- Email: [email protected]
QSO extended emission-line regions
New Astronomy Reviews 50 (2006) 694–700 www.elsevier.com/locate/newastrev
QSO extended emission-line regions Alan Stockton b
a,*
, Hai Fu a, Gabriela Canalizo
q b
a Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA Institute of Geophysics and Planetary Physics and Department of Physics, University of California at Riverside, USA
Available online 17 July 2006
Abstract Extended emission-line regions (EELRs) on scales of a few tens of kpc are found around a substantial fraction of steep-spectrum radio-loud QSOs. In most cases, at least at low redshifts, the distribution of this gas seems to be uncorrelated with either the stellar distribution in the host galaxy or the radio structure. The origin of the gas and the physical processes that control its distribution are still uncertain. We review circumstantial evidence that high densities required to explain the strengths of certain emission lines in spectra of EELRs are due to transient shocks, and we present new Chandra images that show extended X-ray emission associated with two QSOs with known optical EELRs. These discrete X-ray clumps may also be evidence for shocks. We discuss observations that could potentially confirm a model in which EELRs are results of galaxy-wide superwinds that could originate either from starbursts in the host galaxy or from the QSO itself. Finally, we discuss in some detail the QSO 3C 48, which has one of the most luminous EELRs and for which we also have considerable knowledge about both the host galaxy and processes in the immediate vicinity of the QSO. High-velocity ionized gas near the nucleus, but apparently not associated with the radio jet, indicates an outflow that may be the origin of the EELR. 2006 Elsevier B.V. All rights reserved. PACS: 98.54.Aj; 98.62.Nx Keywords: Quasars; Extended emission-line regions; Superwinds
Contents 1. 2. 3. 4. 5.
6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The origin of the extended gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Extended X-ray emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tests of superwind shock models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A case study: 3C 48 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. The nature of 3C 48A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. The blueshifted emission-line component in the nucleus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program # GO-09365.01-A. * Corresponding author. E-mail address: [email protected] (A. Stockton). 1387-6473/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.newar.2006.06.024
A. Stockton et al. / New Astronomy Reviews 50 (2006) 694–700
1. Introduction The first evidence that some QSOs had significant amounts 104 K gas extending well beyond the classical narrow-line region (radius 1 kpc) was the spectroscopic detection of emission lines in the nebulosity around 3C 48 by Wampler et al. (1975). Other examples followed: 4C 37.43 (Stockton, 1976) and 3C 249.1 (Richstone and Oke, 1977). In an important spectroscopic survey for offnuclear emission, Boroson and Oke (1984) and Boroson et al. (1985) not only found additional QSOs with extended emission, but noted that strong extended emission was found essentially exclusively in steep-spectrum radio-loud QSOs. Around the same time, (Stockton and MacKenty, 1983, 1987) carried out a narrow-band imaging survey of a large sample of low-redshift (z 6 0.45) QSOs. This survey confirmed the correlation between extended optical emission and steep-spectrum radio emission. It is also found that these extended emission-line regions (EELRs) seemed especially prevalent in QSOs showing signs of strong interaction. It has gradually become clear that there are at least four general categories of EELRs: (i) Those closely associated with radio jets or radio lobes. These are common at high redshifts but relatively uncommon at low redshifts. The clearest lowredshift example is the z = 0.325 quasar PKS 2251 + 11, where one of two high-surface-brightness [OIII] emission peaks (Stockton and MacKenty, 1987) is coincident with the peak of the southeast radio lobe (Price et al., 1993).
˚Fig. 1. The broad-line radio galaxy 3C 79, imaged through a 30 A bandpass interference filter centered on the redshifted [OIII] k5007 emission line. The inset shows the same image at lower contrast. Linefree continuum images simply show what appears to be a normal elliptical galaxy.
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(ii) Those that involve photoionization by the QSO of the in situ interstellar medium of the host galaxy. An example is PG 0052 + 251, for which the host galaxy is clearly a spiral (Bahcall et al., 1996), and which shows weak extended emission in the outskirts of the disk (Stockton and MacKenty, 1987). (iii) Those in which the QSO photoionizes gas in a nearby object. The clearest example is the compact object to the east of NAB 0205 + 02 (Stockton and MacKenty, 1987); another is the small disk galaxy to the west of MRK 1014 (Canalizo and Stockton, 2000b). (iv) Those that show a complex, filamentary, distribution of ionized gas that appears to be unrelated either to the morphology of the host galaxy or to the extended radio structure. The most luminous EELRs associated with low-redshift QSOs fall into this class. A prime example is 4C 37.43 (Stockton et al., 2002; see Fig. 2). Another example is the broad-line radio galaxy 3C 79 (Fig. 1).
