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Warm ionized gas in the local interstellar medium
Advances in Space Research 34 (2004) 27–34 www.elsevier.com/locate/asr
Warm ionized gas in the local interstellar medium R.J. Reynolds
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Department of Astronomy, University of Wisconsin-Madison, 475 North Charter Street, Madison, WI 53706, USA Received 28 November 2002; received in revised form 13 January 2003; accepted 13 February 2003
Abstract The properties of widespread ionized gas in the disk and halo of the Milky Way have been explored through the detection of faint optical emission lines from hydrogen, helium, and trace atoms and ions within the gas. Observations reveal extensive regions of warm (T 104 K), low density (ne 101 cm3 ) ionized hydrogen spread throughout the interstellar medium, with enormous loops and filaments stretching across the sky and arching far above the midplane of the Milky Way. These observations have provided new clues about the role of massive stars and supernova explosions in shaping the large scale properties of the interstellar medium as well as the possible relationship between this wide spread gas and the small interstellar cloud surrounding the sun. Observed differences between this local cloud and the more extensive regions of ionization appear to be due primarily to differences in the strength and spectrum of the ionizing radiation field. Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Local interstellar medium; Warm ionized gas; Milky way
1. Introduction The interstellar medium plays an important role in the ongoing cycle of stellar birth and death and galactic evolution. It is the material out of which new stars are born and into which dying stars return their nuclear processed remains. The precise role of interstellar matter, from how its properties are influenced by stars to how in turn its properties influence star formation, is poorly understood and is arguably the least understood portion of the cycle. One of the important influences of stars on the interstellar medium is the widespread ionization and heating of the gas by ultraviolet radiation from the youngest and most massive stars. Within the past 15 years, low density photoionized hydrogen has become recognized as a major component of the interstellar medium of our Galaxy (e.g., McKee, 1990; Reynolds, 1991; Ferriere, 2001). The surprisingly high mass, large vertical scale height, and enormous power requirement
*
Tel.: +1-608-262-1249; fax: +1-608-263-6386. E-mail address: [email protected] (R.J. Reynolds).
of this ionized gas has significantly modified our understanding of the composition and structure of the interstellar medium and the distribution and flux of ionizing radiation within the Milky Way’s disk and halo. Widespread ionized hydrogen also has been found in other galaxies (e.g., Rand et al., 1990). The existence of this gas provides clues about the influence of rare massive O type stars on the structure and ionization of the interstellar medium, requiring, for example, that large regions of the galactic disk be free of neutral hydrogen (e.g., Reynolds, 2002; Reynolds et al., 2001). This is contrary to the traditional picture of the interstellar medium, in which photoionized gas was thought to be confined to small, isolated regions (called ‘‘Str€ omgren Spheres’’ or ‘‘H II regions’’) around hot, newly formed stars near the Galactic midplane. It now appears that approximately 90% of the ionized interstellar hydrogen actually resides in this much more extensive Warm Ionized Medium (WIM). The following is a brief review of the properties of diffuse ionized gas in our Galaxy with a discussion of how the low density, partially ionized Local Interstellar Cloud (LIC) immediately surrounding the sun may fit into current understanding of the WIM.
0273-1177/$30 Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.02.059
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R.J. Reynolds / Advances in Space Research 34 (2004) 27–34
2. The warm ionized medium (WIM) The WIM consists of regions of warm (T 104 K), low-density ðne 101 cm3 ) ionized hydrogen (H II) that occupy approximately 20% of the volume within a 2 kpc thick layer about the Galactic midplane (e.g., Haffner et al., 1999; Nordgren et al., 1992). Near the midplane, the space averaged density of the H II is less than 5% that of the neutral hydrogen (H I). However, because of the greater thickness of this ionized layer, the total column density of H II along high Galactic latitude sight lines is relatively large, 1/4 to 1/2 that of the H I, and one kiloparsec above the midplane, this warm H II may be the dominant state of the interstellar medium (Ferriere, 2001; Reynolds, 1991). Its large thickness can have a significant effect upon the interstellar pressure near the midplane (Cox, 1989) and upon the dynamics of hot (106 K) supernova created bubbles (Heiles, 1990). Although originally detected in the 1960s with radio techniques, subsequent developments in high throughput Fabry–Perot spectroscopy have shown that the primary source of information about the large scale distribution, kinematics, and other physical properties of the WIM is obtained through the detection and study of faint, diffuse interstellar emission lines at optical wavelengths.
3. The Wisconsin Ha mapper survey A significant advance in these optical studies has been made by the Wisconsin Ha Mapper (WHAM), a remotely controlled observing facility, funded by the National Science Foundation and dedicated to the detection and study of faint optical emission lines from the Milky Way (e.g., Reynolds et al., 2002). WHAM consists of a 15 cm aperture, dual etalon Fabry–Perot spectrometer coupled to a 0.6 m aperture siderostat, which provide a one degree diameter beam on the sky and produce a 12 km s1 (0.025 nm at Ha) resolution spectrum across a 200 km s1 (0.44 nm) spectral window. This spectral window can be centered on any wavelength between 480 and 730 nm using a gas pressure (optical index) control system and a filter wheel. The tandem etalons greatly extend the effective ‘‘free spectral range’’ of the spectrometer, improve the shape of the response profile, and suppress the multi-order Fabry–Perot ghosts, especially those arising from the relatively bright atmospheric OH emission lines within the pass band of the interference filter. A high quantum efficiency (78% at Ha), low noise (3 e rms) CCD camera serves as a multichannel detector, recording the spectrum as a Fabry–Perot ‘‘ring image’’ without scanning (e.g., Reynolds et al., 1998a). WHAM has made it possible to probe the properties of ionized R interstellar clouds having emission measures (EM ne np ds) as
