The Radio Emission from the Ultra-Luminous Far-Infrared Galaxy NGC 6240

We present new radio observations of the ``prototypical'' ultra-luminous far- infrared galaxy NGC~6240, obtained using the VLA at 20~cm in `B' configuration and at 3.6~cm in `A' configuration. These data, along with those from four previous VLA observations, are used to perform a comprehensive study of the radio emission from NGC~6240. Approximately 70\% (~3 x 10$^{23}$ W~Hz$^{-1}$) of the total radio power at 20 cm originates from the nuclear region ( <~ 1.5 kpc), of which half is emitted by two unresolved ($R <~ 36 pc) cores and half by a diffuse component. The radio spectrum of the nuclear emission is relatively flat ($\alpha$=~ 0.6;$S_{\nu} \propto \nu^{-\alpha}). The supernova rate required to power the diffuse component is consistent with that predicted by the stellar evolution models of Rieke \etal (1985). If the radio emission from the two compact cores is powered by supernova remnants, then either the remnants overlap and form hot bubbles in the cores, or they are very young ( <~ 100 yr). Nearly all of the other 30\% of the total radio power comes from an ``arm-like'' region extending westward from the nuclear region. The western arm emission has a steep spectrum (\alpha$=~ 1.0), suggestive of aging effects from synchrotron or inverse-Compton losses, and is not correlated with starlight; we suggest that it is synchrotron emission from a shell of material driven by a galactic superwind. Inverse-Compton scattering of far-infrared photons in the radio sources is expected to produce an X-ray flux of ~2-6 x 10$^{-14}$ erg/s/cm2 in the 2-10 keV band. No significant radio emission is detected from or near the possible ultra-massive ``dark core'' hypothesized by Bland-Hawthorn, Wilson \& Tully (1991).Comment: 36 pages (text and tables) as an uuencoded compressed postscript file (figures available upon request), accepted for the ApJ (20 Nov issue), STScI preprint no. ?? -- May 199

Old and Young X-ray Point Source Populations in Nearby Galaxies

We analyzed 1441 Chandra X-ray point sources in 32 nearby galaxies. The total point-source X-ray luminosity L_XP is well correlated with B, K, and FIR+UV luminosities of spiral host galaxies, and with the B and K luminosities for ellipticals. This suggests an intimate connection between L_XP and both the old and young stellar populations, for which K and FIR+UV luminosities are proxies for the galaxy mass M and star-formation rate SFR. We derive proportionality constants 1.3E29 erg/s/Msol and 0.7E39 erg/s/(Msol/yr), which can be used to estimate the old and young components from M and SFR, respectively. The cumulative X-ray luminosity functions for the point sources have quite different slopes for the spirals (gamma ~= 0.5-0.8) and ellipticals (gamma ~= 1.4), implying *the most luminous point sources dominate L_XP* for the spirals. Most of the point sources have X-ray colors that are consistent with either LMXBs or Ultraluminous X-ray sources (ULXs a.k.a. IXOs) and we rule out classical HMXBs (e.g. neutron-star X-ray pulsars) as contributing much to L_XP. Thus, for spirals, the ULXs dominate L_XP. We estimate that >~20% of all ULXs found in spirals originate from the older (pop II) stellar populations, indicating that many of the ULXs that have been found in spiral galaxies are in fact pop II ULXs, like those in elliptical galaxies. The linear dependence of L_XP on the SFR argues for either a steepening in the X-ray luminosity function of the young (pop I) X-ray source population at L_X >~10^(38.5-39) erg/s, or a decreasing efficiency for producing all types of young X-ray point sources as the galaxy SFR increases.Comment: 33 pages AASTEX, ApJ accepted. Please download full version with figures from http://www.pha.jhu.edu/~colbert/chps_accepted.p

