 1000 frames each. The FUV camera employs a Cs-I photocathode and six filters for the AA1200— 2000 region. The MUV camera has a Cs-Te photocathode and five filters for .X.X1800—3200 plus a grating providing 30 A resolution spectra covering )i.\1240—3200. The widest filter has a halfpower width of 1150 A. All detector/filter combinations yield excellent visible light rejection. An articulated secondary mirror provides fine guidance; anticipated image diameters are 1.8 arc-sec. The S/N = 10 limiting magnitude in the broad band MUV filter in a 30 minute exposure will be m), -.- 22, corresponding to V 25 for hot unreddened stars. Only one ASTRO mission (9—10 days) is presently on the Shuttle manifest, although a successful flight would produce some 200 target pointings and 2000 exposures. While both FOCA and UIT are powerful imagers by virtue of their faint thresholds and large data fields, which are well matched to nearby galaxies, they are limited by the short duration of their missions. The lIST, on the other hand, is a permanent observatory with a projected lifetime of 20 years. Characteristics of HST instrumentation have been thoroughly documented /23,24/, and generalities need not be repeated here. The one major departure from the planned performance which has been revealed to date (August 1990) is the presence of spherical aberration in the (otherwise excellent) telescope optics. The FWHM of WF/PC and FOC images is 0.1 arc-sec (as specified), but only about 15-20% of the light is in the sharp core, and there are wide wings which contain no more than about 70% of the light within a diameter of 1 arc-sec. The aberration can be corrected in the optics of orbital replacement instruments, but the firstgeneration complement must live with degraded performance. Nonetheless, it will be evident from the preceding discussion that the image quality of liST is superb by earlier standards for UV imaging. The main impact of the aberration will be a brighter
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limiting threshold, by 1-3 mags for stellar sources, owing to the higher detector noise in the larger images. Long-wavelength rejection in the lIST filters is also not entirely satisfactory, especially for FUV observations with the WF/PC, implying a “red leak” problem for cool sources. Original estimates for the S/N = 10 limits for point sources in the MUV were mA 26.5 in 10 hours for the FOC and mA -..~22 in 30 minutes for the WF/PC. The maximum fields of view of these cameras are 44 arc-sec and 2.6 arc-mm, respectively. No imaging has been published yet in the EUV, but the first all-sky surveys in this region will be made soon by the Wide Field Camera on ROSAT and the Extreme Ultraviolet Explorer. These experiments involve mirror and filter technologies which are considerably different from those discussed above. Since they have been presented in detail by Pounds and Bowyer, respectively, elsewhere at this conference, we will not review them here. Finally, there is a central deficiency in UV astronomy which should be apparent from this review, namely the absence of an all-sky FUV/MUV survey with a faint limiting magnitude, say mA ~ 21. No such survey is under active consideration now, and it is unlikely that one will be available within 10-15 years. This will certainly compromise the effectiveness and scientific return in the UV from lIST, Lyman, and other sensitive but narrow-field UV instruments. An all-sky UV survey with a threshold comparable to the ground-based Schmidt surveys should be made a high priority. UV IMAGING OF GLOBULAR CLUSTERS AND HOT LOW-MASS STARS Globular clusters are excellent subjects for UV imaging because they are often in regions of low extinction, they contain a rich population of hot stars of considerable astrophysical interest, and they lack a confusing background of hot main sequence stars. UV studies of clusters will provide the best available information on the evolutionary phases which end the lives of low mass stars. Some of these are so rapid (St iO~ iO~years) that good statistics are not available from field star studies. —
There has been little UV imaging of clusters yet. UV photometry of galactic globulars by OAO-2 and ANS /27,28/ revealed a wide range of UV behavior related to the detailed structure of the horizontal branch (HB). This early work was followed up by extensive IUE spectroscopy, reviewed by Castellani and Cassatella /29/. These studies indicate that filter imagery can easily assess the evolutionary status of clusters insofar as this is reflected in their HB’s. Even relatively brief exposures with FOCA or UIT can yield complete samples of HB stars. Bohlin et a!. published the first imaging study of a globular (M5), based on six exposures ranging up to 25 secs with a UIT prototype /30/. They used (UV—V) colors for 50 HB stars to estimate their helium abundances and core masses. The UV images also allow the hot end of the HB to be determined with confidence, permitting an estimate of the maximum mass loss on the giant branch. A FOCA study of 4 metal poor clusters with similar astrophysical goals is described by Laget, Burgarella, Milliard, and Donas (this conference). UV imaging will also be important for studying multiple populations on the HB induced by chemical mixing, rotation, primordial abundance variations, and other mechanisms (e.g. /31/). The horizontal branch is only the longest-lived of the hot post-giant branch evolutionary phases of low-mass stars. Planetary nebula nuclei, hot subdwarfs, and hot white dwarfs are others, but these are not as well understood astrophysically. Vauclair and Liebert /32/ summarize the existing observational evidence, and Greggio and Renzini /33/ explore the surprisingly large range of evolutionary scenarios (at least six different types) which can produce such objects. UV imaging is one of the best means of identifying objects in these phases. For instance, the hot white dwarf HZ 43, with 7’,, 110,000, could be detected by a UIT-class telescope at a distance of 30 kpc. A broad-band survey of 20-50 galactic globulars would build up a hot star census suitable for making statistical inferences concerning their astrophysics /33/. Indeed, two such objects were discovered on the short exposure M5 frames of Bohlin et a!. /30/. Figure 1 indicates how well a 2000 second FUV exposure with a UIT-class telescope can sample the hot star population of a nearby globular cluster. For objects on the post-giant branch cooling sequence /34/, the threshold will be 9 magnitudes fainter than the horizontal branch. There should be about 250 cooling objects, mostly hot white dwarfs, detected over the face of the cluster; comparable studies of the cooling sequence have proven very difficult at optical wavelengths /37/. A threshold locus for a long grating exposure is also shown. This would provide moderate resolution spectra for all objects on the HB and the vertical part of the cooling curve. Note from the location of the thresholds that the UV exposures will not be strongly contaminated by either the bright red giant branch or the populous main sequence, greatly reducing image crowding.