low as 0.1 cm6 pc, which is 104–6 times fainter than traditional H II regions. The recently completed WHAM Northern Sky Ha Survey (http://astro.wisc.edu/wham/) has provided the first map of the large scale distribution and kinematics of the diffuse interstellar H II through the Balmer-a (Ha) hydrogen recombination line, comparable to earlier surveys of interstellar H I made through the 21 cm line. From 1997 January through 1998 September, WHAM obtained 37,565 spectra with its 1° diameter beam covering 3/4 of the sky on a 0:98°= cos b 0:85° grid ðl; bÞ north of declination )30°. The radial velocity interval for the survey was limited to 100 km s1 with respect to the Galactic local standard of rest (LSR). This range includes nearly all of the interstellar Ha emission except that associated with very faint High Velocity H I Clouds (HVCs), which have radial velocities jvj > 80 km s1 . The emission from HVCs has been the subject of follow-on studies (e.g., Tufte et al., 1998, 2002). Fig. 1 shows the total Ha intensity from the WHAM survey. Interstellar Ha emission extends over virtually the entire sky, with blobs and filaments of enhanced Ha superposed on a more diffuse background. The highest Ha intensities are found near the Galactic equator, with a general decrease toward the poles, consistent with an overall disk-like distribution. The high latitude enhancement at Galactic longitude l ¼ 315°, and latitude b ¼ þ50° is the H II region ionized by the nearby (88 pc) B star Spica, and some of the other high latitude enhancements also seem to be associated with either B stars or hot white dwarf stars, such as p Aqr (Bl V) at 66°, )45° and sdO PHL 6783 at 124°, )74° (see Haffner, 2001). The large, relatively bright region of diffuse and filamentary emission located between longitudes 180° and 240° and latitudes )5° and )50° is the Orion– Eridanus bubble associated with the Orion OB association (Heiles et al., 1999). At high Galactic latitudes the interstellar Ha intensity is typically 0:5 1R ð1R ¼ 106 =4p photon cm2 s1 sr1 ), corresponding to an emission measure EM ¼ 1–2 cm6 pc. In the faintest region near l ¼ 150°, b ¼ þ50° (the Lockman Window), EM 6 0:2 cm6 pc. Of the known sources of ionization in the Galaxy, only the massive O stars produce enough ionizing photons to account for this ionization. However, in the traditional picture of the interstellar medium, widespread photoionized hydrogen cannot exist, because O stars are few and far between and because the interstellar volume was thought to be filled with neutral hydrogen, which is opaque to hydrogen ionizing photons. Although the ionization of the WIM is not yet understood, it can perhaps be explained if significant portions of the interstellar volume have been hollowed out by supernova explosions. Then in some cases, hydrogen ionizing photons may be able to travel large distances from the O stars through these fully ionized, hot (106 K), low density
R.J. Reynolds / Advances in Space Research 34 (2004) 27–34
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Fig. 1. The WHAM Northern Sky Survey total Ha intensity map. The WHAM survey provides for the first time a detailed picture of the distribution and kinematics of the diffuse H II over the sky. This map is centered at Galactic longitude l ¼ 120°, latitude b ¼ 0°, with the Galactic equator running horizontally through the center of the map. Lines of constant longitude and latitude are drawn at 30° and 15° intervals, respectively. An Ha intensity of 1 R (Rayleigh) corresponds to 106 /4p photons cm2 sl sr1 or an emission measure of 2.3 cm6 pc (for T ¼ 8000 K). A higher quality, color version of this map plus the full kinematic survey can be found at http://www.astro.wisc.edu/wham/.
(few 103 cm3 ) cavities (Reynolds et al., 2001). The Orion OB association with its associated Orion– Eridanus bubble provides an excellent example of how this could work. The O stars are just inside the immense cavity, and the ionizing radiation from these stars travels freely across the cavity before striking the distant cavity wall of swept up interstellar gas. If sufficiently large and numerous, such bubbles could explain the extensive photoionization. However, the fraction of the interstellar volume actually filled with these hot cavities is still a subject of debate (e.g., Shelton and Cox, 1994).
4. The local interstellar cloud Another potential source of information about warm ionized interstellar gas is the study of the Local Interstellar Cloud (LIC) that immediately surrounds the sun. Resonantly scattered solar radiation from neutral interstellar hydrogen and helium atoms flowing into the Solar System have revealed that the sun and its surrounding heliosphere are plowing through a small (1 pc), low density interstellar cloud (see review by Fahr, 1973). This cloud is itself immersed in a much larger (100 pc) and lower density region of hot (106 K) gas called the Local Bubble (Frisch, 1998; Cox and Reynolds, 1987). Although no O stars are currently within the Local Bubble, hydrogen ionizing photons from the less luminous B stars and from the hot gas within the bubble are incident on this cloud (e.g., Slavin and Frisch, 2002). New information about the properties of the LIC has recently been obtained from analyses and modeling of
‘‘pickup ions’’ and ‘‘anomalous cosmic rays’’, which originate from the LIC’s neutral interstellar atoms that are ionized and swept up by the solar wind after flowing through the heliospheric boundary (e.g., Cummings et al., 2002; Gloeckler and Geiss, 2002; Slavin and Frisch, 2002). This interplanetary probe of the cloud, combined with more traditional, absorption line observations toward nearby stars (e.g., Gry and Jenkins, 2001; Wood and Linsky, 1997) and a low frequency radio continuum study by Peterson and Webber (2002), have provided detailed information about its physical properties. They reveal that the LIC is significantly ionized and has a temperature and electron density similar to that in the WIM (e.g., Slavin and Frisch, 2002; Gry and Jenkins, 2001). Specifically, T ¼ 7000 1200 K, the neutral hydrogen density n(H0 ) 0.24 cm3 , and the electron density ne 0:13 cm3 . Within the LIC the hydrogen ionization fraction n(Hþ )/n(Htotal ) 1/3, with an upper limit of 0.55 (Gry and Jenkins, 2001). Interestingly, the ionization fraction of helium appears to be greater than that of hydrogen, with n(Heþ )/ n(Hetotal ) 1/2 (Dupuis et al., 1995). Along a typical line of sight, s, through theR cloud the average neutral hydrogen column density nðH0 Þds 4 1017 cm2 , implying an extent 1 pc (Slavin and Frisch, 2002). An important question is whether this cloud is part of the WIM. Unfortunately, the expected emission meaR sure of the LIC, np ne ds 0:01–0:02 cm6 pc, is a factor of ten below that detected in the faintest regions of the WHAM survey (see Fig. 1), which means that its Ha emission is overwhelmed by that from the more distant WIM. However, even though it is not detectable in optical emission, the relationship of the the LIC to the