2. The origin of the extended gas A fundamental question for this last class of EELR is the origin of the extended 104 K gas. Stockton and MacKenty (1987), noting the apparent correlation between EELRs and evidence for strong interaction, suggested that the gas was tidal debris. As pointed out by Crawford et al. (1988), the problem with this suggestion is that some form of confinement is necessary; otherwise, the gas will dissipate within a sound crossing time, typically 106 years or less for the 10-parsec-sized regions indicated by photoionization calculations. A solution offered by Fabian et al. (1987) is that the EELRs comprise material that has cooled from a surrounding hot (107–108 K) medium. In order to check whether pressures are consistent with a short cooling time for this external medium, Crawford et al. (1988) (see also Crawford and Vanderriest, 2000) assume hydrostatic equilibrium between the hot gas and the 104 K gas and use the ratio of [OII] k3727 lines to the [OIII] k5007 line as a measure of the ionization parameter. With an estimate of the ionizing flux from the QSO and the distance to the ionized cloud, the pressure can be found. These authors find high pressures and cooling times much shorter than a Hubble time. A more detailed analysis of the EELR around 4C 37.43 (the most luminous known among low-redshift QSOs) has been given by Stockton et al. (2002). Working from Keck low-resolution imaging spectrograph (LRIS) spectra in the optical and HST faint-object spectrograph (FOS) spectra in the UV, they carried out detailed photoionization calculations, using the MAPPINGS3 package (Dopita and Sutherland, 1995). The overall result was that at least two phases were needed to explain the optical emission: a low-density (2 cm3) phase, with approximately unity filling factor, and a high-density (500 cm3), with 105 filling factor. These phases have roughly similar temperatures,
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so they cannot be in pressure equilibrium. Furthermore, most of the [OIII] emission is produced in the low-density region, whereas most of the [OII] comes from the high-density region. Thus two of the crucial assumptions made by Crawford et al. (1988) are invalid: the dense regions cannot be in hydrostatic equilibrium with their surroundings, and the [OII]/[OIII] ratio cannot be used to infer a useful photoionization parameter or pressure. If the low-density region is assumed to be in pressure equilibrium with a much hotter external medium, the pressure is found to be 3 · 104 cm3 K, over two orders of magnitude lower than those found by Crawford and Vanderriest (2000) for 4C 37.43; the cooling time is then much longer than the Hubble time. Another result of this study is that the high-density regions are ionization bounded, with a thickness of only 0.1 pc. With a typical sound speed of 15 km s1, they will have lifetimes of <104 years. There would seem to be only two possible general scenarios that would allow quasi-steady-state emission from these regions: (1) ‘boiloff,’ as surface photoionization works its way into a large, dense, neutral cloud, and (2) high-speed shocks that propagate through the low-density gas, compressing it to the observed high densities. In the first case, the dense clouds would have to be self gravitating (like giant molecular clouds) in order to be stable against disruption over a sufficiently long time. The presence of such clouds at large distances and in a distribution that is unrelated to the morphology of the host galaxy seems problematical. On the other hand, shocks could be due to the ejection of gas from the host galaxy in a superwind either due to a starburst or to the QSO itself. 3. Extended X-ray emission In deep Chandra ACIS imaging, our group has recently found extended X-ray emission around two QSOs that show extended optical emission, 3C 249.1 and 4C 37.43 (Stockton et al., 2006). This X-ray emission is not aligned along the radio axis, as are the X-ray jets frequently found associated with radio-loud QSOs. In some cases, the extended X-ray emission seems to be roughly coincident with optical emission features; in other cases, it is not. Most of the strongest optical features do not have an X-ray counterpart (see Fig. 2). We can show from measured electron densities that the extended X-ray components cannot be due to Compton scattering of nuclear X-ray emission. We know that there will be some X-ray production from 104 K regions photoionized by a typical powerlaw UV continuum, because there will be X-ray line emission from highly ionized heavy species, notably Fe. (see, e.g., Sako et al., 2000; Kinkhabwala et al., 2002). The question is, how significant is this emission, compared with what we actually observe? We believe that this contribution is unlikely to dominate the observed X-ray emission because many regions that show very high surface brightnesses in optical emission are not
Fig. 2. Chandra ACIS-I and [OIII] images of 4C 37.43. The upper panel shows the residual from the PSF subtraction from the Chandra ACIS image. Several discrete off-nuclear X-ray emission sources are labeled. The lower panel shows a deep HST WFPC2 image obtained through a linear ramp filter centered on the redshifted [OIII] k5007 emission line (Stockton et al. 2002). The sources labeled a 0 and b 0 may be associated with the corresponding X-ray sources. Both images are at the same scale, given in the lower-left corner of the lower panel. This figure is from Stockton et al., 2006.
detected at all in X-rays, whereas some regions of X-ray emission show little or no optical emission. The most common kind of extended X-ray emission is thermal bremsstrahlung from a hot surrounding medium. Such hot haloes are often seen around massive elliptical galaxies. Some of the extended emission we see may have such an origin. However, these haloes normally do not show discrete substructure on very small scales. However, another form of thermal bremsstrahlung that is expected to show small-scale structure is that from local heating by shocks. Since we have already considered the possibility of shocks as an explanation for the presence of the highdensity regions responsible for most of the [OII] emission,
A. Stockton et al. / New Astronomy Reviews 50 (2006) 694–700
these explanations are at least potentially consistent with each other. On the other hand, we clearly cannot have the [OII] emission and the X-rays coming from spatially coextensive regions because of the wide difference in the required temperatures. One possible model is that of Lehnert et al. (1999), in which, as the superwind encounters an ambient cloud, it drives a slow (102 km s1) radiative shock into the cloud as well as producing a fast (103 km s1) stand-off bow shock. The former produces the optical emission, the latter the X-rays. 4. Tests of superwind shock models The evidence for the importance of shocks in QSO EELRs we have been able to give is admittedly circumstantial. It would be gratifying to be able to find a more direct test from emission-line diagnostics. There are two difficulties in doing this. Firstly, photoionization models show that the observed UV flux from a QSO is quite capable of producing the observed optical emission-line spectrum for most EELRs. Thus, even if shocks are important in defining the density structure of the gas, any shock signature may well be overwhelmed by photoionization from the central source. Secondly, it is an unfortunate fact that the strong emission lines in the optical region, which work very well to discriminate between photoionization by the UV continua from hot stars and ionization in AGN (e.g., Veilleux and Osterbrock, 1987), fail completely to separate photoionization by a power-law continuum and ionization by shocks. This problem has been discussed in detail by Allen et al. (1998). Relatively strong lines in the UV can be used not only to separate power-law photoionization from shock ionization, but also ‘‘pure’’ shock ionization from the case in which thermal emission from the shock ionizes upstream gas before the shock