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B-V Figure 1: Color-magnitude diagram for M5 with superposed threshold locations for filter and grating exposures with a UIT-class instrument. Objects above the thresholds will yield S/N > 5. A UV distance modulus of 14.75 is assumed for MS. The white dwarf luminosity fimction derived from such studies can elucidate the physical processes responsible for white dwarf cooling (e.g. neutrino emission, crystallization /34/) and provide a means of independently estimating cluster distances /35/. The temperature and luminosity distribution of the other species of hot low mass stars will constrain basic physical processes on the giant branch, including mass loss /33/. Apart from their intrinsic interest, such objects will be important in spectroscopic studies of the interstellar medium along the line of sight. UV imaging in these areas can be extended to other galaxies as well. Studies of the integrated UV properties of globular clusters as distant as the Virgo cluster should be possible with liST. A preliminary survey of 16 clusters in M31 has been published by the Goddard group /36/. Individual hot low-mass field stars can be detected in nearby galaxies even with small instruments. For example, the FUV broad-band thresholds appropriate for the LMC and M31 with a UIT-class imager in Fig. 1 would lie such that they intercept the cooling curve at V = 23 and V = 17, respectively. Finally, with HST resolution, color-magnitude diagrams of extragalactic globular clusters can be obtained. Applications such as these could fruitfully occupy most of the UV imaging time on either liST or Shuttle-based experiments. Nonetheless, the next section describes a yet more promising set of potential programs.
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UV IMAGING OF GALAXIES Extragalactic astronomy is likely to be the greatest beneficiary of deep UV imaging. There is a fairly obvious set of problems for which UV imaging has long seemed well suited. These include the morphology of spiral structure (massive stars and dust); the dust extinction law in different environments; identification of supernova remnants and planetary nebulae through their FUV emission lines; detection of high temperature halo gas through local FUV emission or resonance scattering; the study of Ly-a line transfer in systems redshifted enough to be clear of geocoronal contamination; massive star populations in elliptical galaxies (which might be associated with cooling flows, the cycling of interstellar gas, or tidal interactions); UV photometry of large samples of galaxies in clusters; surveys for distant star-forming galaxies; and the structure of nonthermal jets from active nuclei. However, one’s approach to many of these problems has been changed by progress in UV imaging and other fields of UV astronomy (principally TUE spectroscopy) during the last 10 years. In some areas there have been surprises, and in others there is a better appreciation of the quantitative contributions possible with UV imaging. The rest of this section will focus on problems where the emphasis has shifted recently. One important result is that interstellar extinction, even in disk galaxies, is usually not a serious hindrance to UV imaging. UIT-prototype images of the nearby Sc spiral M33, for example, are nearly identical in appearance to visible band images. More quantitatively, the effective extinction suffered by the FUV continuum in star-forming galaxies is often less than that expected from optical observations, and the total hot star populations estimated from FUV data are similar within a factor of .-.~3 to those determined from radio thermal continuum, Ha fluxes, or IRAS dust emission measures /38,39,40/. This does not mean that these objects are transparent in the UV, only that a significant fraction of the hot stars in the galaxy is on the near side of any optically thick dust layer /41/. These results indicate that UV luminosities, derived from simple filter imaging, can be used to estimate mean star formation rates over the past 50 Myr (FUV) to 300 Myr (MUV) /39,40/ and to study the correlations between global star formation rate, gas and dust content, and environment. Combined with low resolution UV spectroscopy or Lyman continuum fluxes, UV luminosities can also be used to infer the ages or star-formation history of the regions observed /40,46/. (Filter photometry alone is not sufficiently sensitive to the ages of very young clusters /59/). Integrated FUV/MUV measurements are not good indicators of the shape of the initial mass function (IMF), at least in a multi-generation system, but direct resolution of star forming complexes in nearby systems with the UV imagers on HST will yield invaluable new information on the IMF for large stellar masses and can test whether this depends on the local environment. Another surprise is that UV imaging may be a sensitive probe of the cool interstellar medium of galaxies. The first example was the detection of very low surface brightness UV arms in the spiral galaxy MiOl which were not visible in optical-band images /42,43/. These appear to be cool, dusty regions which scatter UV radiation from the OB stars in the main body of the galaxy. In favorable situations, UV imaging could detect such regions to a limiting H I surface density of 1018_ 1019 cm2, which compares favorably with current 21-cm techniques /1/. Another interesting case is the UV-scattering cloud in the halo of M82 /44/, presumably debris from the tidal interaction with M81 or material ejected from the central starburst region of M82 by a galactic wind. This instance is the more remarkable since the optical depth to the central star-forming regions in M82, presumably the source of the scattered photons, is often estimated from IR observations to be very large, though this is a controversial point /45/. Both these results are based on short UV exposures (60-1200 seconds). Recent results also indicate that UV imaging can directly detect cold H 2 regions by virtue of the UV fluorescence (.XA1550—165o) which is a byproduct of the molecular destruction process /47,48/. Since the night sky at high galactic latitudes is quite dark at these wavelengths, this is a promising new channel for studying the molecular content of galaxies. The detection of a “submerged” starburst in the nucleus of the nearby spiral M83 /54/ emphasized the fact that the contrast between a star-forming region or a nonthermal active nucleus and the cool stellar background is enormously improved in the UV. Such objects can be detected in the UV even if they are 7—9 mags fainter than the background in the V-band. UV imaging can explore the luniinosity function of AGN’s to much fainter levels than currently possible. A considerable surprise is the presence in most elliptical galaxies of a significant hot stellar component, indicated by a strong upturn in the FUV spectrum (/49,50/ and references therein). Because its strength is positively correlated with metal abundance, it is likely that this “UVX” population