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R.J. Reynolds / Advances in Space Research 34 (2004) 27–34
WIM still can be explored. As shown below, the results indicate that, although their temperatures and electron densities are very similar, there are significant differences between the WIM and the LIC in the ionization state of hydrogen and the strength and spectrum of the ionizing radiation. 5. The ionization of hydrogen in the WIM With the Ha survey completed, WHAM observations have been directed toward the detection and study of fainter emission lines from other atoms and ions in order to examine the properties of the gas and the ionizing radiation. One of the important issues is the ionization state of the hydrogen. There appears to be a close relationship, both spatially and kinematically, between diffuse ionized gas and warm neutral hydrogen clouds (Haffner et al., 2001; Reynolds et al., 1995), with the column density of the H II typically about 1/3 that of the H I, just as is found in the LIC. However, unlike the LIC, the Hþ and the H0 , though associated, appear to be in physically separate regions, not mixed together in the form of partially (e.g., 1/3) ionized clouds. The high ionization fraction of hydrogen in the WIM is revealed by the relatively low [O I] k630 nm/Ha line intensity ratios. The ionization state of oxygen is tied closely to that of hydrogen by the resonant charge exchange process Oþ + H0 () O0 + Hþ . Thus the intensity of the [O I] line (which is excited by collisions with warm electrons) relative to Ha measures the neutral hydrogen fraction within the WIM. In particular, the volume photon emissivity of the O 1 D k6300 transition relative to Ha is related to the hydrogen ionization ratio n(Hþ )/n(H0 ) through the relation O I nðH0 Þ nðOÞ T41:85 ¼ 2:63 104 þ n Ha nðH Þ nðHÞ 1 þ 0:605T41:105 2:284 exp ; T4
ð1Þ
where n(O)/n(H) is the gas phase abundance of oxygen, T4 is the electron temperature in units of 104 K, and n is a factor with a value near unity given by ð1 þ rÞ=ð89 þ rÞ,
where r ¼ n(H0 )/n(Hþ ). Eq. (1) incorporates the temperature dependence of the [O I] collision strength given by Pequignot (1990). The [O I] airglow line, which is of order 100 times brighter than the interstellar line, makes these observations difficult. Fortunately, in certain directions at particular times of the year it is possible to resolve these two sources spectrally. For example, the diffuse ionized gas in the Perseus spiral arm in the Galactic longitude range 100° < l < 140° is observed to be Doppler shifted by approximately )40 km s1 (0.08 nm) due to the differential rotation of the Galactic disk, and from July through September the earth’s orbital velocity adds an additional shift of about 25 km sl in this region of the sky. This )65 km s1 shift with respect to the earth allows the interstellar [O I] in the Perseus arm to be resolved cleanly from the much brighter terrestrial line. The results for three sightlines in which these observations have been carried out are presented in Table 1 (from Reynolds et al., 1998a), which lists the values and estimated uncertainties for the radial velocity, width, and intensity for the Perseus arm Ha emission components as well as the estimates for the velocity and width of the corresponding [O I] emission. The resulting [O I]/ Ha line intensity ratio is also listed, followed by the hydrogen ionization ratio calculated for electron temperatures of 6000 and 10,000 K (see Reynolds et al., 1998b), assuming a gas phase oxygen abundance relative to hydrogen of 3 104 (Cardelli and Meyer, 1997). Except for the bright H II region NGC 7000, velocity components near the LSR are not included because of the large contamination associated with the terrestrial [O I] emission. These observations have tightly constrained the possible mechanisms of ionization and provide important information about the relationship between the Hþ and the H0 within the WIM. Specifically, the last two columns of Table 1 show that partially ionized clouds like the LIC (where Hþ /Htotal 0.3) appear to be ruled out; the four WIM clouds all have Hþ /Htotal P 0.7–0.9 for T P 7000 K. The temperature in the WIM has been estimated from measurements of the widths of various emission lines, which indicate a mean temperature near 8000 K (Reynolds, 1985). More recent studies of line
Table 1 Results Direction
114°, 0° 130°, 0° 130°, )7.5° NGC 7000 a
Ha
[O I]
v (km s1 )
I (R)
w (FWHM)
v (km s1 )
w (FWHM)
I[O I]/I(Ha) (energy units)
)36 1 )38 1 )60 3 )31 2 )0.4 1.0
9.8 2.0 3.3 0.7 3.4 0.7 2.5 0.5 800
34 1 35 2 36 4 25a 23.7 0.3
)40 1 )42 1 ()60) ()31) 4.5 1.0
19 5 (27) 18 5 (28) (29) (14) 15.2 0.3 (11.5)
0.020 0.003 0.028 0.009 <0.012 0.044 0.011 0.0033 0.0003
Fixed at this value during the fitting.
n(Hþ )/n(H0 ) 6000 K
10,000 K
2.7 2.0 >4.7 1.2 17
28 20 >48 13 170
R.J. Reynolds / Advances in Space Research 34 (2004) 27–34
intensity ratios suggest that, while 8000 K may be the average, there are significant variations in temperature from sightline to sightline and even from cloud to cloud, ranging from about 7000–10,000 K or more (Haffner et al., 1999; Reynolds et al., 1999). Note that if the total (solar) O/H abundance were used instead of the gas phase abundance in Eq. (1), the derived WIM hydrogen ionization fractions would be larger than those listed in Table 1. Therefore, within the WIM the hydrogen ionization fraction is substantially larger than that deduced for the LIC. It should be pointed out, however, that these [O I] observations sample the WIM near the Galactic midplane, and there is evidence in other galaxies (Rand, 1997) and in the Milky Way (Hausen et al., 2002) for higher [O I]/Ha ratios (lower Hþ /H ionization fractions) farther from the midplane.
6. Heþ /Hetotal and the ionizing photon spectrum Because the first ionization potentials of hydrogen and helium are very different (13.6 and 24.6 eV, respectively), the He I k588 nm/Ha intensity ratio is a measure not only of the ionization state of helium relative to that of hydrogen, but is also an indicator of the hardness of the ionizing spectrum (e.g., Reynolds and Tufte, 1995). Low He I/Ha intensity ratios observed in the WIM relative to the ratios observed in bright O star H II regions, where Heþ /Hetotal and Hþ /Htotal are both near unity (e.g., NGC 7000), implies that in the WIM (unlike the LIC) the ionization fraction of helium is significantly less than that of hydrogen; specifically, in the WIM Heþ /Hetotal 0.3–0.6 Hþ /Htotal (Tufte, 1997). This indicates that the radiation ionizing the WIM is softer than that from O stars. Also, because Hþ /Htotal is near unity in the WIM, these observations suggest that the ionization state of helium in the WIM is comparable to that in the LIC. Combined with the difference in the hydrogen ionization states, this implies that the ionizing spectrum for the LIC is significantly harder than that for the WIM.