arrives (‘‘shock + precursor’’; e.g., Allen et al., 1998). With the demise of STIS on the Hubble Space Telescope, however, there is now essentially no possibility of observing these lines in low-redshift quasars. Fortunately, there are some weaker lines in the optical region that can be used to distinguish between photoionization and shocks. Evans et al. (1999) show that a relatively clean separation can be achieved using the [OIII] k4363/[OIII] k5007 and He II/Hb ratios. The [OIII] k4363/[OIII] k5007 line ratio is an important diagnostic of the electron temperature of the ionized gas. The shock and photoionization models are widely separated in the [OIII] k4363/[OIII] k5007 versus He II/Hb diagram because of the different temperature regimes of the shock and photoionization models. Another likely useful diagnostic is the [S II] kk6717,6731/Ha ratio in combination with the He II/Hb ratio. The use of these weaker optical emission lines has been problematic for many applications because of the difficulty in achieving the required S/N and because of systematic uncertainties. With efficient spectrographs on large telescopes, the S/N issue is less of a problem. The main systematic uncertainty has been in modeling the underlying stellar
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continuum, e.g., in Seyfert galaxies. For QSO EELRs, we have the great advantage that we generally do not have any significant stellar background, and, in the few cases where the emission is projected on the host galaxy, the emission is so clumpy that we almost always will have adjacent emission-line-free host galaxy regions as a reference. We have used the [OIII] k4363/[OIII] k5007 versus He II/ Hb diagram for this test in three regions in 3C 249.1 and one region in 4C 37.43. In all four cases, however, the ratios fall in a region of the diagram that lies between the expected loci for photoionization by a power-law continuum, shock ionization, and shock plus precursor ionization. This ambiguity may indicate a mixture of ionization mechanisms, or it may result from remaining uncertainties of the models; in any case, we do not have a compelling case for the presence of shocks. One hope is that, if shocks really are important, we may be able to find some emission regions that are shielded from the QSO UV continuum and that will show a clearly shock-dominated spectrum. 5. A case study: 3C 48 3C 48 was the first quasar identified (Matthews et al., 1961; Matthews and Sandage, 1963). This early identification was partly a result of the compact nature of its radio source, and 3C 48 remains the only compact steep-spectrum (CSS) radio source among powerful quasars at redshifts <0.5. The host galaxy of 3C 48 is larger and more luminous than those of most other low-redshift quasars (Kristian, 1973), and it is clearly an example of a major merger in progress (Stockton and MacKenty, 1987; Canalizo and Stockton, 2000a). 3C 48 is also one of the few previously identified QSOs to have been found by IRAS to be embedded in ultraluminous IR galaxies. Vigorous star formation is currently underway in the inner part of the host galaxy (Canalizo and Stockton, 2000a), and there are still large reserves of molecular gas (Scoville et al., 1993; Wink et al., 1997). All of these things taken together indicate that we are witnessing a brief but important phase in the history of a quasar, a time when host-galaxy star formation is near its peak, the radio jet is breaking through the dense material in the inner part of the host galaxy, and UV radiation from the central continuum source has just recently become visible along many lines of sight. 3C 48 remains one of the most luminous examples of extended emission among lowredshift QSOs (Stockton and MacKenty, 1987). It is an especially interesting example because the host galaxy and the nuclear regions have been studied in more detail than those of any other low-redshift quasar. Boroson and Oke (1982, 1984) showed the presence of strong Balmer absorption in the 3C 48 host galaxy. More recently, Canalizo and Stockton (2000a) carried out a detailed spectroscopic analysis of the host galaxy. They showed that the stellar population has a classic ‘‘E + A00 spectrum; i.e., it shows features from both young and old stellar populations. This character made it possible to carry out a decomposition of the integrated spectrum into the
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young and old populations, using spectral synthesis models (e.g., Bruzual and Charlot, 2003), and three parameters: normalization factors for the young and old stellar populations, and a luminosity-weighted mean age for the young population. These, of course, apply to summations of the light along various line-of-sight paths through the host galaxy. It was possible to determine starburst ages as well as stellar radial velocities in 32 distinct regions in the host. In general, the regions dominated by very recent or current star formation are closest the quasar nucleus, whereas most of the north-west tidal tail shows only the old stellar population. The region very close to the quasar reveals some fascinating features apart from the CSS radio jet, which extends about 100 north from the quasar (Feng et al., 2005). There is a second luminosity peak (3C 48A) about 100 to the northwest of the quasar (Stockton and Ridgway, 1991) which has been variously interpreted as the distorted nucleus of the merging galaxy (Stockton and Ridgway, 1991; Canalizo and Stockton, 2000a) or as jet-induced star formation from the radio jet (Chatzichristou et al., 1999). Finally, the [OIII] lines, which had long been known to have mean redshifts significantly lower than those of the broad lines (Wampler et al., 1975), actually comprise two components: one at the broad-line redshift, and a second one blueshifted by about 580 km s1 in the quasar frame (Chatzichristou et al., 1999; Canalizo and Stockton, 2000a).
HST archive. The quasar nucleus has been subtracted, and the elongated feature to the northwest is 3C 48A. The two lines indicate the position of a 0.00 5-wide slit we have used to obtain a spectrum of 3C 48A with the HST STIS instrument. Tentative results from the spectrum itself are shown in Fig. 4. The clear signature of Balmer absorption line at the longer-wavelength end of the spectrum (where our S/N is relatively high) shows that the spectrum of 3C 48A is dominated by stars. We are interested primarily in the age of these stars in order to decide whether radio-jet-induced star formation is a reasonable explanation for the feature. The gray traces show Bruzual and Charlot (2003) spectral-synthesis models with ages of 1.0 · 108, 1.4 · 108, and 2.0 · 108 years. While the noise in this spectrum increases dramatically towards shorter wavelengths, the overall spectral slope shortward of the Balmer limit should be fairly reliable; an age younger than 1.0 · 108 appears to be quite unlikely. Furthermore, the obvious presence of the Ca II K line and the fact that the Balmer lines are weaker in the spectrum than in any of the models indicates the presence of an older population as well. Our reduction procedure subtracted adjoining regions of the host galaxy, so these older stars are almost certainly in 3C 48A itself. Even this preliminary analysis seems clearly to show that 3C 48A cannot have been the result of jet-induced star formation and is much more likely to be the distorted nucleus of the merging companion.
5.1. The nature of 3C 48A Fig. 3 shows an HST WFPC2 F555W image of the inner part of the 3C 48 host galaxy, taken from the
Fig. 3. The inner region of the 3C 48 host galaxy, from an HST PC F555W image, showing the position of the STIS slit centered on 3C 48A.
Fig. 4. HST STIS spectrum of 3C 48A is shown as the heavy black line. The spectrum is clearly dominated by moderately young stars, along with some [OII] k3727 emission. The noise increases greatly to shorter wavelengths. The three gray traces are instantaneous-burst spectral synthesis models with ages (from top to bottom) of 100 Myr, 140 Myr, and 200 Myr, respectively.
