1,026 research outputs found
Chandra Snapshot Observations of Low-Luminosity AGNs with a Compact Radio Source
The results of Chandra snapshot observations of 11 LINERs (Low-Ionization
Nuclear Emission-line Regions), three low-luminosity Seyfert galaxies, and one
HII-LINER transition object are presented. Our sample consists of all the
objects with a flat or inverted spectrum compact radio core in the VLA survey
of 48 low-luminosity AGNs (LLAGNs) by Nagar et al. (2000). An X-ray nucleus is
detected in all galaxies except one and their X-ray luminosities are in the
range 5x10^38 to 8x10^41 erg/s. The X-ray spectra are generally steeper than
expected from thermal bremsstrahlung emission from an advection-dominated
accretion flow (ADAF). The X-ray to Halpha luminosity ratios for 11 out of 14
objects are in good agreement with the value characteristic of LLAGNs and more
luminous AGNs, and indicate that their optical emission lines are predominantly
powered by a LLAGN. For three objects, this ratio is less than expected.
Comparing with properties in other wavelengths, we find that these three
galaxies are most likely to be heavily obscured AGN. We use the ratio RX = \nu
L\nu (5 GHz)/LX, where LX is the luminosity in the 2-10 keV band, as a measure
of radio loudness. In contrast to the usual definition of radio loudness (RO =
L\nu(5 GHz)/L\nu(B)), RX can be used for heavily obscured (NH >~ 10^23 cm^-2,
AV>50 mag) nuclei. Further, with the high spatial resolution of Chandra, the
nuclear X-ray emission of LLAGNs is often easier to measure than the nuclear
optical emission. We investigate the values of RX for LLAGNs, luminous Seyfert
galaxies, quasars and radio galaxies and confirm the suggestion that a large
fraction of LLAGNs are radio loud.Comment: 15 pages, accepted for publication in Ap
Variable Iron K-alpha Lines in Seyfert 1 Galaxies
We find that variability of the iron K-alpha line is common in Seyfert 1
galaxies. Using data from the ASCA archive for objects that have been observed
more than once during the mission, we study the time-averaged spectra from
individual observations, thereby probing variability on timescales that range
from days to years. Since the statistics of the data do not warrant searches
for line variability in terms of a complex physical model, we use a a simple
Gaussian to model the gross shape of the line, and then use the centroid
energy, intensity and equivalent width as robust indicators of changes in the
line profile. We find that ~70% of Seyfert 1s (ten out of fifteen) show
variability in at least one of these parameters: the centroid energy,
intensity, and equivalent width vary in six, four, and eight sources
respectively. Due to the low S/N, limited sampling and time averaging, we
consider these results to represent lower limits to the rate of incidence of
variability. In most cases changes in the line do not appear to track changes
in the continuum. In particular, we find no evidence for variability of the
line intensity in NGC 4151, suggesting an origin in a region larger than the
putative accretion disk, where most of the iron line has been thought to
originate. Mkn 279 is investigated on short timescales. The time-averaged
effective line energy is 6.5 keV in the galaxy rest frame. As the continuum
flux increases by 20% in a few hours, the Fe K line responds with the effective
line energy increasing by 0.22 keV (~10,500 km s^-1). Problems with the ASCA
and Rosat calibration that affect simultaneous spectral fits are discussed in
an appendix.Comment: 26 pages, 30 figures, accepted for publication in the Astrophysical
Journa
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
Broad Band X-Ray Observations of the Narrow Line X-Ray Galaxy NGC 5506
We present a detailed analysis of broad band X-ray data of the Seyfert 2
galaxy NGC5506. 2-10 keV band are detected during a 1-day ASCA observation,
while no significant change in the 2-10 keV continuum shape is found. The ASCA
spectrum consists of an absorbed power-law, a 'soft excess' below 2 keV, and an
Fe K emission line at 6.4 keV. The 'soft excess' can be well described
by either thermal emission from very low abundance material at a temperature
kT0.8 keV, or scattered/leaking flux from the primary power-law plus a
small amount of thermal emission. Analysis of ROSAT HRI data reveals that the
soft X-ray emission is extended on kpc scales in this object, and the extended
component may account for most of the soft X-ray excess observed by the ASCA.