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The heterogeneous nature of the WIM is revealed in Fig. 2, which shows emission lines from H I (21 cm) plus seven optical lines obtained by WHAM toward the direction l ¼ 130:°0, b ¼ 7:5° (Madsen, 2002). This particular sightline was selected because the Ha emission is relatively bright, making it possible to detect optical emission lines, including the exceedingly faint, temperature sensitive [N II]k576 nm line, in a number of radial velocity components (clouds), and because the [N II]k658 nm/Ha and [S II]k672 nm/Ha intensity ratios are characteristic of diffuse low density gas (i.e., are significantly higher than the ratios in the bright, traditional H II regions; Haffner et al., 1999). This line of sight was also observed in [O I] emission (see Table 1). Radial velocity components ranging from near the local standard of rest (LSR) out to about )80 km s1 are clearly present in the Ha and [N II]k658 nm profiles, indicating that the ionized gas is spread along a large portion of the sight line out to a distance of a least 2000 pc from the sun and 300 pc from the midplane (see Haffner et al., 1999). Fig. 2 shows the close correspondence between the neutral and ionized gas along the sightline, with the four ionized clouds each having associated (but not intermixed) H I. Large variations in the line ratios are apparent between some of the clouds in this direction. For example, between components 1 and 3 there is a large difference in the [O III]/[S II] intensity ratio. These variations in [O III] (ionization potential of Oþ2 ¼ 35 eV) relative to lines from ions with much lower ionization potentials like [S II] (IP of S ¼ 10 eV) indicate significant differences from cloud to cloud in the ionization state of oxygen. Also, because the [N II]k576 nm/ [N II]k658 nm intensity ratio is a direct measure of electron temperature (Osterbrock, 1989), the low k576/ k658 nm ratio in component three implies that that cloud has a lower temperature in addition to its higher Oþ2 abundance. There are also clear variations in the hydrogen ionization fraction in this direction. The hydrogen in cloud 3 is significantly more ionized than cloud two (see Table 1), and the H I cloud at )30 km s1 must be primarily neutral, since it has no associated optical lines from ionized gas (see Fig. 2).
7. Variations in ionization and excitation conditions Recent WHAM observations of faint optical emission lines from other ions indicate that conditions within the WIM are not homogeneous. Variations in excitation and ionization are apparent with distance from the Galactic midplane, from sightline to sightline, and even from one cloud to the next along a single sightline. This complexity of the WIM has important implications for the nature of interstellar clouds and their environments as well as the possible relationship between this widespread ionization and the LIC.
8. Summary and conclusions Regions of warm ionized interstellar gas (the WIM), with electron densities near 0.1 cm3 and temperatures near 8000 K, occupy a significant fraction of the Galactic disk and halo. While the density and temperature of the WIM are very close to what is found for the LIC, it appears that the hydrogen in most of the WIM is nearly fully ionized, with Hþ /Htotal well above the value of 1/3 in the LIC.
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R.J. Reynolds / Advances in Space Research 34 (2004) 27–34
Fig. 2. The neutral hydrogen 21 cm spectrum from Leiden/Dwingeloo (Hartmann and Burton, 1997) plus seven optical emission lines from WHAM toward l ¼ 130°, b ¼ 7:5°. The accompanying table compares typical line intensity ratios (relative to Ha in photon units) for the WIM and for much brighter, traditional H II regions surrounding luminous O stars. Wavelengths are in angstroms (figure: courtesy of G. Madsen).
The ratio of the ionizing flux to the electron density (i.e., the ionization parameter) determines ionization ratios under photoionization equilibrium conditions. Therefore, the LICs relatively low hydrogen ionization fraction may be due at least in part to the fact that the local ionizing radiation field is much weaker than that associated with WIM clouds. Slavin and Frisch (2002) calculate a hydrogen ionizing flux of approximately 104 photons cm2 s1 for the LIC. The Ha intensities of interstellar clouds, which provide a direct measure of the incident ionizing flux (Tufte et al., 1998), indicate fluxes for the WIM that are 105–6 photons cm2 s1 (e.g., Reynolds et al., 1995). This factor of 10–100 in the hydrogen ionizing flux between the LIC and the WIM is more than sufficient to account for the observed differ-
ence in their hydrogen ionization. Interestingly, since the LIC and WIM appear to have roughly similar helium ionization fractions, the flux of helium ionizing radiation in the WIM appears to be not very different from the local (LIC) value. Due to the much higher radiation field in the WIM, optical emission from the WIM clouds will mask any emission from LIC-like clouds. Furthermore, even if the LIC were fully ionized, its emission measure would still be too small to be detected, because its hydrogen column density is so low (<1018 cm2 ) compared to WIM clouds (few 1018 cm2 ; Reynolds et al., 1995). Thus it is possible that weakly ionized clouds similar to the LIC are common, but just have column densities and emission measures that are too low to be detected over the
R.J. Reynolds / Advances in Space Research 34 (2004) 27–34
brighter, more highly ionized clouds along the line of sight. While clouds associated with the WIM appear to be located in parts of the low density interstellar medium exposed directly to O star radiation, the LIC may represent a class of clouds residing in the shadows, exposed only to much weaker B star fluxes and to the weak, but spectrally hard, radiation produced by the hot (106 K) phase of the interstellar medium (Slavin and Frisch, 2002). In this case, the LIC offers a unique opportunity to explore a type of interstellar cloud, and a part of the WIM, not accessible by more traditional astronomical observations.
Acknowledgements The WHAM results were made possible by many people, including Matt Haffner, Greg Madsen, Steve Tufte, Nikki Hausen, and Kurt Jaehnig. WHAM is supported by the National Science Foundation through grants AST96-19424 and AST02-04973.
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the galactic plane: Ha-emitting HI clouds. The Astrophysical Journal 448, 715, 1995. Shelton, R.L., Cox, D.P. Hot gas in the interstellar medium: a reanalysis of O VI absorption data. The Astrophysical Journal 434, 599, 1994. Slavin, J.D., Frisch, P.C. The lonization of nearby interstellar gas. The Astrophysical Journal 565, 364, 2002. Tufte, S.L. The WHAM spectrometer: design, performance characteristics, and first results. Ph.D. Thesis, University of Wisconsin – Madison, 1997.