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5.2. The blueshifted emission-line component in the nucleus The Hb–[OIII]-line region in 3C 48 is shown in Fig. 5. The ‘‘anomalous’’ blueshifted component to the [O III] lines actually has about 5 times the flux of the classical narrowline component at the broad-line redshift. Chatzichristou et al. (1999) suggest that the blue-shifted emission is associated with the radio jet, which extends to the north side of the quasar (Feng et al., 2005). We have obtained a deep GMOS Integral-Field Unit (IFU) spectrum of 3C 48 with the Gillett Gemini North telescope. By subtracting a scaled PSF, derived from the nearby continuum, we can explore the distribution of any off-nuclear emission with velocity by taking cuts through
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the IFU data cube at various parts of the line profile. ˚ Fig. 6 shows the distribution of the emission for a 10 A bandpass near the blueshifted emission peak. It is apparent that the bulk of this emission, peaked mostly to the west of the quasar, cannot be associated with the radio jet to the north. Nor is it likely to be connected with a putative counterjet to the south, which may recently have been detected (Feng et al., 2005). The most likely explanation for this high-velocity gas very close to the quasar is that it is part of a wind generated either by the quasar accretion disk or by an intense compact starburst at the center of 3C 48. In either case, this result give additional credence to the suggestion that the EELR around 3C 48 and other quasars is likely to be due to a superwind. 6. Summary
Fig. 5. The spectrum of 3C 48, obtained with GMOS in the IFU mode on ˚ is the the Gillett Gemini North telescope. The feature longward of 6880 A terrestrial atmospheric B-band.
Fig. 6. The distribution of [OIII] emission dominated by blueshifted narrow-line region in 3C 48. The white cross shows the position of the quasar, where a scaled PSF derived from the nearby continuum has been subtracted to give a smooth residual.
While we do not yet have a detailed understanding of how quasar EELRs are related to physical processes in the quasar, the host galaxy, and the immediate environment, we are beginning to accumulate a number of pieces of the puzzle. The presence of dense regions in EELRs with very short estimated lifetimes, together with the demonstration that these cannot be anywhere near hydrostatic equilibrium with the surrounding low-density phase, requires either gravitational confinement (which seems unlikely) or shocks. Discrete (but resolved) X-ray sources around some of these quasars also suggest shocks. Highvelocity gas near the center of 3C 48 seems not to be associated with the steep-spectrum compact radio jet. It may, instead, be an example of an outflow from either the quasar accretion disk or a very compact central starburst, which ultimately produces the EELR seen in this object. Acknowledgments Partial support for this work under proposal number GO-09365 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Partial support was also provided by NASA through Chandra Award Number GO3-4126X issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060. Additional support was given by the National Science Foundation, under grant number AST 03-07335. Some observations were obtained under Program ID GN-2003B-C-5 at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Particle Physics and Astronomy Research Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil) and CONICET (Argentina).
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References Allen, M.G., Dopita, M.A., Tsvetanov, Z.I., 1998. ApJ 493, 571. Bahcall, J.N., Kirhakos, S., Schneider, D.P., 1996. ApJ 457, 557. Boroson, T.A., Oke, J.B., 1982. Nature 296, 397. Boroson, T.A., Oke, J.B., 1984. ApJ 281, 535. Boroson, T.A., Persson, S.E., Oke, J.B., 1985. ApJ 293, 120. Bruzual, G., Charlot, S., 2003. MNRAS 344, 1000. Canalizo, G., Stockton, A., 2000a. ApJ 528, 201. Canalizo, G., Stockton, A., 2000b. AJ 120, 1750. Chatzichristou, E.T., Vanderriest, C., Jaffe, W., 1999. A&A 343, 407. Crawford, C.S., Fabian, A.C., Johnstone, R.M., 1988. MNRAS 235, 183. Crawford, C.S., Vanderriest, C., 2000. MNRAS 315, 433. Dopita, M.A., Sutherland, R.S., 1995. ApJ 455, 468. Evans, I., Koratkar, A., Allen, M., Dopita, M., Tsvetanov, Z., 1999. ApJ 521, 531. Fabian, A.C., Crawford, C.S., Johnstone, R.M., Thomas, P.A., 1987. MNRAS 228, 963. Feng, W.X., An, T., Hong, X.Y., Zhao, J.-H., Venturi, T., Shen, Z.-Q., Wang, W.H., 2005. A&A 434, 101. Kinkhabwala, A., Sako, M., Behar, E., Kahn, S.M., Paerels, F., Brinkman, A.C., Kaastra, J.S., Gu, M.F., Liedahl, D.A., 2002. ApJ 575, 732.
Kristian, J., 1973. ApJ 179, L61. Lehnert, M.D., Heckman, T.M., Weaver, K.A., 1999. ApJ 523, 575. Matthews, T.A., Bolton, J.G., Greenstein, J.L., Munch, G., Sandage, A.R., 1961. Sky Telesc 21, 148. Matthews, T.A., Sandage, A.R., 1963. ApJ 138, 30. Price, R., Gower, A.C., Hutchings, J.B., Talon, S., Duncan, D., Ross, G., 1993. ApJS 86, 365. Richstone, D.O., Oke, J.B., 1977. ApJ 213, 8. Sako, M., Kahn, S.M., Paerels, F., Liedahl, D.A., 2000. ApJ 543, L115. Scoville, N.Z., Padin, S., Sanders, D.B., Soifer, B.T., Yun, M.S., 1993. ApJ 415, L75. Stockton, A., 1976. ApJ 205, L113. Stockton, A., Fu, H., Henry, J.P., Canalizo, G., 2006. ApJ 638, 635. Stockton, A., MacKenty, J.W., 1983. Nature 305, 678. Stockton, A., MacKenty, J.W., 1987. ApJ 316, 584. Stockton, A., MacKenty, J.W., Hu, E.M., Kim, T.-S., 2002. ApJ 572, 735. Stockton, A., Ridgway, S.E., 1991. AJ 102, L131. Veilleux, S., Osterbrock, D.E., 1987. ApJS 63, 295. Wampler, E.J., Robinson, L.B., Burbidge, E.M., Baldwin, J.A., 1975. ApJ 198, L49. Wink, J.E., Guilloteau, S., Wilson, T.L., 1997. A&A 322, 427.
