46 research outputs found
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 W~Hz)
of the total radio power at 20 cm originates from the nuclear region ( <~ 1.5
kpc), of which half is emitted by two unresolved (\alphaS_{\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^{-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 . 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 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 of Seyferts have good evidence for
minor-axis galactic outflows. Kinetic luminosities of the galactic outflows in
our sample Seyferts are 1010 erg~s, assuming all
of the observed minor-axis emission is produced by the outflow. These values
are, in general, 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 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- emission from the
galaxies underlying the absorbers at and their companion
galaxies. The 3 sensitivity levels ranged between
and erg s cm in
observed-frame Ly- flux. No significant Ly- emitters were
detected at level. The absence of significant Ly- emission
implies limits on the star formation rate (SFR) of 0.9-2.7
yr 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- 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