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Figure 2: The interacting system M51/NGC5195 in the )~6500R-band (left) and .A2650 (right), taken with a UIT rocket prototype. In the MUV, the spiral arms become patchy and less symmetrical, the nuclear region is less concentrated, and the companion galaxy, which is responsible for the tidal disturbance, is nearly invisible. consists of highly evolved low-mass stars of the type discussed in the last section /33,50/. If these are low mass objects, then their properties will be related to the global chemical and age structure of the galaxy, and they will be clues to its evolutionary history. Several UV imaging programs are obvious here. First, with HST one can attempt to resolve the individual UVX stars in nearby galaxies. Even with its reduced imaging capabilities, HST should certainly be able to decide whether massive main sequence stars are involved. (UIT-prototype images appear to exclude high-mass stars in the case of the M31 bulge /51/.) With HST or UIT-class instruments one can study the brightness and distribution of the UVX population in galaxies of various morphological types and luminosities. Recent work indicates that the UVX population is brighter in ellipticals with boxy isophotes /52/ and that there may be more than one UVX component present in some of the Virgo ellipticals /58/. The darkness of the UV sky suggests that UV imaging surveys with moderate to large fields will add significantly to our understanding of the faint end of the luminosity function for dwarf irregular galaxies and the nature of very low surface brightness systems like Malin 1 / 1/. Very strong Ly-a emission features have recently been detected in some high redshift systems /53/ and may be characteristic of protogalaxies. One can take advantage of the dark UV sky and make imaging surveys for systems like these based on Ly-a or the .X912 absorption edge, as long as the redshift moves the feature of interest longward of geocoronal Ly-a. UV imaging will be useful for 0.1—2.5 based on these features /1/. It is possible that the He I and lie II EUV edges can be used to identify even higher redshift systems (z 2—8). Such objects would have very peculiar UV colors because of the zc~absorption cross-section shortward of the Lyman edge and might be identifiable from broad-band exposures. Finally, a major result of UV imaging studies to date is that the morphology of galaxies is a strong function of wavelength (/55,56/ and references therein). As the observing wavelength shifts to the restframe UV, hot populations are enhanced while cool populations are suppressed. Thus, one encounters remarkable “morphological transformations” in the appearance of galaxies (see Figure 2). In the UV, barred galaxies become unbarred, early type spirals become late types or irregulars, interacting systems become single (as in the case of M51 in Fig. 2). These effects are of intrinsic interest, but they are also relevant to attempts to understand galaxy evolution at high redshifts, since most observations of distant systems are made in the restframe UV. (The bright
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night sky for )~>6000 A inhibits observations of distant galaxies from the ground in the restframe B,V.) The interpretation of galaxies in early evolutionary phases will therefore require extensive UV observations of nearby systems. Clusters of galaxies represent one of the best arenas for such studies, and both FOCA and UIT programs are under way here /22/. Bohlin et a!. have simulated the appearance of high redshift galaxies to lIST and ground-based telescopes using images from the UIT prototype program /56/. CONCLUSION There is a great deal of exciting science to be done with ultraviolet imaging. Imaging and full field spectroscopy represent the primary means by which new phenomena or astrophysically important examples of known classes of objects will be recognized in the UV. Were it not for setbacks in the US space program many of the projects above would be completed by now, and it is likely that the character of UV astronomy would be very different. We trust that qualifying remarks of this type will not be necessary the next time the field is reviewed. This work was supported in part by NASA grant NAG 5-700. REFERENCES /1/ /2/ /3/ /4/ /5/ /6/
O’Connell, R.W. 1987, Astron. J., 94, 876. Bohlin, R.C., Hill, J.K., Stecher, T.P., and Witt, A.N. 1982, Astrophys. .1., 255, 87. Hurwitz, M., Bowyer, S., and Martin, C. 1990, Astrophys. J., in press. Fanelli, M.N., O’Connell, R.W., Burstein, D., and Wu, C.-C. 1990. Astrophys. J., in press. Simon, T., Herbig, G., and Boesgaard, A.M. 1985, Astrophys. J., 293, 551. Smith, G.H., Burstein, D., Fanelli, M.N., O’Connell, R.W., and Wu, C.-C. 1990, Astron. J., in press. /7/ Linsky, 3., this conference. /8/ Davis, R.J., Deutschman, W.A., Lundquist, C.A., Nozawa, Y., and Bass, S.D. 1972, in: Scientific Results from OAO-t~,ed. A.D. Code (NASA SP-310), p. 1. /9/ Carruthers, G.R., and Page, T. 1984, PubI. Astron. Soc. Pac., 96, 447. /10/ Page, T., and Carruthers, G.R. 1981, Astrophys. J., 248, 906. /11/ Carruthers, G.R., Heckathorn, H.M., and Opal, C.B. 1978, Astrophys. J., 225, 346. /12/ Smith, A.M., and Cornett, R.H. 1982, Astrophys. .1., 261, 1. /13/ Smith, A.M., Cornett, R.H., and Hill, R.S. 1987, Astrophys. J., 320, 609. /14/ Bixler, 3., Bowyer, S., Deharveng, J.M., Courtès, G., Malina, R., Martin, C., and Lampton, M. 1984, Science, 225, 184. /15/ Courtès, G., Viton, M., Sivan, 3.P., Decher, R., and Gary, A. 1984, Science, 225, 179. /16/ 4breu, V.3., Eastes, R.W., Yee, J.H., Solomon, S.C., and Chakrabarti, S. 1986, J. Geophys. Res., 91, No. AlO, 11365. /17/ Pierre, M., Viton, M., Sivan, J.P., and Courtès, G. 1986, Astron. Astrophys., 154, 249. /18/ Henize, K.G., Wray, J.D., Parsons, S.B., Benedict, G.F., Bruhweiler, F.C., Rybski, P.M., and O’Callaghan, F.G. 1975, Astrophys. J. Lett., 199, L119. /19/ Parsons, S.B., Wray, J.D., Kondo, Y., Heniz, K.G., and Benedict, G.F. 1976, Astrophys. J., 203, 435. /20/ Martin, C., and Bowyer, S. 1989, Astrophys. J., 338, 677. /21/ Tovmasyan, G.M., Khodzhayants, Yu.M., Krmoyan, M.N., Kashin, A.L., Zakharyan, A.Z., Oganesyan, R.Kh., Mkrtchyan, M.A., Tovmasyan, G.G., Butov, V.V., Romanenko, Yu.V., Laveikin, A.I., Aleksandrov, A.P., and Huguenin, D. 1988, Soy. Astron. Lett., 14, 123. /22/ Donas, 3., Buat, V., Milliard, B., and Laget, M. 1990, Astr. Ap., in press. /23/ Hall, D.N.B. 1982, The Space Telescope Observatory, (NASA CP-2244; Baltimore: Space Telescope Science Institute). /24/ Hubble Space Telescope Handbooks 1990, (Baltimore: Space Telescope Science Institute). /25/ Cebula, R.P., and Feldman, P.D. 1984, J. Geophys. Res., 89, No. AlO, 9080. /26/ Hettrick, M.C., and Bowyer, S. 1983, AppI. Optics, 22, 3921. /27/ Welch, G.A., and Code, A.D. 1980, Astrophys. J., 236, 798. /28/ van Albada, T.S., de Boer, K.S., and Dickens, R.J. 1981, Mon. Not. R. Astron. Soc., 195, 591. /29/ Castellani, V., and Cassatella, A. 1987, in: Exploring the Universe with the IUE Satellite, ed. Y. Kondo (Dordrecht: Reidel), p. 637. /30/ Boblin, R.C., Cornett, R.H., Hill, J.K., Smith, A.M., and Stecher, T.P. 1985, Astrophys. J., 292, 687.