QSO extended emission-line regions Alan Stockton b
a,*
, Hai Fu a, Gabriela Canalizo
q b
a Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA Institute of Geophysics and Planetary Physics and Department of Physics, University of California at Riverside, USA
Available online 17 July 2006
Abstract Extended emission-line regions (EELRs) on scales of a few tens of kpc are found around a substantial fraction of steep-spectrum radio-loud QSOs. In most cases, at least at low redshifts, the distribution of this gas seems to be uncorrelated with either the stellar distribution in the host galaxy or the radio structure. The origin of the gas and the physical processes that control its distribution are still uncertain. We review circumstantial evidence that high densities required to explain the strengths of certain emission lines in spectra of EELRs are due to transient shocks, and we present new Chandra images that show extended X-ray emission associated with two QSOs with known optical EELRs. These discrete X-ray clumps may also be evidence for shocks. We discuss observations that could potentially confirm a model in which EELRs are results of galaxy-wide superwinds that could originate either from starbursts in the host galaxy or from the QSO itself. Finally, we discuss in some detail the QSO 3C 48, which has one of the most luminous EELRs and for which we also have considerable knowledge about both the host galaxy and processes in the immediate vicinity of the QSO. High-velocity ionized gas near the nucleus, but apparently not associated with the radio jet, indicates an outflow that may be the origin of the EELR. 2006 Elsevier B.V. All rights reserved. PACS: 98.54.Aj; 98.62.Nx Keywords: Quasars; Extended emission-line regions; Superwinds
Contents 1. 2. 3. 4. 5.
6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The origin of the extended gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Extended X-ray emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tests of superwind shock models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A case study: 3C 48 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. The nature of 3C 48A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. The blueshifted emission-line component in the nucleus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program # GO-09365.01-A. * Corresponding author. E-mail address: [email protected] (A. Stockton). 1387-6473/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.newar.2006.06.024
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1. Introduction The first evidence that some QSOs had significant amounts 104 K gas extending well beyond the classical narrow-line region (radius 1 kpc) was the spectroscopic detection of emission lines in the nebulosity around 3C 48 by Wampler et al. (1975). Other examples followed: 4C 37.43 (Stockton, 1976) and 3C 249.1 (Richstone and Oke, 1977). In an important spectroscopic survey for offnuclear emission, Boroson and Oke (1984) and Boroson et al. (1985) not only found additional QSOs with extended emission, but noted that strong extended emission was found essentially exclusively in steep-spectrum radio-loud QSOs. Around the same time, (Stockton and MacKenty, 1983, 1987) carried out a narrow-band imaging survey of a large sample of low-redshift (z 6 0.45) QSOs. This survey confirmed the correlation between extended optical emission and steep-spectrum radio emission. It is also found that these extended emission-line regions (EELRs) seemed especially prevalent in QSOs showing signs of strong interaction. It has gradually become clear that there are at least four general categories of EELRs: (i) Those closely associated with radio jets or radio lobes. These are common at high redshifts but relatively uncommon at low redshifts. The clearest lowredshift example is the z = 0.325 quasar PKS 2251 + 11, where one of two high-surface-brightness [OIII] emission peaks (Stockton and MacKenty, 1987) is coincident with the peak of the southeast radio lobe (Price et al., 1993).
˚Fig. 1. The broad-line radio galaxy 3C 79, imaged through a 30 A bandpass interference filter centered on the redshifted [OIII] k5007 emission line. The inset shows the same image at lower contrast. Linefree continuum images simply show what appears to be a normal elliptical galaxy.
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(ii) Those that involve photoionization by the QSO of the in situ interstellar medium of the host galaxy. An example is PG 0052 + 251, for which the host galaxy is clearly a spiral (Bahcall et al., 1996), and which shows weak extended emission in the outskirts of the disk (Stockton and MacKenty, 1987). (iii) Those in which the QSO photoionizes gas in a nearby object. The clearest example is the compact object to the east of NAB 0205 + 02 (Stockton and MacKenty, 1987); another is the small disk galaxy to the west of MRK 1014 (Canalizo and Stockton, 2000b). (iv) Those that show a complex, filamentary, distribution of ionized gas that appears to be unrelated either to the morphology of the host galaxy or to the extended radio structure. The most luminous EELRs associated with low-redshift QSOs fall into this class. A prime example is 4C 37.43 (Stockton et al., 2002; see Fig. 2). Another example is the broad-line radio galaxy 3C 79 (Fig. 1).
