46 research outputs found
A Chandra view of Naked Active Galactic Nuclei
We present the first X-ray observations of three sources belonging to a new
AGN class: the naked AGNs. Based on optical spectroscopic studies, these
sources appear as classical type 2 (obscured) AGNs, with only narrow emission
lines. However, long-term optical monitoring campaigns, carried out over more
than two decades, show that the same sources are strongly variable, like type 1
(un-obscured) AGNs. Based on short Chandra observations, the sources appear to
be fairly bright in the X-rays, with typical Seyfert 1s values for the photon
index (Gamma~1.8) and without significant intrinsic absorption, supporting the
conclusion that some bright AGNs may genuinely lack a broad line region.
Future, broad-band studies as well as deeper X-ray observations, probing both
the spectral and the temporal properties of the naked AGNs, are crucial to shed
light on the central engine of these sources, which may be representative of a
large class of AGNs.Comment: 7 pages, 3 figures, accepted for publication in Ap
Assessing indirect methods to determine black hole masses using NGC 4151
Accurately determining the black hole mass () in active
galactic nuclei (AGN) is crucial to constraining their properties and to
studying their evolution. While direct methods yield reliable measurements of
in unobscured type 1 AGN, where the dynamics of stellar or gas
components can be directly observed, only indirect methods can be applied to
the vast majority of heavily absorbed type 2 AGN, which represent most of the
AGN population. Since it is difficult to evaluate the accuracy and precision of
these indirect methods, we utilize the nearby X-ray bright Seyfert galaxy NGC
4151, whose has been tightly constrained with several
independent direct methods, as a laboratory to assess the reliability of three
indirect methods that have been applied to obscured AGN. All three, the X-ray
scaling method, the fundamental plane of black hole activity, and the
M- correlation, yield values consistent with those
inferred from direct methods and can therefore be considered accurate. However,
only the X-ray scaling method and the M- correlation are precise
because the substantial scatter in the fundamental plane of BH activity allows
only for crude estimates. Of the four M- correlations we used, only the
one from Kormendy and Ho yields a value consistent with the dynamical
estimates. This study suggests that the best approach to estimating the black
hole mass in systems where direct dynamical methods cannot be applied is to
utilize a combination of indirect methods, taking into account their different
ranges of applicability.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Societ
Buried AGNs in Advanced Mergers:Mid-infrared color selection as a dual AGN finder
A direct consequence of hierarchical galaxy formation is the existence of
dual supermassive black holes (SMBHs), which may be preferentially triggered as
active galactic nuclei (AGN) during galaxy mergers. Despite decades of
searching, however, dual AGNs are extremely rare, and most have been discovered
serendipitously. Using the all-sky WISE survey, we identified a population of
over 100 morphologically identified interacting galaxies or mergers that
display red mid-infrared colors often associated in extragalactic sources with
powerful AGNs. The vast majority of these advanced mergers are optically
classified as star-forming galaxies suggesting that they may represent an
obscured population of AGNs that cannot be found through optical studies. In
this work, we present Chandra/ACIS observations and near-infrared spectra with
the Large Binocular Telescope of six advanced mergers with projected pair
separations less than ~ 10 kpc. The combined X-ray, near-infrared, and
mid-infrared properties of these mergers provide confirmation that four out of
the six mergers host at least one AGN, with four of the mergers possibly
hosting dual AGNs with projected separations less than ~10 kpc, despite showing
no firm evidence for AGNs based on optical spectroscopic studies. Our results
demonstrate that 1) optical studies miss a significant fraction of single and
dual AGNs in advanced mergers, and 2) mid-infrared pre-selection is extremely
effective in identifying dual AGN candidates in late-stage mergers. Our
multi-wavelength observations suggest that the buried AGNs in these mergers are
highly absorbed, with intrinsic column densities in excess of N_H >10^24cm^-2,
consistent with hydrodynamic simulations.Comment: 23 pages, 11 figures, accepted for publication to Ap
The Jet/Disk Connection in AGN: Chandra and XMM-Newton Observations of Three Powerful Radio-Loud Quasars