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/31/ Rood, R.T., and Crocker, D.A. 1989, in: The Use of Pulsating Stars in Fundamental Problems in Astronomy, ed. E.G. Schmidt (Cambridge: Cambridge Univ. Press), p. 103. /32/ Vauclair, G., and Liebert, 3. 1987, in: Exploring the Universe with the IUE Satellite, ed. Y. Kondo (Dordrecht: Reidel), p. 355. /33/ Greggio, L., and Renzini, A. 1990, Astrophys. J., in press. /34/ Iben, I., and Tutukov, A.V. 1984, Astrophys. J., 282, 615. /35/ Fusi Pecci, F., and Renzini, A. 1979, in: Astronomical Uses of the Space Telescope, eds. F. Macchetto, F. Pacini, and M. Tarenghi (Geneva: ESO), p. 181. /36/ Bohlin, R.C., Cornett, R.H., Hill, J.K., Hill, R.S., and Stecher, T.P. 1988, Astrophys. J., 334, 657. /37/ Richer, H.B., and Fahlman, G.G. 1988, Astrophys. J., 325, 218. /38/ Fanelli, M.N., O’Connell, R.W., and Thuan, T.X. 1988, Astrophys. J., 334, 665. /39/ Donas, 3., Deharveng, J.M., Laget, M., Milliard, B., and Huguenin, D. 1987, Astr. Ap., 180, 12. /40/ O’Connell, R.W., 1990, in: Windows on Galaxies, eds. G. Fabbiano, J.S. Gallagher, and A. Renzini (Dordrecht: Kluwer), p. 39. /41/ Disney, M. Davies, 3., and Phillipps, S. 1989, Mon. Not. H. Astron. Soc., 239, 939. /42/ Stecher, T., Bohlin, R., Hill, 3., and Jura, M. 1982, Astrophys. J. Lett., 225, L99. /43/ Donas, J., Milliard, B., Laget, M., and Deharveng, J.M. 1981, Astron. Astrophys., 97, L7. /44/ Courvoisier, T.J.-L., Reichen, M., Blecha, A., Golay, M., and Huguenin, D. 1990, Astron. Astrophys., in press. /45/ Telesco, C.M., Campins, H., Joy, M., Dietz, K., and Decher, R. 1990, Astrophys. J., in press. /46/ Lequeux, 3., Maucherat-Joubert, M., Deharveng, J.M., and Kunth, D. 1981, Astron. Astrophys.., 103, 305. /47/ Witt, A., Stecher, T., Boroson, T., and Bohlin, R. 1989. Astrophys. J. (Letters), 336, L21. /48/ Martin, C., Hurwitz, M., and Bowyer, S. 1990, Astrophys. .1., 354, 220. /49/ Code, A.D., and Welch, G.A. 1979, Astrophys. J., 228, 95. /50/ Burstein, D., Bertola, F., Buson, L.M., Faber, S.M., and Lauer, T.R. 1988, Astrophys. J., 328, 440. /51/ Bohlin, R.C., Cornett, R.H., Hill, J.K., Hill, R.S., O’Connell, R.W., and Stecher, T.P. 1985, Astrophys. 3. Lett., 298, L37. /52/ Longo, G., Capaccioli, M., Bender, R., and Busarello, G. 1989, Astron. Astrophys., 225, L17. /53/ Djorgovski, S., Spinrad, H., McCarthy, P., and Strauss, M. 1985, Astrophys. J. (Letters), 299, Li. /54/ Bohlin, R.C., Cornett, R.H., Hill, J.K., Smith, A.M., and Stecher, T.P. 1983, Astrophys. J. (Letters), 274, L53. /55/ Vuillemin, A., Buat, V., Donas, 3., Milliard, B., Courtès, G., and Golay, M. 1990, this conference. /56/ Bohlin, R.C., Cornett, R.H., Hill, J.K., Hill, R.S., Landsman, W.B., O’Connell, R.W., Neff, S.G., Smith, A.M., and Stecher, T.P. 1990, Astrophys. 3., in press. /57/ Onaka, T., Tanaka, W., Watanabe, T., Watanabe, J., Yamaguchi, A., Nakagiri, M., Kodaira, K., Nakano, M., Sasaki, M., Tsujimura, T., and Yamashita, K. 1989, Astrophys. J., 342, 238. /58/ Kodaira, K., Watanabe, T., Onaka, T., and Tanaka, W. 1990, Astrophys. J., in press. /59/ Meurer, G.R., Cacciari, C., arsd Freesnait, K.C. 1990, Astron. 3., 99, 1124. V
0273-4177/91 $0.00 +.50 Copyflght © 1991 COSPAR
ULTRAVIOLET IMAGThIG OF GLOBULAR CLUSTERS AND GALAXIES Robert W. O’Connell Astronomy Department, Universily of Virginia, Charlottesville, VA 22903-0818, U.S.A. ABSTRACT Ultraviolet imaging of star clusters and galaxies is a subject still in its infancy. Here we review the techniques of UV imaging and its scientific potential for studies of advanced stellar evolution, the star formation history and interstellar medium of galaxies, active nuclei, and the evolutionary state of systems observed at high redshift. Early Hubble Space Telescope imaging is expected to emphasize such programs since its image quality is excellent by earlier standards for the UV, despite the presence of spherical aberration. INTRODUCTION Ultraviolet imaging is roughly in the position now of UV spectroscopy in 1978 just prior to the launch of the International Ultraviolet Explorer (IUE). That is to say, enough preliminary work has been done to demonstrate the great promise of the technique, but astronomers still await facilities capable of producing large numbers of observations on faint sources. If things go well over the next few years, there should be a tremendous increase in the availability of high quality UV images of the universe. Deep imaging will move UV astronomy in new directions. Some of the more obvious of these are explored in this review, but the most important probably cannot be anticipated. This article is being written in the wake of the realization that the first generation Hubble Space Telescope (liST) instruments will not achieve their intended image quality. Accordingly, the emphasis in early liST imaging programs is expected to shift from the visible to the ultraviolet. As an introduction, it might therefore be useful to discuss imaging in the context both of UV and ground-based astronomy. THE NEW CAPABILITIES OF ULTRAVIOLET IMAGING UV astronomy to date has been overwhelmingly based on spectroscopy. Over 100,000 UV observations have been made (mostly by OAO-2, Copernicus, TD-1, ANS, and IUE), the vast majority of them being spectroscopy or photometry of stars. Only 1-2% are of star clusters or galaxies, and fewer still are direct images of the UV sky. (Imaging is distinguished from photometry in this discussion by having more than a few resolution elements across extended sources of interest.) Considering the enormous contributions made to optical astronomy by imaging (first visual, later photographic) and that special techniques are not required to form good images in the mid- to far-UV, unlike X-ray astronomy, it is surprising that UV imaging has not received more attention. Astronomers appear to have assumed that interstellar extinction might be a serious handicap and that optical imaging would suffice because of the proximity of the optical band to the UV. Neither is a good assumption, as we shall see. Spectroscopy has had such a strong influence on the course of UV astronomy that it is worth reviewing briefly the special advantages which imaging brings to the UV: Large fields of view: UV imaging experiments have offered field areas ranging up to thousands of square-degrees. This means that many targets can be studied simultaneously. Structural or morphological studies which would be impractical or impossible with small field spectroscopic experiments become straightforward. Better background (sky or source) sampling is possible, increasing S/N. Flux calibration is usually more difficult than for spectrographs, but precisions of order 1% are anticipated for modern UV imagers.