2. The origin of the extended gas A fundamental question for this last class of EELR is the origin of the extended 104 K gas. Stockton and MacKenty (1987), noting the apparent correlation between EELRs and evidence for strong interaction, suggested that the gas was tidal debris. As pointed out by Crawford et al. (1988), the problem with this suggestion is that some form of confinement is necessary; otherwise, the gas will dissipate within a sound crossing time, typically 106 years or less for the 10-parsec-sized regions indicated by photoionization calculations. A solution offered by Fabian et al. (1987) is that the EELRs comprise material that has cooled from a surrounding hot (107–108 K) medium. In order to check whether pressures are consistent with a short cooling time for this external medium, Crawford et al. (1988) (see also Crawford and Vanderriest, 2000) assume hydrostatic equilibrium between the hot gas and the 104 K gas and use the ratio of [OII] k3727 lines to the [OIII] k5007 line as a measure of the ionization parameter. With an estimate of the ionizing flux from the QSO and the distance to the ionized cloud, the pressure can be found. These authors find high pressures and cooling times much shorter than a Hubble time. A more detailed analysis of the EELR around 4C 37.43 (the most luminous known among low-redshift QSOs) has been given by Stockton et al. (2002). Working from Keck low-resolution imaging spectrograph (LRIS) spectra in the optical and HST faint-object spectrograph (FOS) spectra in the UV, they carried out detailed photoionization calculations, using the MAPPINGS3 package (Dopita and Sutherland, 1995). The overall result was that at least two phases were needed to explain the optical emission: a low-density (2 cm3) phase, with approximately unity filling factor, and a high-density (500 cm3), with 105 filling factor. These phases have roughly similar temperatures,
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so they cannot be in pressure equilibrium. Furthermore, most of the [OIII] emission is produced in the low-density region, whereas most of the [OII] comes from the high-density region. Thus two of the crucial assumptions made by Crawford et al. (1988) are invalid: the dense regions cannot be in hydrostatic equilibrium with their surroundings, and the [OII]/[OIII] ratio cannot be used to infer a useful photoionization parameter or pressure. If the low-density region is assumed to be in pressure equilibrium with a much hotter external medium, the pressure is found to be 3 · 104 cm3 K, over two orders of magnitude lower than those found by Crawford and Vanderriest (2000) for 4C 37.43; the cooling time is then much longer than the Hubble time. Another result of this study is that the high-density regions are ionization bounded, with a thickness of only 0.1 pc. With a typical sound speed of 15 km s1, they will have lifetimes of <104 years. There would seem to be only two possible general scenarios that would allow quasi-steady-state emission from these regions: (1) ‘boiloff,’ as surface photoionization works its way into a large, dense, neutral cloud, and (2) high-speed shocks that propagate through the low-density gas, compressing it to the observed high densities. In the first case, the dense clouds would have to be self gravitating (like giant molecular clouds) in order to be stable against disruption over a sufficiently long time. The presence of such clouds at large distances and in a distribution that is unrelated to the morphology of the host galaxy seems problematical. On the other hand, shocks could be due to the ejection of gas from the host galaxy in a superwind either due to a starburst or to the QSO itself. 3. Extended X-ray emission In deep Chandra ACIS imaging, our group has recently found extended X-ray emission around two QSOs that show extended optical emission, 3C 249.1 and 4C 37.43 (Stockton et al., 2006). This X-ray emission is not aligned along the radio axis, as are the X-ray jets frequently found associated with radio-loud QSOs. In some cases, the extended X-ray emission seems to be roughly coincident with optical emission features; in other cases, it is not. Most of the strongest optical features do not have an X-ray counterpart (see Fig. 2). We can show from measured electron densities that the extended X-ray components cannot be due to Compton scattering of nuclear X-ray emission. We know that there will be some X-ray production from 104 K regions photoionized by a typical powerlaw UV continuum, because there will be X-ray line emission from highly ionized heavy species, notably Fe. (see, e.g., Sako et al., 2000; Kinkhabwala et al., 2002). The question is, how significant is this emission, compared with what we actually observe? We believe that this contribution is unlikely to dominate the observed X-ray emission because many regions that show very high surface brightnesses in optical emission are not
Fig. 2. Chandra ACIS-I and [OIII] images of 4C 37.43. The upper panel shows the residual from the PSF subtraction from the Chandra ACIS image. Several discrete off-nuclear X-ray emission sources are labeled. The lower panel shows a deep HST WFPC2 image obtained through a linear ramp filter centered on the redshifted [OIII] k5007 emission line (Stockton et al. 2002). The sources labeled a 0 and b 0 may be associated with the corresponding X-ray sources. Both images are at the same scale, given in the lower-left corner of the lower panel. This figure is from Stockton et al., 2006.
detected at all in X-rays, whereas some regions of X-ray emission show little or no optical emission. The most common kind of extended X-ray emission is thermal bremsstrahlung from a hot surrounding medium. Such hot haloes are often seen around massive elliptical galaxies. Some of the extended emission we see may have such an origin. However, these haloes normally do not show discrete substructure on very small scales. However, another form of thermal bremsstrahlung that is expected to show small-scale structure is that from local heating by shocks. Since we have already considered the possibility of shocks as an explanation for the presence of the highdensity regions responsible for most of the [OII] emission,
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these explanations are at least potentially consistent with each other. On the other hand, we clearly cannot have the [OII] emission and the X-rays coming from spatially coextensive regions because of the wide difference in the required temperatures. One possible model is that of Lehnert et al. (1999), in which, as the superwind encounters an ambient cloud, it drives a slow (102 km s1) radiative shock into the cloud as well as producing a fast (103 km s1) stand-off bow shock. The former produces the optical emission, the latter the X-rays. 4. Tests of superwind shock models The evidence for the importance of shocks in QSO EELRs we have been able to give is admittedly circumstantial. It would be gratifying to be able to find a more direct test from emission-line diagnostics. There are two difficulties in doing this. Firstly, photoionization models show that the observed UV flux from a QSO is quite capable of producing the observed optical emission-line spectrum for most EELRs. Thus, even if shocks are important in defining the density structure of the gas, any shock signature may well be overwhelmed by photoionization from the central source. Secondly, it is an unfortunate fact that the strong emission lines in the optical region, which work very well to discriminate between photoionization by the UV continua from hot stars and ionization in AGN (e.g., Veilleux and Osterbrock, 1987), fail completely to separate photoionization by a power-law continuum and ionization by shocks. This problem has been discussed in detail by Allen et al. (1998). Relatively strong lines in the UV can be used not only to separate power-law photoionization from shock ionization, but also ‘‘pure’’ shock ionization from the case in which thermal emission from the shock ionizes upstream gas before the shock