Extranuclear X-ray Emission in the Edge-on Seyfert Galaxy NGC 2992

We found several extranuclear (r >~ 3") X-ray nebulae within 40" (6.3 kpc at 32.5 Mpc) of the nucleus of the Seyfert galaxy NGC 2992. The net X-ray luminosity from the extranuclear sources is ~2-3 E39 erg/s (0.3-8.0 keV). The X-ray core itself (r <~ 1") is positioned at 9:45:41.95 -14:19:34.8 (J2000) and has a remarkably simple power-law spectrum with photon index Gamma=1.86 and Nh=7E21 /cm2. The near-nuclear (3" <~ r <~ 18") Chandra spectrum is best modelled by three components: (1) a direct AGN component with Gamma fixed at 1.86, (2) cold Compton reflection of the AGN component, and (3) a 0.5 keV low-abundance (Z < 0.03 Zsolar) "thermal plasma," with ~10% of the flux of either of the first two components. The X-ray luminosity of the 3rd component (the "soft excess") is ~1.4E40 erg/s, or ~5X that of all of the detected extranuclear X-ray sources. We suggest that most (~75-80%) of the soft excess emission originates from 1" < r < 3", which is not imaged in our observation due to severe CCD pile-up. We also require the cold reflector to be positioned at least 1" (158 pc) from the nucleus, since there is no reflection component in the X-ray core spectrum. Much of the extranuclear X-ray emission is coincident with radio structures (nuclear radio bubbles and large-scale radio features), and its soft X-ray luminosity is generally consistent with luminosities expected from a starburst-driven wind (with the starburst scaled from L_FIR). However, the AGN in NGC 2992 seems equally likely to power the galactic wind in that object. Furthermore, AGN photoionization and photoexcitation processes could dominate the soft excess, especially the \~75-80% which is not imaged by our observations.Comment: 34 pages AASTEX, 9 (low-res) PS figures, ApJ, in press. For full-resolution postscript file, visit http://www.pha.jhu.edu/~colbert/n2992_chandra.ps.g

Large-Scale Outflows in Edge-on Seyfert Galaxies. III. Kiloparsec-Scale Soft X-ray Emission

We present ROSAT PSPC and HRI images of eight galaxies selected from a distance-limited sample of 22 edge-on Seyfert galaxies. Kiloparsec-scale soft X-ray nebulae extend along the galaxy minor axes in three galaxies (NGC 2992, NGC 4388 and NGC 5506). The extended X-ray emission has 0.2-2.4 keV X-ray luminosities of $0.4-3.5 \times 10^{40} erg s^{-1}$. The X-ray nebulae are roughly co-spatial with the large-scale radio emission, suggesting that both are produced by large-scale galactic outflows. Assuming pressure balance between the radio and X-ray plasmas, the X-ray filling factor is \gapprox 10^4 times larger than the radio plasma filling factor, suggesting that large-scale outflows in Seyfert galaxies are predominantly winds of thermal X-ray emitting gas. We favor an interpretation in which large-scale outflows originate as AGN-driven jets that entrain and heat gas on kpc scales as they make their way out of the galaxy. AGN- and starburst-driven winds are also possible explanations in cases where the winds are oriented along the rotation axis of the galaxy disk.Comment: 24 pages, 7 ps figures, AASTEX 4.0, accepted for ApJ April 1, 199

Large-Scale Outflows in Edge-on Seyfert Galaxies. I. Optical Emission- Line Imaging and Optical Spectroscopy

We have launched a search for large-scale (\gapprox1 kpc) minor-axis outflows in edge-on Seyfert galaxies in order to assess their frequency of occurrence and study their properties. Here we present optical continuum and \han2 line images and/or minor-axis long-slit spectra of 22 edge-on Seyfert galaxies. Six of these galaxies show at least one of the following: (i) bi-symmetric H$\alpha$ halos extending along the minor axis, (ii) bright emission-line complexes at distances \gapprox4 kpc (in projection) out of the disk, and (iii) double-peaked emission-line profiles from the gas along the minor-axis, suggesting that a wind-blown bubble is present. Our results indicate that \gapprox${{1}\over{4}}$ of Seyferts have good evidence for minor-axis galactic outflows. Kinetic luminosities of the galactic outflows in our sample Seyferts are $\sim$10$^{40}$$-$10$^{42}$ erg~s$^{-1}$, assuming all of the observed minor-axis emission is produced by the outflow. These values are, in general, $\sim$0.1 as large as those for well-studied cases of superwinds in starburst galaxies (Heckman, Armus \& Miley 1990). However, far-infrared luminosities of our sample Seyferts are also $\sim$0.1 as large. Both starburst-driven superwinds and wide-angled outflows from the active galactic nucleus are possible explanations for the observed large-scale outflows.Comment: 34 pages (text and tables) AASTEX, figures available from [email protected], ApJ Supp., June 199