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Improved spatial re.soIntlon: In-iagers generally produce higher spatial resolution than spectrographs or photometers, allowing one to work in crowded or complex fields. Early platforms for imaging experiments, such as sounding rockets, did not yield image diameters better than about 15 arc-sec. However, the ASTRO Ultraviolet Imaging Telescope (UIT) on Spacelab should produce 1.8 arc-sec diameter images, and the liST, even with its degraded optics, produces images with cores of about 0.1 arc-sec. Widely adjustable bandwidths: Using filters or special detector/optics combinations, imaging experiments can employ very broad bands (up to ~A 2000 A) to detect very faint objects in the UV. Alternatively, interference filters can provide relatively narrow bands in the range 50-300 A centered on features of special interest such as C IV .X1550 or redshifted Ly-c~. One of the powerful applications of imaging is full field speciroscopy, where one uses gratings or prisms to simultaneously generate low resolution (R 50—100) spectra of many sources across the field /26/. Such devices are carried on the liST Wide Field/Planetary Camera (WF/PC) and the Faint Object Camera (FOC) and also on UIT. ‘-~
One of the obvious consequences of these features is that UV imaging will reach much fainter thresholds than spectroscopy. The working limit* of TUE is mA (2000 A) 14, and we know almost nothing about the UV sky at significantly fainter levels. With FOCA, UJT and liST one can reach to rn>, 18—25 or fainter, corresponding to a spatial volume as much as 4 x 106 times larger for a given class of UV source. Indeed, this leap in UV source threshold is far larger than that at visible wavelengths between ground-based and liST instruments. Let us now turn to the relationship of UV to ground-based imaging. The particular astrophysical advantages of the ultraviolet for imaging are based on five special considerations: (i) Hot stars (T~> 10000) have their flux maxima in the far-UV. UV—V colors will be about 10 times more sensitive than optical colors in determining the temperatures of hot sources. Further, one can isolate such sources since their contrast against a cool star background is much larger in the UV than the optical. For instance, a hot horizontal branch star in a globular cluster will be brighter than a cool giant of equal V magnitude by a factor of 106 at 1500 A. Smaller, though still very considerable, advantages apply to detecting nonthermal nuclei against a cool star background. (ii) There is a deep minimum in the night sky background for 1600—2400 A, where the sky brightness at high galactic latitudes reaches p~ 26 mag arcsec2 This is the faintest in the entire UV-optical-IR spectrum, and it permits identification and study of ultra low surface brightness objects, perhaps up to 100,000 times fainter than the V-band brightness of the ground-based night sky /1/. Other applications, especially full-field spectroscopy, benefit from the dark sky as well. .
(iii) The dust extinction function is largest in the UV. This, together with the local maximum in the function at .X2175, makes the UV of central importance in understanding dust in our own and other galaxies. Absorption studies can be efficiently pursued with broad or intermediate band imaging. The UV albedo of dust is high enough that scattering from grains can also be easily observed with imagers, providing independent information on grain properties as well as a method for detecting cool regions which do not contain illuminating stars /2,3/. (iv) The UV has a rich emission/absorption line spectrum, including a number of astrophysically unique features. Important cool-star absorption lines in the midUV (MUV), 2000—3200 A, include Fe I, Fe II, Mg I, and Mg II. The confluence of cool metallic features makes intermediate-band colors such as (2600 V) about 10 times as sensitive to metal abundance as the familiar optical ~(U—B)index /4/. Chromospheric emission in Mg II and several far-UV (FUV), 912—2000 A, features can be used as an age indicator for cool dwarf stars even at low spectral resolution / 5,6/. There are many hot-star or ISM lines in the FUV, including the H I Lyman series, C —
IV, Si IV, 0 VI, N V, and the IT *
2 fluorescence features. In the extreme-UV (EUV), We quote magnitudes in the monochromatic system, defined as mA = —2.5 log FA 21.1, where 2 A’. [FA}= erg ~ cm —
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100—912 A, there are other important features of highly ionized species /7/ as well as the resonance lines (228—584 A) of He I and He II. The highly ionized lines are unique tracers of interstellar gas at temperatures of i0~~K. (v) The EUV contains the hydrogen ionizing continuum, which is, of course, of fundamental importance for the physical state of the interstellar medium. Little is known yet about the EUV since the local ISM is opaque at these wavelengths for pathlengths ~ 1 kpc. It is possible that a few percent of the sky has sufficiently low opacity that globular clusters or galaxies could be observed in the EUV. However, the rest-frame EUV of extragalactic objects with high redshifts (z ~ 0.2) can be observed with FUV instruments, and the )t912 Lyman discontinuity can be used in intermediate-band or grism imagery to identify such objects. Prospects for a rich scientific return, even with small aperture instruments, are therefore excellent. UV imaging not only permits exploration of an uncharted domain of the electromagnetic spectrum, but it also promises fundamentally new scientific capabilities which are not available at other wavelengths. HISTORY Small UV instruments (mostly 15-in diameter) have been used to attack a wide variety of imaging problems. One of the earliest orbiting UV instruments was the Celescope experiment on OAO-2 /8/, which employed Uvicons in four 12-in Schwarzschild telescopes to image 2°fields in four broad MUV-FUV bands. Celescope had relatively low spatial resolution and was limited to photometric measurements of some 5000 bright stars over about 10% of the sky. Another important early imaging experiment was the Naval Research Laboratory S201 camera, which was operated from the lunar surface by the crew of Apollo 16 /9/. S201 was an electrographic Schmidt with a 20° field of view and 3 arc-mm spatial resolution. Exposures up to 30 mm were made in the )~A1250—1600band; scattered Ly-x radiation (1215.7 A) contaminated exposures in the .A.