arrives (‘‘shock + precursor’’; e.g., Allen et al., 1998). With the demise of STIS on the Hubble Space Telescope, however, there is now essentially no possibility of observing these lines in low-redshift quasars. Fortunately, there are some weaker lines in the optical region that can be used to distinguish between photoionization and shocks. Evans et al. (1999) show that a relatively clean separation can be achieved using the [OIII] k4363/[OIII] k5007 and He II/Hb ratios. The [OIII] k4363/[OIII] k5007 line ratio is an important diagnostic of the electron temperature of the ionized gas. The shock and photoionization models are widely separated in the [OIII] k4363/[OIII] k5007 versus He II/Hb diagram because of the different temperature regimes of the shock and photoionization models. Another likely useful diagnostic is the [S II] kk6717,6731/Ha ratio in combination with the He II/Hb ratio. The use of these weaker optical emission lines has been problematic for many applications because of the difficulty in achieving the required S/N and because of systematic uncertainties. With efficient spectrographs on large telescopes, the S/N issue is less of a problem. The main systematic uncertainty has been in modeling the underlying stellar
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continuum, e.g., in Seyfert galaxies. For QSO EELRs, we have the great advantage that we generally do not have any significant stellar background, and, in the few cases where the emission is projected on the host galaxy, the emission is so clumpy that we almost always will have adjacent emission-line-free host galaxy regions as a reference. We have used the [OIII] k4363/[OIII] k5007 versus He II/ Hb diagram for this test in three regions in 3C 249.1 and one region in 4C 37.43. In all four cases, however, the ratios fall in a region of the diagram that lies between the expected loci for photoionization by a power-law continuum, shock ionization, and shock plus precursor ionization. This ambiguity may indicate a mixture of ionization mechanisms, or it may result from remaining uncertainties of the models; in any case, we do not have a compelling case for the presence of shocks. One hope is that, if shocks really are important, we may be able to find some emission regions that are shielded from the QSO UV continuum and that will show a clearly shock-dominated spectrum. 5. A case study: 3C 48 3C 48 was the first quasar identified (Matthews et al., 1961; Matthews and Sandage, 1963). This early identification was partly a result of the compact nature of its radio source, and 3C 48 remains the only compact steep-spectrum (CSS) radio source among powerful quasars at redshifts <0.5. The host galaxy of 3C 48 is larger and more luminous than those of most other low-redshift quasars (Kristian, 1973), and it is clearly an example of a major merger in progress (Stockton and MacKenty, 1987; Canalizo and Stockton, 2000a). 3C 48 is also one of the few previously identified QSOs to have been found by IRAS to be embedded in ultraluminous IR galaxies. Vigorous star formation is currently underway in the inner part of the host galaxy (Canalizo and Stockton, 2000a), and there are still large reserves of molecular gas (Scoville et al., 1993; Wink et al., 1997). All of these things taken together indicate that we are witnessing a brief but important phase in the history of a quasar, a time when host-galaxy star formation is near its peak, the radio jet is breaking through the dense material in the inner part of the host galaxy, and UV radiation from the central continuum source has just recently become visible along many lines of sight. 3C 48 remains one of the most luminous examples of extended emission among lowredshift QSOs (Stockton and MacKenty, 1987). It is an especially interesting example because the host galaxy and the nuclear regions have been studied in more detail than those of any other low-redshift quasar. Boroson and Oke (1982, 1984) showed the presence of strong Balmer absorption in the 3C 48 host galaxy. More recently, Canalizo and Stockton (2000a) carried out a detailed spectroscopic analysis of the host galaxy. They showed that the stellar population has a classic ‘‘E + A00 spectrum; i.e., it shows features from both young and old stellar populations. This character made it possible to carry out a decomposition of the integrated spectrum into the
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young and old populations, using spectral synthesis models (e.g., Bruzual and Charlot, 2003), and three parameters: normalization factors for the young and old stellar populations, and a luminosity-weighted mean age for the young population. These, of course, apply to summations of the light along various line-of-sight paths through the host galaxy. It was possible to determine starburst ages as well as stellar radial velocities in 32 distinct regions in the host. In general, the regions dominated by very recent or current star formation are closest the quasar nucleus, whereas most of the north-west tidal tail shows only the old stellar population. The region very close to the quasar reveals some fascinating features apart from the CSS radio jet, which extends about 100 north from the quasar (Feng et al., 2005). There is a second luminosity peak (3C 48A) about 100 to the northwest of the quasar (Stockton and Ridgway, 1991) which has been variously interpreted as the distorted nucleus of the merging galaxy (Stockton and Ridgway, 1991; Canalizo and Stockton, 2000a) or as jet-induced star formation from the radio jet (Chatzichristou et al., 1999). Finally, the [OIII] lines, which had long been known to have mean redshifts significantly lower than those of the broad lines (Wampler et al., 1975), actually comprise two components: one at the broad-line redshift, and a second one blueshifted by about 580 km s1 in the quasar frame (Chatzichristou et al., 1999; Canalizo and Stockton, 2000a).
HST archive. The quasar nucleus has been subtracted, and the elongated feature to the northwest is 3C 48A. The two lines indicate the position of a 0.00 5-wide slit we have used to obtain a spectrum of 3C 48A with the HST STIS instrument. Tentative results from the spectrum itself are shown in Fig. 4. The clear signature of Balmer absorption line at the longer-wavelength end of the spectrum (where our S/N is relatively high) shows that the spectrum of 3C 48A is dominated by stars. We are interested primarily in the age of these stars in order to decide whether radio-jet-induced star formation is a reasonable explanation for the feature. The gray traces show Bruzual and Charlot (2003) spectral-synthesis models with ages of 1.0 · 108, 1.4 · 108, and 2.0 · 108 years. While the noise in this spectrum increases dramatically towards shorter wavelengths, the overall spectral slope shortward of the Balmer limit should be fairly reliable; an age younger than 1.0 · 108 appears to be quite unlikely. Furthermore, the obvious presence of the Ca II K line and the fact that the Balmer lines are weaker in the spectrum than in any of the models indicates the presence of an older population as well. Our reduction procedure subtracted adjoining regions of the host galaxy, so these older stars are almost certainly in 3C 48A itself. Even this preliminary analysis seems clearly to show that 3C 48A cannot have been the result of jet-induced star formation and is much more likely to be the distorted nucleus of the merging companion.
5.1. The nature of 3C 48A Fig. 3 shows an HST WFPC2 F555W image of the inner part of the 3C 48 host galaxy, taken from the
Fig. 3. The inner region of the 3C 48 host galaxy, from an HST PC F555W image, showing the position of the STIS slit centered on 3C 48A.
Fig. 4. HST STIS spectrum of 3C 48A is shown as the heavy black line. The spectrum is clearly dominated by moderately young stars, along with some [OII] k3727 emission. The noise increases greatly to shorter wavelengths. The three gray traces are instantaneous-burst spectral synthesis models with ages (from top to bottom) of 100 Myr, 140 Myr, and 200 Myr, respectively.