The X-ray Reflectors in the Nucleus of the Seyfert Galaxy NGC 1068

(abridged) Based on observations of the Seyfert nucleus in NGC1068 with ASCA, RXTE and BeppoSAX, we report the discovery of a flare (increase in flux by a factor of ~1.6) in the 6.7 keV Fe K line component between observations obtained 4 months apart, with no significant change in the other (6.21, 6.4, and 6.97 keV) Fe K_alpha line components. During this time, the continuum flux decreased by ~20%. The RXTE spectrum requires an Fe K absorption edge near 8.6 keV (Fe XXIII - XXV). The spectral data indicate that the 2-10 keV continuum emission is dominated (~2/3 of the luminosity) by reflection from a previously unidentified region of warm, ionized gas located <~ 0.2 pc from the AGN. The remaining ~1/3 of the observed X-ray emission is reflected from optically thick, neutral gas. The inferred properties of the warm reflector (WR) are: size (diameter) ~ 10^{5.5} /cm3, ionization parameter xi approx 10^{3.5} erg cm/s, and covering fraction 0.003 (L_0/10^{43.5} erg/s)^{-1} < (Omega/4 pi) < 0.024 (L_0/10^{43.5})^{-1}, where L_0 is the intrinsic 2-10 keV X-ray luminosity of the AGN. We suggest that the WR gas is the source of the (variable) 6.7 keV Fe line emission, and the 6.97 keV Fe line emission. The 6.7 keV line flare is assumed to be due to an increase in the emissivity of the WR gas from a decrease (by 20-30%) in L_0. The properties of the WR are most consistent with an intrinsically X-ray weak AGN with L_0 approx 10^{43.0} erg/s. The optical and UV emission that scatters from the WR into our line of sight is required to suffer strong extinction, which can be reconciled if the line-of-sight skims the outer surface of the torus. Thermal bremsstrahlung radio emission from the WR may be detectable in VLBA radio maps of the NGC 1068 nucleus.Comment: 39 pages (9 postscript figures) AASTEX, ApJ, accepte

Another intermediate mass black hole in a starburst galaxy?: The luminous X-ray source in NGC 3628 reappears

In a 52 ks-long Chandra ACIS-S observation of the nearby starburst galaxy NGC 3628, obtained to study the starburst-driven outflow from this galaxy, we have detected a very luminous (L_X = 1.1e40 erg/s in the 0.3-8.0 keV energy band) point source located at least 20 arcsec (~970 pc) from the nucleus of the galaxy. No radio, optical or near-IR counterpart to this source has been found. This is most probably the reappearance of the strongly-variable X-ray-luminous source discovered by Dahlem et al (1995), which faded by a factor >27 between December 1991 and March 1994 (at which point it had faded below the detection limit in a ROSAT HRI observation). This source is clearly a member of an enigmatic class of X-ray sources that are considerably more luminous than conventional X-ray binaries but less luminous than AGN, and which are not found at the dynamical center of the host galaxy. The Chandra spectrum is best-fit by an absorbed power law model with a photon index of Gamma = 1.8+/-0.2, similar to that seen in Galactic BH binary candidates in their hard state. Bremsstrahlung models or multi-color disk models (the favored spectral model for objects in this class based on ASCA observations) can provide statistically acceptable fits only if the data at energies E > 5 keV is ignored. This is one of the first X-ray spectra of such an object that is unambiguously that of the source alone, free from the spectral contamination by X-ray emission from the rest of the galaxy that affects previous spectral studies of these objects using ASCA.Comment: Accepted for publication in the Ap