Tufte, S.L., Reynolds, R.J., Haffher, L.M. WHAM Observations of Halpha emission from high-velocity clouds in the M, A, and C complexes. The Astrophysical Journal 504, 773, 1998. Tufte, S.L., Wilson, J.D., Madsen, G.J., Haffner, L.M., Reynolds, R.J. WHAM Observations of H-alpha from high-velocity clouds: are they galactic or extragalactic? The Astrophysical Journal 572, L153, 2002. Wood, B.E., Linsky, J.L. A new measurement of the electron density in the local interstellar medium. The Astrophysical Journal 474, L39, 1997.
Warm ionized gas in the local interstellar medium R.J. Reynolds
*
Department of Astronomy, University of Wisconsin-Madison, 475 North Charter Street, Madison, WI 53706, USA Received 28 November 2002; received in revised form 13 January 2003; accepted 13 February 2003
Abstract The properties of widespread ionized gas in the disk and halo of the Milky Way have been explored through the detection of faint optical emission lines from hydrogen, helium, and trace atoms and ions within the gas. Observations reveal extensive regions of warm (T 104 K), low density (ne 101 cm3 ) ionized hydrogen spread throughout the interstellar medium, with enormous loops and filaments stretching across the sky and arching far above the midplane of the Milky Way. These observations have provided new clues about the role of massive stars and supernova explosions in shaping the large scale properties of the interstellar medium as well as the possible relationship between this wide spread gas and the small interstellar cloud surrounding the sun. Observed differences between this local cloud and the more extensive regions of ionization appear to be due primarily to differences in the strength and spectrum of the ionizing radiation field. Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Local interstellar medium; Warm ionized gas; Milky way
1. Introduction The interstellar medium plays an important role in the ongoing cycle of stellar birth and death and galactic evolution. It is the material out of which new stars are born and into which dying stars return their nuclear processed remains. The precise role of interstellar matter, from how its properties are influenced by stars to how in turn its properties influence star formation, is poorly understood and is arguably the least understood portion of the cycle. One of the important influences of stars on the interstellar medium is the widespread ionization and heating of the gas by ultraviolet radiation from the youngest and most massive stars. Within the past 15 years, low density photoionized hydrogen has become recognized as a major component of the interstellar medium of our Galaxy (e.g., McKee, 1990; Reynolds, 1991; Ferriere, 2001). The surprisingly high mass, large vertical scale height, and enormous power requirement
*
Tel.: +1-608-262-1249; fax: +1-608-263-6386. E-mail address: [email protected] (R.J. Reynolds).
of this ionized gas has significantly modified our understanding of the composition and structure of the interstellar medium and the distribution and flux of ionizing radiation within the Milky Way’s disk and halo. Widespread ionized hydrogen also has been found in other galaxies (e.g., Rand et al., 1990). The existence of this gas provides clues about the influence of rare massive O type stars on the structure and ionization of the interstellar medium, requiring, for example, that large regions of the galactic disk be free of neutral hydrogen (e.g., Reynolds, 2002; Reynolds et al., 2001). This is contrary to the traditional picture of the interstellar medium, in which photoionized gas was thought to be confined to small, isolated regions (called ‘‘Str€ omgren Spheres’’ or ‘‘H II regions’’) around hot, newly formed stars near the Galactic midplane. It now appears that approximately 90% of the ionized interstellar hydrogen actually resides in this much more extensive Warm Ionized Medium (WIM). The following is a brief review of the properties of diffuse ionized gas in our Galaxy with a discussion of how the low density, partially ionized Local Interstellar Cloud (LIC) immediately surrounding the sun may fit into current understanding of the WIM.
0273-1177/$30 Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.02.059
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R.J. Reynolds / Advances in Space Research 34 (2004) 27–34
2. The warm ionized medium (WIM) The WIM consists of regions of warm (T 104 K), low-density ðne 101 cm3 ) ionized hydrogen (H II) that occupy approximately 20% of the volume within a 2 kpc thick layer about the Galactic midplane (e.g., Haffner et al., 1999; Nordgren et al., 1992). Near the midplane, the space averaged density of the H II is less than 5% that of the neutral hydrogen (H I). However, because of the greater thickness of this ionized layer, the total column density of H II along high Galactic latitude sight lines is relatively large, 1/4 to 1/2 that of the H I, and one kiloparsec above the midplane, this warm H II may be the dominant state of the interstellar medium (Ferriere, 2001; Reynolds, 1991). Its large thickness can have a significant effect upon the interstellar pressure near the midplane (Cox, 1989) and upon the dynamics of hot (106 K) supernova created bubbles (Heiles, 1990). Although originally detected in the 1960s with radio techniques, subsequent developments in high throughput Fabry–Perot spectroscopy have shown that the primary source of information about the large scale distribution, kinematics, and other physical properties of the WIM is obtained through the detection and study of faint, diffuse interstellar emission lines at optical wavelengths.