The result suggests that in this type 2 AGN, the 'soft excess' at least partly
comes from an extended region, imposing serious problem for the model in which
the source is partially covered. Fe K profile is complex and can not be
satisfactorily modeled by a single gaussian. Models of either double gaussians,
or a narrow gaussian plus a line from a relativistic accretion disk viewed at
an inclination of about 40 provide good fits to the data. However,
the inclination of the disk can be substantially larger if there is a small
amount of excessive Fe K edge absorption. The intermediate inclinations for
NLXGs are consistent with the ideas that the inner accretion disk is aligned
with the outer obscuring torus.Comment: 8 pages, 5 postscript figures. to appear in Astrophy. J., 1999, April
2
A Hubble Space Telescope Imaging Survey of Nearby Active Glactic Nuclei
We obtained 500-second F606W WFPC2 images of 256 of the nearest (z<0.035)
Seyfert 1,Seyfert 2, and starburst galaxies. Less than 10% show tidal features
or multiple nuclei. The incidence of inner starburst rings is about 10% in both
classes of Sy galaxies. In contrast, galaxies with H II region emission line
spectra appear substantially more irregular because of their much higher
specific rates of star formation. An unresolved central continuum source in our
HST images is a virtually perfect indicator of a Sy1 spectrum. 52% of these Sy1
point sources are saturated in our images; we use their wings to estimate their
magnitudes. The converse is not however true, as over a third of Sy's with
direct spectroscopic evidence for broad Balmer wings show no nuclear point
source. Like the Sy2's, they have central surface brightnesses consistent with
those expected for the bulges of normal galaxies. The frequency of bars in
Sy1's and 2's and non-Sys are the same. The Sy2 galaxies are significantly more
likely to show nuclear dust absorption, especially in lanes and patches which
are irregular or reach close to the nucleus. The difference cannot be explained
by different average redshifts or selection techniques. This is confirmed by
our morphology classifications, which show that Sy1 nuclei reside in earlier
type galaxies than Sy2 nuclei. This intrinsic difference in host galaxy
properties may undermine the strong unification hypothesis for Sy galaxies that
they appear different due to the orientation of their central engine. The
excess galactic dust we see in Sy2's may cause substantial absorption which
obscures their hypothesized broad emission-line regions and central nonstellar
continua. This galactic dust could produce much of the absorption in Sy2 nuclei
which had instead been attributed to a thick dusty accretion torus.Comment: The text of the paper is 23 pages (ms.tex), there are 8 tables, and 9
figures. Figures 1, 2, and 3 are the image gallery (45 pages) and are NOT
included here. They can be ftp'ed from ftp.astro.ucla.edu. Log in as
anonymous and give your e-mail address as the password. The images are in the
/pub/submit/vg/AGNgallery . Figures 4-9 are in eps format and are included
here and can be printed using the lpr command in unix system
VLBI Polarimetry of 177 Sources from the Caltech-Jodrell Bank Flat-spectrum Survey
We present VLBA observations and a statistical analysis of 5 GHz VLBI
polarimetry data from 177 sources in the Caltech-Jodrell Bank flat-spectrum
(CJF) survey. The CJF survey, a complete, flux-density-limited sample of 293
extragalactic radio sources, gives us the unique opportunity to compare a broad
range of source properties for quasars, galaxies and BL Lacertae objects. We
focus primarily on jet properties, specifically the correlation between the jet
axis angle and the polarization angle in the core and jet. A strong correlation
is found for the electric vector polarization angle in the cores of quasars to
be perpendicular to the jet axis. Contrary to previous claims, no correlation
is found between the jet polarization angle and the jet axis in either quasars
or BL Lac objects. With this large, homogeneous sample we are also able to
investigate cosmological issues and AGN evolution.Comment: Accepted to the Astrophysical Journal: 37 pages, 14 figure
Coevolution of Supermassive Black Holes and Circumnuclear Disks
We propose a new evolutionary model of a supermassive black hole (SMBH) and a
circumnuclear disk (CND), taking into account the mass-supply from a host
galaxy and the physical states of CND. In the model, two distinct accretion
modes depending on gravitational stability of the CND play a key role on
accreting gas to a SMBH. (i) If the CMD is gravitationally unstable, energy
feedback from supernovae (SNe) supports a geometrically thick, turbulent gas
disk. The accretion in this mode is dominated by turbulent viscosity, and it is
significantly larger than that in the mode (ii), i.e., the CMD is supported by
gas pressure. Once the gas supply from the host is stopped, the high accretion
phase () changes to the low one (mode
(ii), ), but there is a delay with yr. Through this evolution, the gas-rich CND turns into the gas poor
stellar disk. We found that not all the gas supplied from the host galaxy
accrete onto the SMBH even in the high accretion phase (mode (i)), because the
part of gas is used to form stars. As a result, the final SMBH mass () is not proportional to the total gas mass supplied from the host
galaxy (); decreases with .This would indicate that it is difficult to form a SMBH with observed at high- QSOs. The evolution of the SMBH and CND would