\1050—1600 band. On the longest exposures the limiting stellar magnitude was mA -~ 11. S201 provided information on the relative numbers of UV-bright stars at high and low galactic latitudes and showed that H II regions are dominated by dust scattering in the UV. It also provided the first UV images of another galaxy, the Large Magellanic Cloud /10/, and demonstrated the remarkable morphological change caused by the suppression of the cool stars in the bar, which dominate the LMC’s appearance at visible wavelengths. Later versions of S201 were flown on sounding rockets by Carruthers and his colleagues and produced, for example, FUV imagery of M31 /11/. The photometric quality of these early experiments suffered because of the difficulty of calibrating their nonlinear detectors. A number of other wide field experiments have been constructed. Smith and his colleagues used a 12-in Schwarzschild telescope with an 11.4° field of view on sounding rockets to observe the Virgo cluster of galaxies and the LMC /12,13/. The detector was a microchannel plate with Cs-Te photocathode coupled to film. The experiment yielded 50 arc-sec FWHM resolution and a limiting magnitude for the broad-band (1100 A) Virgo exposures of m>,(2420A) 16.3, with an uncertainty in the absolute photometry for the brighter sources of 0.3 mag. In exposures ranging up to 200 sec, they detected some 200 galaxies in the center of Virgo and discovered a strong negative correlation between the UV—V colors and V for the integrated light of SO galaxies (consistent with the metallicity-luminosity effect seen in the visible). The luminous E galaxies in the sample clearly show the effects of the “UVX” component, which is responsible for the FUV upturn (see below), in that their MUV fluxes are higher than expected. This work was followed up by .~\1560imaging of Virgo by a Japanese sounding rocket package including two 17-cm telescopes with 4° fields of view /57/. This yielded spatial resolutions of 6 arc-mm and a threshold of mA 13. Comparison with TD-1 fluxes for field stars in the 7— 10 mag range suggests the presence of significant UV variability in some cases; this possibility emphasizes how little is known about the UV sky. The “UVX” component was detected in two bright ellipticals. Two wide field experiments, FAUST (8°)and VWFC (66°),were carried on Spacelab 1 /14,15/. Both employed image intensifiers coupled to film. Unfortunately, owing to the low altitude (240 km) and high inclination (57°)of the Shuttle orbit, both of these small f-number experiments were seriously compromised by twilight airgiow, notably the 0 I )~1304,1356lines in the tropical JASR 11:U—5~
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TJV arcs /16,25/. Nonetheless, each experiment returned useful images of faint sources, including the Cygnus Loop and the intergalactic “wing” of the Small Magellanic Cloud /17/. The Skylab space station carried a 15-cm objective-prism imaging telescope /18/ which obtained low dispersion UV spectra for nearly 6000 stars in 188 4°x 5°fields. This experiment demonstrated the efficiency of full-field spectroscopy for low resolution work and helped to establish FUV spectral classification criteria for hot stars. It also identified a number of cool stars with previously unrecognized hot subluminous companions /19/. An interesting special purpose FUV imaging experiment with a 95-cm aperture and a 1.6°field was flown by Martin and Bowyer /20/ on a sounding rocket to study the extragalactic background light. Imaging is important for two reasons in this application: to remove the effects of foreground stars and to detect potential background fluctuations associated with distant galaxies. In this case, the experiment provided a strong constraint on the evolution of the FUV light of galaxies over the last third of a Bubble time and may have detected individual distant star-forming systems. CURRENT INSTRUMENTATION Currently in orbit is the Glazar UV imaging telescope in the Kvant astrophysics module of the Mir space station /21/. This is a 40-cm telescope with a 1.3°field and a microchannel plate detector coupled to film. The system is intended for extragalactic surveys in selected fields (extending the well-known Markarian surveys) using an intermediate-width band at )d64O. Most of the published UV imaging studies of globular clusters and galaxies have come from two instrument series: the SCAP/FOCA balloon-borne telescopes developed by the Marseille Laboratoire d’Astronomie Spatiale and the Geneva Observatory and the sounding rocket prototypes for the UIT developed by Stecher and colleagues at NASA—Goddard Space Flight Center. Scientific results from these instruments will be discussed in later sections. The SCAP/FOCA program takes advantage of a minimum in the opacity of the Earth’s atmosphere near 2000 A which permits UV observations to be made from balloon altitudes (.— 40 km). Several versions of the telescopes have been flown with apertures between 13 and 40-cm and fields of view between 1.6°and 6°.The detector is a microchannel plate coupled to film. The bandpass (z~.\-~ 150 A centered at -.~2O20A) is defined on the short-wave side by 02 absorption and on the long-wave side by a multilayer coating on the mirrors. Additional long-wave rejection is provided by the Cs-Te photocathode. Spatial resolution, determined by the star tracking system, is 15 arc-sec. The current FOCA system /22/ has a limiting magnitude of mA —. 18. Further details are given by Milliard, Donas, and Laget (this conference). The UIT, which is one of the three UV instruments on the ASTRO Spacelab payload, is a 38-cm telescope with a 40 arc-mm diameter field of view. There are two selectable cameras, consisting of two-stage magnetically focussed image intensifiers coupled to film transports which carry about 1000 frames each. The FUV camera employs a Cs-I photocathode and six filters for the AA1200— 2000 region. The MUV camera has a Cs-Te photocathode and five filters for .X.X1800—3200 plus a grating providing 30 A resolution spectra covering )i.\1240—3200. The widest filter has a halfpower width of 1150 A. All detector/filter combinations yield excellent visible light rejection. An articulated secondary mirror provides fine guidance; anticipated image diameters are 1.8 arc-sec. The S/N = 10 limiting magnitude in the broad band MUV filter in a 30 minute exposure will be m), -.- 22, corresponding to V 25 for hot unreddened stars. Only one ASTRO mission (9—10 days) is presently on the Shuttle manifest, although a successful flight would produce some 200 target pointings and 2000 exposures. While both FOCA and UIT are powerful imagers by virtue of their faint thresholds and large data fields, which are well matched to nearby galaxies, they are limited by the short duration of their missions. The lIST, on the other hand, is a permanent observatory with a projected lifetime of 20 years. Characteristics of HST instrumentation have been thoroughly documented /23,24/, and generalities need not be repeated here. The one major departure from the planned performance which has been revealed to date (August 1990) is the presence of spherical aberration in the (otherwise excellent) telescope optics. The FWHM of WF/PC and FOC images is 0.1 arc-sec (as specified), but only about 15-20% of the light is in the sharp core, and there are wide wings which contain no more than about 70% of the light within a diameter of 1 arc-sec. The aberration can be corrected in the optics of orbital replacement instruments, but the firstgeneration complement must live with degraded performance. Nonetheless, it will be evident from the preceding discussion that the image quality of liST is superb by earlier standards for UV imaging. The main impact of the aberration will be a brighter