3. The Wisconsin Ha mapper survey A significant advance in these optical studies has been made by the Wisconsin Ha Mapper (WHAM), a remotely controlled observing facility, funded by the National Science Foundation and dedicated to the detection and study of faint optical emission lines from the Milky Way (e.g., Reynolds et al., 2002). WHAM consists of a 15 cm aperture, dual etalon Fabry–Perot spectrometer coupled to a 0.6 m aperture siderostat, which provide a one degree diameter beam on the sky and produce a 12 km s1 (0.025 nm at Ha) resolution spectrum across a 200 km s1 (0.44 nm) spectral window. This spectral window can be centered on any wavelength between 480 and 730 nm using a gas pressure (optical index) control system and a filter wheel. The tandem etalons greatly extend the effective ‘‘free spectral range’’ of the spectrometer, improve the shape of the response profile, and suppress the multi-order Fabry–Perot ghosts, especially those arising from the relatively bright atmospheric OH emission lines within the pass band of the interference filter. A high quantum efficiency (78% at Ha), low noise (3 e rms) CCD camera serves as a multichannel detector, recording the spectrum as a Fabry–Perot ‘‘ring image’’ without scanning (e.g., Reynolds et al., 1998a). WHAM has made it possible to probe the properties of ionized R interstellar clouds having emission measures (EM ne np ds) as
low as 0.1 cm6 pc, which is 104–6 times fainter than traditional H II regions. The recently completed WHAM Northern Sky Ha Survey (http://astro.wisc.edu/wham/) has provided the first map of the large scale distribution and kinematics of the diffuse interstellar H II through the Balmer-a (Ha) hydrogen recombination line, comparable to earlier surveys of interstellar H I made through the 21 cm line. From 1997 January through 1998 September, WHAM obtained 37,565 spectra with its 1° diameter beam covering 3/4 of the sky on a 0:98°= cos b 0:85° grid ðl; bÞ north of declination )30°. The radial velocity interval for the survey was limited to 100 km s1 with respect to the Galactic local standard of rest (LSR). This range includes nearly all of the interstellar Ha emission except that associated with very faint High Velocity H I Clouds (HVCs), which have radial velocities jvj > 80 km s1 . The emission from HVCs has been the subject of follow-on studies (e.g., Tufte et al., 1998, 2002). Fig. 1 shows the total Ha intensity from the WHAM survey. Interstellar Ha emission extends over virtually the entire sky, with blobs and filaments of enhanced Ha superposed on a more diffuse background. The highest Ha intensities are found near the Galactic equator, with a general decrease toward the poles, consistent with an overall disk-like distribution. The high latitude enhancement at Galactic longitude l ¼ 315°, and latitude b ¼ þ50° is the H II region ionized by the nearby (88 pc) B star Spica, and some of the other high latitude enhancements also seem to be associated with either B stars or hot white dwarf stars, such as p Aqr (Bl V) at 66°, )45° and sdO PHL 6783 at 124°, )74° (see Haffner, 2001). The large, relatively bright region of diffuse and filamentary emission located between longitudes 180° and 240° and latitudes )5° and )50° is the Orion– Eridanus bubble associated with the Orion OB association (Heiles et al., 1999). At high Galactic latitudes the interstellar Ha intensity is typically 0:5 1R ð1R ¼ 106 =4p photon cm2 s1 sr1 ), corresponding to an emission measure EM ¼ 1–2 cm6 pc. In the faintest region near l ¼ 150°, b ¼ þ50° (the Lockman Window), EM 6 0:2 cm6 pc. Of the known sources of ionization in the Galaxy, only the massive O stars produce enough ionizing photons to account for this ionization. However, in the traditional picture of the interstellar medium, widespread photoionized hydrogen cannot exist, because O stars are few and far between and because the interstellar volume was thought to be filled with neutral hydrogen, which is opaque to hydrogen ionizing photons. Although the ionization of the WIM is not yet understood, it can perhaps be explained if significant portions of the interstellar volume have been hollowed out by supernova explosions. Then in some cases, hydrogen ionizing photons may be able to travel large distances from the O stars through these fully ionized, hot (106 K), low density
R.J. Reynolds / Advances in Space Research 34 (2004) 27–34
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Fig. 1. The WHAM Northern Sky Survey total Ha intensity map. The WHAM survey provides for the first time a detailed picture of the distribution and kinematics of the diffuse H II over the sky. This map is centered at Galactic longitude l ¼ 120°, latitude b ¼ 0°, with the Galactic equator running horizontally through the center of the map. Lines of constant longitude and latitude are drawn at 30° and 15° intervals, respectively. An Ha intensity of 1 R (Rayleigh) corresponds to 106 /4p photons cm2 sl sr1 or an emission measure of 2.3 cm6 pc (for T ¼ 8000 K). A higher quality, color version of this map plus the full kinematic survey can be found at http://www.astro.wisc.edu/wham/.
(few 103 cm3 ) cavities (Reynolds et al., 2001). The Orion OB association with its associated Orion– Eridanus bubble provides an excellent example of how this could work. The O stars are just inside the immense cavity, and the ionizing radiation from these stars travels freely across the cavity before striking the distant cavity wall of swept up interstellar gas. If sufficiently large and numerous, such bubbles could explain the extensive photoionization. However, the fraction of the interstellar volume actually filled with these hot cavities is still a subject of debate (e.g., Shelton and Cox, 1994).
4. The local interstellar cloud Another potential source of information about warm ionized interstellar gas is the study of the Local Interstellar Cloud (LIC) that immediately surrounds the sun. Resonantly scattered solar radiation from neutral interstellar hydrogen and helium atoms flowing into the Solar System have revealed that the sun and its surrounding heliosphere are plowing through a small (1 pc), low density interstellar cloud (see review by Fahr, 1973). This cloud is itself immersed in a much larger (100 pc) and lower density region of hot (106 K) gas called the Local Bubble (Frisch, 1998; Cox and Reynolds, 1987). Although no O stars are currently within the Local Bubble, hydrogen ionizing photons from the less luminous B stars and from the hot gas within the bubble are incident on this cloud (e.g., Slavin and Frisch, 2002). New information about the properties of the LIC has recently been obtained from analyses and modeling of