The connection between the accretion process that powers AGN and the formation of jets is still poorly understood. Here we tackle this issue using new, deep Chandra and XMM-Newton observations of tlie cores of three powerful radio loud quasars: 1136-135, 1150+497 (Chandra), and 0723+679 (XMM-Newton), in the redshift range z=0.3-0.8. These sources are known from our previous Chandra siiapsliot survey to liave kpc-scale X-ray jets. In 1136-135 and 1150-1+497; evidence is found for the presence of diffuse thermal X-ray emission around the cores; on scales of 40-50 kpc and with luminosity L(sub 0.3-2 kev approx. 10(sup 43) erg per second, suggesting thermal emission from the host galaxy or a galaxy group. The X-ray continua of the cores in the three sources are described by an upward-curved (concave) broken power law, with photon indices GAMMA (sub soft) approx. 1.8 - 2.1 and GAMMA (sub hard) approx. 1.7 below and above approx. equal to 2 keV, respectively. There is evidence for an uiiresolved Fe K alpha line with EW approx. 70 eV in the three quasars. The Spectral Energy Distributions of the sources can be well described by a mix of jet and disk emission, with the jet dominating the radio and hard X-rays (via synchrotron and external Compton) and the disk dominating the optical/UV through soft X-rays. The ratio of the jet-to-disk powers is approx. 1, consistent with those derived for a number of gamma ray emitting blazars. This indicates that near equality of accretion and jet power may be common in powerful radio-loud AGN
Buried Black Hole Growth in IR-selected Mergers: New Results from Chandra
Observations and theoretical simulations suggest that a significant fraction
of merger-triggered accretion onto supermassive black holes is highly obscured,
particularly in late-stage galaxy mergers, when the black hole is expected to
grow most rapidly. Starting with the Wide-Field Infrared Survey Explorer
all-sky survey, we identified a population of galaxies whose morphologies
suggest ongoing interaction and which exhibit red mid-infrared colors often
associated with powerful active galactic nuclei (AGNs). In a follow-up to our
pilot study, we now present Chandra/ACIS and XMM-Newton X-ray observations for
the full sample of the brightest 15 IR-preselected mergers. All mergers reveal
at least one nuclear X-ray source, with 8 out of 15 systems exhibiting dual
nuclear X-ray sources, highly suggestive of single and dual AGNs. Combining
these X-ray results with optical line ratios and with near-IR coronal emission
line diagnostics, obtained with the near-IR spectrographs on the Large
Binocular Telescope, we confirm that 13 out of the 15 mergers host AGNs, two of
which host dual AGNs. Several of these AGNs are not detected in the optical.
All X-ray sources appear X-ray weak relative to their mid-infrared continuum,
and of the nine X-ray sources with sufficient counts for spectral analysis,
eight reveal strong evidence of high absorption with column densities of
~cm. These observations demonstrate that a
significant population of single and dual AGNs are missed by optical studies,
due to high absorption, adding to the growing body of evidence that the epoch
of peak black hole growth in mergers occurs in a highly obscured phase.Comment: 29 pages, 22 figures; (Main text: 17 pages, 4 figures
NuSTAR Observations of Four Mid-IR Selected Dual AGN Candidates in Galaxy Mergers
Mergers of galaxies are a ubiquitous phenomenon in the Universe and represent
a natural consequence of the ``bottom-up'' mass accumulation and galaxy
evolution cosmological paradigm. It is generally accepted that the peak of AGN
accretion activity occurs at nuclear separations of kpc for major
mergers. Here we present new NuSTAR and XMM-Newton observations for a subsample
of mid-IR preselected dual AGN candidates in an effort to better constrain the
column densities along the line-of-sight for each system. Only one dual AGN
candidate, J0841+0101, is detected as a single, unresolved source in the
XMM-Newton and NuSTAR imaging, while the remaining three dual AGN candidates,
J0122+0100, J1221+1137, and J1306+0735, are not detected with NuSTAR; if these
non-detections are due to obscuration alone, these systems are consistent with
being absorbed by column densities of log()
24.9, 24.8, and 24.6, which are roughly consistent with previously inferred
column densities in these merging systems. In the case of J0841+0101, the
analysis of the 0.3-30 keV spectra reveal a line-of-sight column density of
cm, significantly larger than the column
densities previously reported for this system and demonstrating the importance
of the higher signal-to-noise XMM-Newton spectra and access to the keV
energies via NuSTAR. Though it is unclear if J0841+0101 truly hosts a dual AGN,
these results are in agreement with the high obscuring columns expected in AGNs
in late-stage mergers.Comment: 21 pages, 9 tables, 4 figures. Accepted for publication in Ap