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5.2. The blueshifted emission-line component in the nucleus The Hb–[OIII]-line region in 3C 48 is shown in Fig. 5. The ‘‘anomalous’’ blueshifted component to the [O III] lines actually has about 5 times the flux of the classical narrowline component at the broad-line redshift. Chatzichristou et al. (1999) suggest that the blue-shifted emission is associated with the radio jet, which extends to the north side of the quasar (Feng et al., 2005). We have obtained a deep GMOS Integral-Field Unit (IFU) spectrum of 3C 48 with the Gillett Gemini North telescope. By subtracting a scaled PSF, derived from the nearby continuum, we can explore the distribution of any off-nuclear emission with velocity by taking cuts through
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the IFU data cube at various parts of the line profile. ˚ Fig. 6 shows the distribution of the emission for a 10 A bandpass near the blueshifted emission peak. It is apparent that the bulk of this emission, peaked mostly to the west of the quasar, cannot be associated with the radio jet to the north. Nor is it likely to be connected with a putative counterjet to the south, which may recently have been detected (Feng et al., 2005). The most likely explanation for this high-velocity gas very close to the quasar is that it is part of a wind generated either by the quasar accretion disk or by an intense compact starburst at the center of 3C 48. In either case, this result give additional credence to the suggestion that the EELR around 3C 48 and other quasars is likely to be due to a superwind. 6. Summary
Fig. 5. The spectrum of 3C 48, obtained with GMOS in the IFU mode on ˚ is the the Gillett Gemini North telescope. The feature longward of 6880 A terrestrial atmospheric B-band.
Fig. 6. The distribution of [OIII] emission dominated by blueshifted narrow-line region in 3C 48. The white cross shows the position of the quasar, where a scaled PSF derived from the nearby continuum has been subtracted to give a smooth residual.
While we do not yet have a detailed understanding of how quasar EELRs are related to physical processes in the quasar, the host galaxy, and the immediate environment, we are beginning to accumulate a number of pieces of the puzzle. The presence of dense regions in EELRs with very short estimated lifetimes, together with the demonstration that these cannot be anywhere near hydrostatic equilibrium with the surrounding low-density phase, requires either gravitational confinement (which seems unlikely) or shocks. Discrete (but resolved) X-ray sources around some of these quasars also suggest shocks. Highvelocity gas near the center of 3C 48 seems not to be associated with the steep-spectrum compact radio jet. It may, instead, be an example of an outflow from either the quasar accretion disk or a very compact central starburst, which ultimately produces the EELR seen in this object. Acknowledgments Partial support for this work under proposal number GO-09365 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Partial support was also provided by NASA through Chandra Award Number GO3-4126X issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060. Additional support was given by the National Science Foundation, under grant number AST 03-07335. Some observations were obtained under Program ID GN-2003B-C-5 at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Particle Physics and Astronomy Research Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil) and CONICET (Argentina).
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References Allen, M.G., Dopita, M.A., Tsvetanov, Z.I., 1998. ApJ 493, 571. Bahcall, J.N., Kirhakos, S., Schneider, D.P., 1996. ApJ 457, 557. Boroson, T.A., Oke, J.B., 1982. Nature 296, 397. Boroson, T.A., Oke, J.B., 1984. ApJ 281, 535. Boroson, T.A., Persson, S.E., Oke, J.B., 1985. ApJ 293, 120. Bruzual, G., Charlot, S., 2003. MNRAS 344, 1000. Canalizo, G., Stockton, A., 2000a. ApJ 528, 201. Canalizo, G., Stockton, A., 2000b. AJ 120, 1750. Chatzichristou, E.T., Vanderriest, C., Jaffe, W., 1999. A&A 343, 407. Crawford, C.S., Fabian, A.C., Johnstone, R.M., 1988. MNRAS 235, 183. Crawford, C.S., Vanderriest, C., 2000. MNRAS 315, 433. Dopita, M.A., Sutherland, R.S., 1995. ApJ 455, 468. Evans, I., Koratkar, A., Allen, M., Dopita, M., Tsvetanov, Z., 1999. ApJ 521, 531. Fabian, A.C., Crawford, C.S., Johnstone, R.M., Thomas, P.A., 1987. MNRAS 228, 963. Feng, W.X., An, T., Hong, X.Y., Zhao, J.-H., Venturi, T., Shen, Z.-Q., Wang, W.H., 2005. A&A 434, 101. Kinkhabwala, A., Sako, M., Behar, E., Kahn, S.M., Paerels, F., Brinkman, A.C., Kaastra, J.S., Gu, M.F., Liedahl, D.A., 2002. ApJ 575, 732.
Kristian, J., 1973. ApJ 179, L61. Lehnert, M.D., Heckman, T.M., Weaver, K.A., 1999. ApJ 523, 575. Matthews, T.A., Bolton, J.G., Greenstein, J.L., Munch, G., Sandage, A.R., 1961. Sky Telesc 21, 148. Matthews, T.A., Sandage, A.R., 1963. ApJ 138, 30. Price, R., Gower, A.C., Hutchings, J.B., Talon, S., Duncan, D., Ross, G., 1993. ApJS 86, 365. Richstone, D.O., Oke, J.B., 1977. ApJ 213, 8. Sako, M., Kahn, S.M., Paerels, F., Liedahl, D.A., 2000. ApJ 543, L115. Scoville, N.Z., Padin, S., Sanders, D.B., Soifer, B.T., Yun, M.S., 1993. ApJ 415, L75. Stockton, A., 1976. ApJ 205, L113. Stockton, A., Fu, H., Henry, J.P., Canalizo, G., 2006. ApJ 638, 635. Stockton, A., MacKenty, J.W., 1983. Nature 305, 678. Stockton, A., MacKenty, J.W., 1987. ApJ 316, 584. Stockton, A., MacKenty, J.W., Hu, E.M., Kim, T.-S., 2002. ApJ 572, 735. Stockton, A., Ridgway, S.E., 1991. AJ 102, L131. Veilleux, S., Osterbrock, D.E., 1987. ApJS 63, 295. Wampler, E.J., Robinson, L.B., Burbidge, E.M., Baldwin, J.A., 1975. ApJ 198, L49. Wink, J.E., Guilloteau, S., Wilson, T.L., 1997. A&A 322, 427.