be related to the evolutionary tracks of different type of AGNs.Comment: 11 pages, 11 figures, accepted for publication in Ap
Multicolour Optical Imaging of IR-Warm Seyfert Galaxies. I. Introduction and Sample Selection
The standard AGN unification models attempt to explain the diversity of
observed AGN types by a few fundamental parameters, where orientation effects
play a paramount role. Whether other factors, such as the evolutionary stage
and the host galaxy properties are equally important parameters for the AGN
diversity, is a key issue that we are addressing with the present data. Our
sample of IR-selected Seyfert galaxies is based on the important discovery that
their integrated IR spectrum contains an AGN signature. This being an almost
isotropic property, our sample is much less affected by orientation/obscuration
effects compared to most Seyfert samples. It therefore provides a test-bed for
the orientation-dependent models of Seyferts, involving dusty tori. We have
obtained multi-colour broad and narrow band imaging for a sample of mid-IR
``warm'' Seyferts and for a control sample of mid-IR ``cold'' galaxies. In the
present paper we describe the sample selection and briefly discuss their IR
properties. We then give an overview of the data collected and present
broad-band images for all our objects. Finally, we summarize the main issues
that will be addressed with these data in a series of forthcoming papers.Comment: 18 pages including 3 figures and 5 tables (tables 1,4,5 are included
as independent files
Relativistic Iron K Emission and absorption in the Seyfert 1.9 galaxy MCG-5-23-16
We present the results of the simultaneous deep XMM and Chandra observations
of the bright Seyfert 1.9 galaxy MCG-5-23-16, which is thought to have one of
the best known examples of a relativistically broadened iron K-alpha line. The
time averaged spectral analysis shows that the iron K-shell complex is best
modeled with an unresolved narrow emission component (FWHM < 5000 km/s, EW ~ 60
eV) plus a broad component. This latter component has FWHM ~ 44000 km/s and EW
~ 50 eV. Its profile is well described by an emission line originating from an
accretion disk viewed with an inclination angle ~ 40^\circ and with the
emission arising from within a few tens of gravitational radii of the central
black hole. The time-resolved spectral analysis of the XMM EPIC-pn spectrum
shows that both the narrow and broad components of the Fe K emission line
appear to be constant in time within the errors. We detected a narrow sporadic
absorption line at 7.7 keV which appears to be variable on a time-scale of 20
ksec. If associated with Fe XXVI Ly-alpha this absorption is indicative of a
possibly variable, high ionization, high velocity outflow. The variability of
this absorption feature appears to rule out a local (z=0) origin. The analysis
of the XMM RGS spectrum reveals that the soft X-ray emission of MCG-5-23-16 is
likely dominated by several emission lines superimposed on an unabsorbed
scattered power-law continuum. The lack of strong Fe L shell emission together
with the detection of a strong forbidden line in the O VII triplet is
consistent with a scenario where the soft X-ray emission lines are produced in
a plasma photoionized by the nuclear emission.Comment: 45 pages, 12 figures, 4 tables, accepted for publication in Ap
A Survey for HO Megamasers in Active Galactic Nuclei -- II. A Comparison of Detected and Undetected Galaxies
A survey for H2O megamaser emission from 354 active galaxies has resulted in
the detection of 10 new sources, making 16 known altogether. The galaxies
surveyed include a distance-limited sample (covering Seyferts and LINERs with
recession velocities < 7000 km s^{-1}) and a magnitude-limited sample (covering
Seyferts and LINERs with m(B) <= 14.5). In order to determine whether the
H2O-detected galaxies are "typical" AGN or have special properties which
facilitate the production of powerful masers, we have accumulated a database of
physical, morphological, and spectroscopic properties of the observed galaxies.
The most significant finding is that H2O megamasers are detected only in
Seyfert 2 and LINER galaxies, not Seyfert 1s. Implications for this finding are
discussed. LINERs are detected at a similar rate to Seyfert 2s, constituting a
strong argument that at least some nuclear LINERs are AGN rather than
starbursts, since starbursts have not been detected as H2O megamasers. We
preferentially detect H2O emission from the nearer galaxies and from those
which are apparently brighter at mid- and far-infrared and centimeter radio
wavelengths. There is also a possible trend for the H2O-detected galaxies to be
more intrinsically luminous in nuclear 6 cm radio emission than the undetected
ones, though these data are incomplete. We find evidence that Seyfert 2s with
very high (N(H) > 10^{24} cm^{-2}) X-ray absorbing columns of gas are more
often detected as H2O maser emitters than Seyfert 2s with lower columns. It may
be that the probability of detecting H2O maser emission in Seyfert galaxies
increases with increasing column of cool gas to the nucleus, from Seyfert 1s
through NLXGs to Seyfert 2s.Comment: 19 pages, 35 figures, 3 of the tables are contained in separate LaTeX
files: Table 1-a, 1-b, and 3. Also, figure captions are contained in a
separate file which must be compiled with plain TeX, not LaTe
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