‘‘pickup ions’’ and ‘‘anomalous cosmic rays’’, which originate from the LIC’s neutral interstellar atoms that are ionized and swept up by the solar wind after flowing through the heliospheric boundary (e.g., Cummings et al., 2002; Gloeckler and Geiss, 2002; Slavin and Frisch, 2002). This interplanetary probe of the cloud, combined with more traditional, absorption line observations toward nearby stars (e.g., Gry and Jenkins, 2001; Wood and Linsky, 1997) and a low frequency radio continuum study by Peterson and Webber (2002), have provided detailed information about its physical properties. They reveal that the LIC is significantly ionized and has a temperature and electron density similar to that in the WIM (e.g., Slavin and Frisch, 2002; Gry and Jenkins, 2001). Specifically, T ¼ 7000 1200 K, the neutral hydrogen density n(H0 ) 0.24 cm3 , and the electron density ne 0:13 cm3 . Within the LIC the hydrogen ionization fraction n(Hþ )/n(Htotal ) 1/3, with an upper limit of 0.55 (Gry and Jenkins, 2001). Interestingly, the ionization fraction of helium appears to be greater than that of hydrogen, with n(Heþ )/ n(Hetotal ) 1/2 (Dupuis et al., 1995). Along a typical line of sight, s, through theR cloud the average neutral hydrogen column density nðH0 Þds 4 1017 cm2 , implying an extent 1 pc (Slavin and Frisch, 2002). An important question is whether this cloud is part of the WIM. Unfortunately, the expected emission meaR sure of the LIC, np ne ds 0:01–0:02 cm6 pc, is a factor of ten below that detected in the faintest regions of the WHAM survey (see Fig. 1), which means that its Ha emission is overwhelmed by that from the more distant WIM. However, even though it is not detectable in optical emission, the relationship of the the LIC to the
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R.J. Reynolds / Advances in Space Research 34 (2004) 27–34
WIM still can be explored. As shown below, the results indicate that, although their temperatures and electron densities are very similar, there are significant differences between the WIM and the LIC in the ionization state of hydrogen and the strength and spectrum of the ionizing radiation. 5. The ionization of hydrogen in the WIM With the Ha survey completed, WHAM observations have been directed toward the detection and study of fainter emission lines from other atoms and ions in order to examine the properties of the gas and the ionizing radiation. One of the important issues is the ionization state of the hydrogen. There appears to be a close relationship, both spatially and kinematically, between diffuse ionized gas and warm neutral hydrogen clouds (Haffner et al., 2001; Reynolds et al., 1995), with the column density of the H II typically about 1/3 that of the H I, just as is found in the LIC. However, unlike the LIC, the Hþ and the H0 , though associated, appear to be in physically separate regions, not mixed together in the form of partially (e.g., 1/3) ionized clouds. The high ionization fraction of hydrogen in the WIM is revealed by the relatively low [O I] k630 nm/Ha line intensity ratios. The ionization state of oxygen is tied closely to that of hydrogen by the resonant charge exchange process Oþ + H0 () O0 + Hþ . Thus the intensity of the [O I] line (which is excited by collisions with warm electrons) relative to Ha measures the neutral hydrogen fraction within the WIM. In particular, the volume photon emissivity of the O 1 D k6300 transition relative to Ha is related to the hydrogen ionization ratio n(Hþ )/n(H0 ) through the relation O I nðH0 Þ nðOÞ T41:85 ¼ 2:63 104 þ n Ha nðH Þ nðHÞ 1 þ 0:605T41:105 2:284 exp ; T4
ð1Þ
where n(O)/n(H) is the gas phase abundance of oxygen, T4 is the electron temperature in units of 104 K, and n is a factor with a value near unity given by ð1 þ rÞ=ð89 þ rÞ,
where r ¼ n(H0 )/n(Hþ ). Eq. (1) incorporates the temperature dependence of the [O I] collision strength given by Pequignot (1990). The [O I] airglow line, which is of order 100 times brighter than the interstellar line, makes these observations difficult. Fortunately, in certain directions at particular times of the year it is possible to resolve these two sources spectrally. For example, the diffuse ionized gas in the Perseus spiral arm in the Galactic longitude range 100° < l < 140° is observed to be Doppler shifted by approximately )40 km s1 (0.08 nm) due to the differential rotation of the Galactic disk, and from July through September the earth’s orbital velocity adds an additional shift of about 25 km sl in this region of the sky. This )65 km s1 shift with respect to the earth allows the interstellar [O I] in the Perseus arm to be resolved cleanly from the much brighter terrestrial line. The results for three sightlines in which these observations have been carried out are presented in Table 1 (from Reynolds et al., 1998a), which lists the values and estimated uncertainties for the radial velocity, width, and intensity for the Perseus arm Ha emission components as well as the estimates for the velocity and width of the corresponding [O I] emission. The resulting [O I]/ Ha line intensity ratio is also listed, followed by the hydrogen ionization ratio calculated for electron temperatures of 6000 and 10,000 K (see Reynolds et al., 1998b), assuming a gas phase oxygen abundance relative to hydrogen of 3 104 (Cardelli and Meyer, 1997). Except for the bright H II region NGC 7000, velocity components near the LSR are not included because of the large contamination associated with the terrestrial [O I] emission. These observations have tightly constrained the possible mechanisms of ionization and provide important information about the relationship between the Hþ and the H0 within the WIM. Specifically, the last two columns of Table 1 show that partially ionized clouds like the LIC (where Hþ /Htotal 0.3) appear to be ruled out; the four WIM clouds all have Hþ /Htotal P 0.7–0.9 for T P 7000 K. The temperature in the WIM has been estimated from measurements of the widths of various emission lines, which indicate a mean temperature near 8000 K (Reynolds, 1985). More recent studies of line
Table 1 Results Direction
114°, 0° 130°, 0° 130°, )7.5° NGC 7000 a
Ha
[O I]
v (km s1 )
I (R)
w (FWHM)
v (km s1 )
w (FWHM)
I[O I]/I(Ha) (energy units)
)36 1 )38 1 )60 3 )31 2 )0.4 1.0
9.8 2.0 3.3 0.7 3.4 0.7 2.5 0.5 800
34 1 35 2 36 4 25a 23.7 0.3
)40 1 )42 1 ()60) ()31) 4.5 1.0
19 5 (27) 18 5 (28) (29) (14) 15.2 0.3 (11.5)
0.020 0.003 0.028 0.009 <0.012 0.044 0.011 0.0033 0.0003
Fixed at this value during the fitting.
n(Hþ )/n(H0 ) 6000 K
10,000 K
2.7 2.0 >4.7 1.2 17
28 20 >48 13 170
R.J. Reynolds / Advances in Space Research 34 (2004) 27–34
intensity ratios suggest that, while 8000 K may be the average, there are significant variations in temperature from sightline to sightline and even from cloud to cloud, ranging from about 7000–10,000 K or more (Haffner et al., 1999; Reynolds et al., 1999). Note that if the total (solar) O/H abundance were used instead of the gas phase abundance in Eq. (1), the derived WIM hydrogen ionization fractions would be larger than those listed in Table 1. Therefore, within the WIM the hydrogen ionization fraction is substantially larger than that deduced for the LIC. It should be pointed out, however, that these [O I] observations sample the WIM near the Galactic midplane, and there is evidence in other galaxies (Rand, 1997) and in the Milky Way (Hausen et al., 2002) for higher [O I]/Ha ratios (lower Hþ /H ionization fractions) farther from the midplane.