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limiting threshold, by 1-3 mags for stellar sources, owing to the higher detector noise in the larger images. Long-wavelength rejection in the lIST filters is also not entirely satisfactory, especially for FUV observations with the WF/PC, implying a “red leak” problem for cool sources. Original estimates for the S/N = 10 limits for point sources in the MUV were mA 26.5 in 10 hours for the FOC and mA -..~22 in 30 minutes for the WF/PC. The maximum fields of view of these cameras are 44 arc-sec and 2.6 arc-mm, respectively. No imaging has been published yet in the EUV, but the first all-sky surveys in this region will be made soon by the Wide Field Camera on ROSAT and the Extreme Ultraviolet Explorer. These experiments involve mirror and filter technologies which are considerably different from those discussed above. Since they have been presented in detail by Pounds and Bowyer, respectively, elsewhere at this conference, we will not review them here. Finally, there is a central deficiency in UV astronomy which should be apparent from this review, namely the absence of an all-sky FUV/MUV survey with a faint limiting magnitude, say mA ~ 21. No such survey is under active consideration now, and it is unlikely that one will be available within 10-15 years. This will certainly compromise the effectiveness and scientific return in the UV from lIST, Lyman, and other sensitive but narrow-field UV instruments. An all-sky UV survey with a threshold comparable to the ground-based Schmidt surveys should be made a high priority. UV IMAGING OF GLOBULAR CLUSTERS AND HOT LOW-MASS STARS Globular clusters are excellent subjects for UV imaging because they are often in regions of low extinction, they contain a rich population of hot stars of considerable astrophysical interest, and they lack a confusing background of hot main sequence stars. UV studies of clusters will provide the best available information on the evolutionary phases which end the lives of low mass stars. Some of these are so rapid (St iO~ iO~years) that good statistics are not available from field star studies. —
There has been little UV imaging of clusters yet. UV photometry of galactic globulars by OAO-2 and ANS /27,28/ revealed a wide range of UV behavior related to the detailed structure of the horizontal branch (HB). This early work was followed up by extensive IUE spectroscopy, reviewed by Castellani and Cassatella /29/. These studies indicate that filter imagery can easily assess the evolutionary status of clusters insofar as this is reflected in their HB’s. Even relatively brief exposures with FOCA or UIT can yield complete samples of HB stars. Bohlin et a!. published the first imaging study of a globular (M5), based on six exposures ranging up to 25 secs with a UIT prototype /30/. They used (UV—V) colors for 50 HB stars to estimate their helium abundances and core masses. The UV images also allow the hot end of the HB to be determined with confidence, permitting an estimate of the maximum mass loss on the giant branch. A FOCA study of 4 metal poor clusters with similar astrophysical goals is described by Laget, Burgarella, Milliard, and Donas (this conference). UV imaging will also be important for studying multiple populations on the HB induced by chemical mixing, rotation, primordial abundance variations, and other mechanisms (e.g. /31/). The horizontal branch is only the longest-lived of the hot post-giant branch evolutionary phases of low-mass stars. Planetary nebula nuclei, hot subdwarfs, and hot white dwarfs are others, but these are not as well understood astrophysically. Vauclair and Liebert /32/ summarize the existing observational evidence, and Greggio and Renzini /33/ explore the surprisingly large range of evolutionary scenarios (at least six different types) which can produce such objects. UV imaging is one of the best means of identifying objects in these phases. For instance, the hot white dwarf HZ 43, with 7’,, 110,000, could be detected by a UIT-class telescope at a distance of 30 kpc. A broad-band survey of 20-50 galactic globulars would build up a hot star census suitable for making statistical inferences concerning their astrophysics /33/. Indeed, two such objects were discovered on the short exposure M5 frames of Bohlin et a!. /30/. Figure 1 indicates how well a 2000 second FUV exposure with a UIT-class telescope can sample the hot star population of a nearby globular cluster. For objects on the post-giant branch cooling sequence /34/, the threshold will be 9 magnitudes fainter than the horizontal branch. There should be about 250 cooling objects, mostly hot white dwarfs, detected over the face of the cluster; comparable studies of the cooling sequence have proven very difficult at optical wavelengths /37/. A threshold locus for a long grating exposure is also shown. This would provide moderate resolution spectra for all objects on the HB and the vertical part of the cooling curve. Note from the location of the thresholds that the UV exposures will not be strongly contaminated by either the bright red giant branch or the populous main sequence, greatly reducing image crowding.
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B-V Figure 1: Color-magnitude diagram for M5 with superposed threshold locations for filter and grating exposures with a UIT-class instrument. Objects above the thresholds will yield S/N > 5. A UV distance modulus of 14.75 is assumed for MS. The white dwarf luminosity fimction derived from such studies can elucidate the physical processes responsible for white dwarf cooling (e.g. neutrino emission, crystallization /34/) and provide a means of independently estimating cluster distances /35/. The temperature and luminosity distribution of the other species of hot low mass stars will constrain basic physical processes on the giant branch, including mass loss /33/. Apart from their intrinsic interest, such objects will be important in spectroscopic studies of the interstellar medium along the line of sight. UV imaging in these areas can be extended to other galaxies as well. Studies of the integrated UV properties of globular clusters as distant as the Virgo cluster should be possible with liST. A preliminary survey of 16 clusters in M31 has been published by the Goddard group /36/. Individual hot low-mass field stars can be detected in nearby galaxies even with small instruments. For example, the FUV broad-band thresholds appropriate for the LMC and M31 with a UIT-class imager in Fig. 1 would lie such that they intercept the cooling curve at V = 23 and V = 17, respectively. Finally, with HST resolution, color-magnitude diagrams of extragalactic globular clusters can be obtained. Applications such as these could fruitfully occupy most of the UV imaging time on either liST or Shuttle-based experiments. Nonetheless, the next section describes a yet more promising set of potential programs.