A Fabry-Perot Imaging Search for Lyman-alpha Emission in Quasar Absorbers at z ~ 2.4

We have carried out a deep narrow-band imaging survey of six fields with heavy-element quasar absorption lines, using the Goddard Fabry-Perot (FP) system at the Apache Point Observatory (APO) 3.5-meter telescope. The aim of these observations was to search for redshifted Ly-$\alpha$ emission from the galaxies underlying the absorbers at $z = 2.3-2.5$ and their companion galaxies. The 3 $\sigma$ sensitivity levels ranged between $1.9 \times 10^{-17}$ and $5.4 \times 10^{-17}$ erg s$^{-1}$ cm$^{-2}$ in observed-frame Ly-$\alpha$ flux. No significant Ly-$\alpha$ emitters were detected at $> 3 \sigma$ level. The absence of significant Ly-$\alpha$ emission implies limits on the star formation rate (SFR) of 0.9-2.7 $M_{\odot}$ yr$^{-1}$ per 2-pixel x 2-pixel region, if no dust attenuation is assumed. We compare our results with those from other emission-line studies of absorber fields and with predictions for global average SFR based on the models of cosmic chemical evolution. Our limits are among the tightest existing constraints on Ly-$\alpha$ emission from galaxies in absorber fields, but are consistent with many other studies. In the absence of dust attenuation, these studies suggest that SFRs in a large fraction of objects in the absorber fields may lie below the global mean SFR. However, it is possible that dust attenuation is responsible for the low emission line fluxes in some objects. It is also possible that the star-forming regions are compact and at smaller angular separations from the quasar than the width of our point spread function and, get lost in the quasar emission. We outline future observations that could help to distinguish between the various possibilities.Comment: Accepted for Publication in the Astrophysical Journal, 32 pages, 8 figures. NOTE: 25 of the 29 subpanels in the figures are included here at lower resolution to keep the astroph submission size within allowed limits. Please see http://boson.physics.sc.edu/~kulkarni/fpimaging.html for a pdf file of the complete paper including all subpanels of all figures in the original higher resolutio

A high spatial resolution X-ray and H-alpha study of hot gas in the halos of star-forming disk galaxies. II. Quantifying supernova feedback

We investigate how the empirical properties of hot X-ray-emitting gas in a sample of seven starburst and three normal edge-on spiral galaxies (a sample which covers the full range of star-formation intensity found in disk galaxies) correlate with the size, mass, star formation rate and star formation intensity in the host galaxies. Intriguingly, the diffuse X-ray properties of the normal spirals (both in their disks and halos) fall where extrapolation of the trends from the starburst galaxies with superwinds would predict. We demonstrate that the luminosity of diffuse X-ray emission in both disk and halo is directly proportional to the rate of mechanical energy feedback from massive stars. Nevertheless, with only three non-starburst normal spiral galaxies it is hard to exclude an accretion-based origin for extra-planar diffuse X-ray emission around normal star-forming galaxies. Larger galaxies have more extended X-ray-emitting halos, but galaxy mass appears to play no role in determining the properties of the disk or extra-planar X-ray emitting plasma. The combination of these luminosity and size correlations leads to a correlation between the surface brightness of the diffuse X-ray emission and the mean star formation rate per unit area in the disk (L_FIR/D_25^2). We argue that the crucial spatial region around a galaxy that controls whether gas in starburst-driven superwinds will escape into the IGM is not the outer halo ~100 kpc from the host galaxy, but the inner few halo scale heights, within ~20 kpc of the galaxy plane. Given the properties of the gaseous halos we observe, superwind outflows from disk galaxies of mass M ~ 10^10 -- 10^11 Msun should still eject some fraction of their material into the IGM. (abstract abridged)Comment: To appear in 2004 May 10 edition of ApJ. For slightly higher resolution version, see http://proteus.pha.jhu.edu/~dks/dks_published.htm