6. Heþ /Hetotal and the ionizing photon spectrum Because the first ionization potentials of hydrogen and helium are very different (13.6 and 24.6 eV, respectively), the He I k588 nm/Ha intensity ratio is a measure not only of the ionization state of helium relative to that of hydrogen, but is also an indicator of the hardness of the ionizing spectrum (e.g., Reynolds and Tufte, 1995). Low He I/Ha intensity ratios observed in the WIM relative to the ratios observed in bright O star H II regions, where Heþ /Hetotal and Hþ /Htotal are both near unity (e.g., NGC 7000), implies that in the WIM (unlike the LIC) the ionization fraction of helium is significantly less than that of hydrogen; specifically, in the WIM Heþ /Hetotal 0.3–0.6 Hþ /Htotal (Tufte, 1997). This indicates that the radiation ionizing the WIM is softer than that from O stars. Also, because Hþ /Htotal is near unity in the WIM, these observations suggest that the ionization state of helium in the WIM is comparable to that in the LIC. Combined with the difference in the hydrogen ionization states, this implies that the ionizing spectrum for the LIC is significantly harder than that for the WIM.
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The heterogeneous nature of the WIM is revealed in Fig. 2, which shows emission lines from H I (21 cm) plus seven optical lines obtained by WHAM toward the direction l ¼ 130:°0, b ¼ 7:5° (Madsen, 2002). This particular sightline was selected because the Ha emission is relatively bright, making it possible to detect optical emission lines, including the exceedingly faint, temperature sensitive [N II]k576 nm line, in a number of radial velocity components (clouds), and because the [N II]k658 nm/Ha and [S II]k672 nm/Ha intensity ratios are characteristic of diffuse low density gas (i.e., are significantly higher than the ratios in the bright, traditional H II regions; Haffner et al., 1999). This line of sight was also observed in [O I] emission (see Table 1). Radial velocity components ranging from near the local standard of rest (LSR) out to about )80 km s1 are clearly present in the Ha and [N II]k658 nm profiles, indicating that the ionized gas is spread along a large portion of the sight line out to a distance of a least 2000 pc from the sun and 300 pc from the midplane (see Haffner et al., 1999). Fig. 2 shows the close correspondence between the neutral and ionized gas along the sightline, with the four ionized clouds each having associated (but not intermixed) H I. Large variations in the line ratios are apparent between some of the clouds in this direction. For example, between components 1 and 3 there is a large difference in the [O III]/[S II] intensity ratio. These variations in [O III] (ionization potential of Oþ2 ¼ 35 eV) relative to lines from ions with much lower ionization potentials like [S II] (IP of S ¼ 10 eV) indicate significant differences from cloud to cloud in the ionization state of oxygen. Also, because the [N II]k576 nm/ [N II]k658 nm intensity ratio is a direct measure of electron temperature (Osterbrock, 1989), the low k576/ k658 nm ratio in component three implies that that cloud has a lower temperature in addition to its higher Oþ2 abundance. There are also clear variations in the hydrogen ionization fraction in this direction. The hydrogen in cloud 3 is significantly more ionized than cloud two (see Table 1), and the H I cloud at )30 km s1 must be primarily neutral, since it has no associated optical lines from ionized gas (see Fig. 2).
7. Variations in ionization and excitation conditions Recent WHAM observations of faint optical emission lines from other ions indicate that conditions within the WIM are not homogeneous. Variations in excitation and ionization are apparent with distance from the Galactic midplane, from sightline to sightline, and even from one cloud to the next along a single sightline. This complexity of the WIM has important implications for the nature of interstellar clouds and their environments as well as the possible relationship between this widespread ionization and the LIC.
8. Summary and conclusions Regions of warm ionized interstellar gas (the WIM), with electron densities near 0.1 cm3 and temperatures near 8000 K, occupy a significant fraction of the Galactic disk and halo. While the density and temperature of the WIM are very close to what is found for the LIC, it appears that the hydrogen in most of the WIM is nearly fully ionized, with Hþ /Htotal well above the value of 1/3 in the LIC.
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R.J. Reynolds / Advances in Space Research 34 (2004) 27–34
Fig. 2. The neutral hydrogen 21 cm spectrum from Leiden/Dwingeloo (Hartmann and Burton, 1997) plus seven optical emission lines from WHAM toward l ¼ 130°, b ¼ 7:5°. The accompanying table compares typical line intensity ratios (relative to Ha in photon units) for the WIM and for much brighter, traditional H II regions surrounding luminous O stars. Wavelengths are in angstroms (figure: courtesy of G. Madsen).
The ratio of the ionizing flux to the electron density (i.e., the ionization parameter) determines ionization ratios under photoionization equilibrium conditions. Therefore, the LICs relatively low hydrogen ionization fraction may be due at least in part to the fact that the local ionizing radiation field is much weaker than that associated with WIM clouds. Slavin and Frisch (2002) calculate a hydrogen ionizing flux of approximately 104 photons cm2 s1 for the LIC. The Ha intensities of interstellar clouds, which provide a direct measure of the incident ionizing flux (Tufte et al., 1998), indicate fluxes for the WIM that are 105–6 photons cm2 s1 (e.g., Reynolds et al., 1995). This factor of 10–100 in the hydrogen ionizing flux between the LIC and the WIM is more than sufficient to account for the observed differ-
ence in their hydrogen ionization. Interestingly, since the LIC and WIM appear to have roughly similar helium ionization fractions, the flux of helium ionizing radiation in the WIM appears to be not very different from the local (LIC) value. Due to the much higher radiation field in the WIM, optical emission from the WIM clouds will mask any emission from LIC-like clouds. Furthermore, even if the LIC were fully ionized, its emission measure would still be too small to be detected, because its hydrogen column density is so low (<1018 cm2 ) compared to WIM clouds (few 1018 cm2 ; Reynolds et al., 1995). Thus it is possible that weakly ionized clouds similar to the LIC are common, but just have column densities and emission measures that are too low to be detected over the
R.J. Reynolds / Advances in Space Research 34 (2004) 27–34
brighter, more highly ionized clouds along the line of sight. While clouds associated with the WIM appear to be located in parts of the low density interstellar medium exposed directly to O star radiation, the LIC may represent a class of clouds residing in the shadows, exposed only to much weaker B star fluxes and to the weak, but spectrally hard, radiation produced by the hot (106 K) phase of the interstellar medium (Slavin and Frisch, 2002). In this case, the LIC offers a unique opportunity to explore a type of interstellar cloud, and a part of the WIM, not accessible by more traditional astronomical observations.
Acknowledgements The WHAM results were made possible by many people, including Matt Haffner, Greg Madsen, Steve Tufte, Nikki Hausen, and Kurt Jaehnig. WHAM is supported by the National Science Foundation through grants AST96-19424 and AST02-04973.
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