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UV IMAGING OF GALAXIES Extragalactic astronomy is likely to be the greatest beneficiary of deep UV imaging. There is a fairly obvious set of problems for which UV imaging has long seemed well suited. These include the morphology of spiral structure (massive stars and dust); the dust extinction law in different environments; identification of supernova remnants and planetary nebulae through their FUV emission lines; detection of high temperature halo gas through local FUV emission or resonance scattering; the study of Ly-a line transfer in systems redshifted enough to be clear of geocoronal contamination; massive star populations in elliptical galaxies (which might be associated with cooling flows, the cycling of interstellar gas, or tidal interactions); UV photometry of large samples of galaxies in clusters; surveys for distant star-forming galaxies; and the structure of nonthermal jets from active nuclei. However, one’s approach to many of these problems has been changed by progress in UV imaging and other fields of UV astronomy (principally TUE spectroscopy) during the last 10 years. In some areas there have been surprises, and in others there is a better appreciation of the quantitative contributions possible with UV imaging. The rest of this section will focus on problems where the emphasis has shifted recently. One important result is that interstellar extinction, even in disk galaxies, is usually not a serious hindrance to UV imaging. UIT-prototype images of the nearby Sc spiral M33, for example, are nearly identical in appearance to visible band images. More quantitatively, the effective extinction suffered by the FUV continuum in star-forming galaxies is often less than that expected from optical observations, and the total hot star populations estimated from FUV data are similar within a factor of .-.~3 to those determined from radio thermal continuum, Ha fluxes, or IRAS dust emission measures /38,39,40/. This does not mean that these objects are transparent in the UV, only that a significant fraction of the hot stars in the galaxy is on the near side of any optically thick dust layer /41/. These results indicate that UV luminosities, derived from simple filter imaging, can be used to estimate mean star formation rates over the past 50 Myr (FUV) to 300 Myr (MUV) /39,40/ and to study the correlations between global star formation rate, gas and dust content, and environment. Combined with low resolution UV spectroscopy or Lyman continuum fluxes, UV luminosities can also be used to infer the ages or star-formation history of the regions observed /40,46/. (Filter photometry alone is not sufficiently sensitive to the ages of very young clusters /59/). Integrated FUV/MUV measurements are not good indicators of the shape of the initial mass function (IMF), at least in a multi-generation system, but direct resolution of star forming complexes in nearby systems with the UV imagers on HST will yield invaluable new information on the IMF for large stellar masses and can test whether this depends on the local environment. Another surprise is that UV imaging may be a sensitive probe of the cool interstellar medium of galaxies. The first example was the detection of very low surface brightness UV arms in the spiral galaxy MiOl which were not visible in optical-band images /42,43/. These appear to be cool, dusty regions which scatter UV radiation from the OB stars in the main body of the galaxy. In favorable situations, UV imaging could detect such regions to a limiting H I surface density of 1018_ 1019 cm2, which compares favorably with current 21-cm techniques /1/. Another interesting case is the UV-scattering cloud in the halo of M82 /44/, presumably debris from the tidal interaction with M81 or material ejected from the central starburst region of M82 by a galactic wind. This instance is the more remarkable since the optical depth to the central star-forming regions in M82, presumably the source of the scattered photons, is often estimated from IR observations to be very large, though this is a controversial point /45/. Both these results are based on short UV exposures (60-1200 seconds). Recent results also indicate that UV imaging can directly detect cold H 2 regions by virtue of the UV fluorescence (.XA1550—165o) which is a byproduct of the molecular destruction process /47,48/. Since the night sky at high galactic latitudes is quite dark at these wavelengths, this is a promising new channel for studying the molecular content of galaxies. The detection of a “submerged” starburst in the nucleus of the nearby spiral M83 /54/ emphasized the fact that the contrast between a star-forming region or a nonthermal active nucleus and the cool stellar background is enormously improved in the UV. Such objects can be detected in the UV even if they are 7—9 mags fainter than the background in the V-band. UV imaging can explore the luniinosity function of AGN’s to much fainter levels than currently possible. A considerable surprise is the presence in most elliptical galaxies of a significant hot stellar component, indicated by a strong upturn in the FUV spectrum (/49,50/ and references therein). Because its strength is positively correlated with metal abundance, it is likely that this “UVX” population
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R.W. O’Connell
,~ .~l~waV~V
V
~
V
~
V V V
V
VVV~L~r~V
V:-V~~.:
Figure 2: The interacting system M51/NGC5195 in the )~6500R-band (left) and .A2650 (right), taken with a UIT rocket prototype. In the MUV, the spiral arms become patchy and less symmetrical, the nuclear region is less concentrated, and the companion galaxy, which is responsible for the tidal disturbance, is nearly invisible. consists of highly evolved low-mass stars of the type discussed in the last section /33,50/. If these are low mass objects, then their properties will be related to the global chemical and age structure of the galaxy, and they will be clues to its evolutionary history. Several UV imaging programs are obvious here. First, with HST one can attempt to resolve the individual UVX stars in nearby galaxies. Even with its reduced imaging capabilities, HST should certainly be able to decide whether massive main sequence stars are involved. (UIT-prototype images appear to exclude high-mass stars in the case of the M31 bulge /51/.) With HST or UIT-class instruments one can study the brightness and distribution of the UVX population in galaxies of various morphological types and luminosities. Recent work indicates that the UVX population is brighter in ellipticals with boxy isophotes /52/ and that there may be more than one UVX component present in some of the Virgo ellipticals /58/. The darkness of the UV sky suggests that UV imaging surveys with moderate to large fields will add significantly to our understanding of the faint end of the luminosity function for dwarf irregular galaxies and the nature of very low surface brightness systems like Malin 1 / 1/. Very strong Ly-a emission features have recently been detected in some high redshift systems /53/ and may be characteristic of protogalaxies. One can take advantage of the dark UV sky and make imaging surveys for systems like these based on Ly-a or the .X912 absorption edge, as long as the redshift moves the feature of interest longward of geocoronal Ly-a. UV imaging will be useful for 0.1—2.5 based on these features /1/. It is possible that the He I and lie II EUV edges can be used to identify even higher redshift systems (z 2—8). Such objects would have very peculiar UV colors because of the zc~absorption cross-section shortward of the Lyman edge and might be identifiable from broad-band exposures. Finally, a major result of UV imaging studies to date is that the morphology of galaxies is a strong function of wavelength (/55,56/ and references therein). As the observing wavelength shifts to the restframe UV, hot populations are enhanced while cool populations are suppressed. Thus, one encounters remarkable “morphological transformations” in the appearance of galaxies (see Figure 2). In the UV, barred galaxies become unbarred, early type spirals become late types or irregulars, interacting systems become single (as in the case of M51 in Fig. 2). These effects are of intrinsic interest, but they are also relevant to attempts to understand galaxy evolution at high redshifts, since most observations of distant systems are made in the restframe UV. (The bright
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night sky for )~>6000 A inhibits observations of distant galaxies from the ground in the restframe B,V.) The interpretation of galaxies in early evolutionary phases will therefore require extensive UV observations of nearby systems. Clusters of galaxies represent one of the best arenas for such studies, and both FOCA and UIT programs are under way here /22/. Bohlin et a!. have simulated the appearance of high redshift galaxies to lIST and ground-based telescopes using images from the UIT prototype program /56/. CONCLUSION There is a great deal of exciting science to be done with ultraviolet imaging. Imaging and full field spectroscopy represent the primary means by which new phenomena or astrophysically important examples of known classes of objects will be recognized in the UV. Were it not for setbacks in the US space program many of the projects above would be completed by now, and it is likely that the character of UV astronomy would be very different. We trust that qualifying remarks of this type will not be necessary the next time the field is reviewed. This work was supported in part by NASA grant NAG 5-700. REFERENCES /1/ /2/ /3/ /4/ /5/ /6/
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