1,860 research outputs found
The orientation of the nuclear obscurer of the AGNs
We examine the distribution of axis ratios of a large sample of disk galaxies
hosting type 2 AGNs selected from the Sloan Digital Sky Survey and compare it
with a well-defined control sample of non-active galaxies. We find them
significantly different, where the type 2 AGNs show both an excess of edge-on
objects and deficit of round objects. This systematical bias can not be
explained by a nuclear obscurer oriented randomly with respect to the stellar
disk. However, a nuclear obscurer coplanar with the stellar disk also does not
fit the data very well. By assuming that the nuclear obscurer having an opening
angle of ~60 degree, we find the observed axis ratio distribution can be nicely
reproduced by a mean tilt angle of ~30 degree between the nuclear obscurer and
the stellar disk.Comment: 5 pages, 3 figures, to appear in ApJ
The Role of Mergers in Early-type Galaxy Evolution and Black Hole Growth
Models of galaxy formation invoke the major merger of gas-rich progenitor
galaxies as the trigger for significant phases of black hole growth and the
associated feedback that suppresses star formation to create red spheroidal
remnants. However, the observational evidence for the connection between
mergers and active galactic nucleus (AGN) phases is not clear. We analyze a
sample of low-mass early-type galaxies known to be in the process of migrating
from the blue cloud to the red sequence via an AGN phase in the green valley.
Using deeper imaging from SDSS Stripe 82, we show that the fraction of objects
with major morphological disturbances is high during the early starburst phase,
but declines rapidly to the background level seen in quiescent early-type
galaxies by the time of substantial AGN radiation several hundred Myr after the
starburst. This observation empirically links the AGN activity in low-redshift
early-type galaxies to a significant merger event in the recent past. The large
time delay between the merger-driven starburst and the peak of AGN activity
allows for the merger features to decay to the background and hence may explain
the weak link between merger features and AGN activity in the literature.Comment: 5 pages, 3 figures. ApJ Letters, in press
Difference in Narrow Emission Line Spectra of Seyfert 1 and 2 galaxies
In the unification scheme of Seyfert galaxies, a dusty torus blocks the
continuum source and broad line region in Seyfert 2 galaxies. However it is not
clear whether or not and to what extent the torus affects the narrow line
spectra. In this paper, we show that Seyfert 1 and Seyfert 2 galaxies have
different distributions on the [OIII]/H vs [NII]/H diagram (BPT
diagram) for narrow lines. Seyfert 2 galaxies display a clear left boundary on
the BPT diagram and only 7.3% of them lie on the left. By contrast, Seyfert 1
galaxies do not show such a cutoff and 33.0% of them stand on the left side of
the boundary. Among Seyfert 1 galaxies, the distribution varies with the
extinction to broad lines. As the extinction increases, the distribution on BPT
diagram moves to larger [NII]/H value. We interpret this as an evidence
for the obscuration of inner dense narrow line region by the dusty torus. We
also demonstrate that the [OIII] and broad line luminosity correlation depends
on the extinction of broad lines in the way that high extinction objects have
lower uncorrected [OIII] luminosities, suggesting that [OIII] is partially
obscured in these objects. Therefore, using [OIII] as an indicator for the
nuclear luminosity will systematically under-estimate the nuclear luminosity of
Seyfert 2 galaxies.Comment: 12 pages, 2 figures, accepted for publication in ApJ Lette
Stellar X-ray sources in the Chandra COSMOS survey
We present an analysis of the X-ray properties of a sample of solar- and
late-type field stars identified in the Chandra Cosmic Evolution Survey
(COSMOS), a deep (160ks) and wide (0.9 deg2) extragalactic survey. The sample
of 60 sources was identified using both morphological and photometric
star/galaxy separation methods. We determine X-ray count rates, extract spectra
and light curves and perform spectral fits to determine fluxes and plasma
temperatures. Complementary optical and near-IR photometry is also presented
and combined with spectroscopy for 48 of the sources to determine spectral
types and distances for the sample. We find distances ranging from 30pc to
~12kpc, including a number of the most distant and highly active stellar X-ray
sources ever detected. This stellar sample extends the known coverage of the
L_X-distance plane to greater distances and higher luminosities, but we do not
detect as many intrinsically faint X-ray sources compared to previous surveys.
Overall the sample is typically more luminous than the active Sun, representing
the high-luminosity end of the disk and halo X-ray luminosity functions. The
halo population appears to include both low-activity spectrally hard sources
that may be emitting through thermal bremsstrahlung, as well as a number of
highly active sources in close binaries.Comment: 13 pages, 5 figures, accepted for publication in Ap
Dusty Structure Around Type-I Active Galactic Nuclei: Clumpy Torus Narrow Line Region and Near-Nucleus Hot Dust
We fitted Spitzer/IRS ~2-35 um spectra of 26 luminous QSOs in attempt to
define the main emission components. Our model has three major components: a
clumpy torus, dusty narrow line region (NLR) clouds and a blackbody-like dust.
The models utilize the clumpy torus of Nenkova et al. (2008) and are the first
to allow its consistent check in type-I AGNs. Single torus models and combined
torus-NLR models fail to fit the spectra of most sources but three component
models adequately fit the spectra of all sources. We present torus inclination,
cloud distribution, covering factor and torus mass for all sources and compare
them with bolometric luminosity, black hole mass and accretion rate. The torus
covering factor and mass are found to be correlated with the bolometric
luminosity of the sources. We find that a substantial amount of the ~2-7 um
radiation originates from a hot dust component, which likely situated in the
innermost part of the torus. The luminosity radiated by this component and its
covering factor are comparable to those of the torus. We quantify the emission
by the NLR clouds and estimate their distance from the center. The distances
are ~700 times larger than the dust sublimation radius and the NLR covering
factor is about 0.07. The total covering factor by all components is in good
agreement with the known AGN type-I:type-II ratio.Comment: 14 pages, including 11 figures, submitted to ApJ and revised
following the referee's repor
Exploring the disk-jet connection from the properties of narrow line regions in powerful young radio-loud AGNs
We investigate the optical emission-line flux ratios of narrow-line regions,
in order to determine whether the formation of AGN jets requires specific
accretion conditions. We find that bright compact radio galaxies, which are
powerful radio galaxies in the early stage of the jet activity, exhibit
systematically larger flux ratios of [O{\sc i}]6300/[O{\sc
iii}]5007 and smaller flux ratios of [O{\sc iii}]5007/[O{\sc
iii}]4363 than radio-quiet (RQ) Seyfert 2 galaxies. Comparing the
observed line ratios with photoionization models, it is found that the
difference in the flux ratio of low- to high-ionization lines (e.g., [O{\sc
i}]6300/[O{\sc iii}]5007) can be well understood by the
difference in the spectral energy distribution (SED) of ionizing sources.
Powerful young radio-loud (YRL) AGNs favor SED without a strong big blue bump,
i.e., a radiatively inefficient accretion flow (RIAF), while RQ AGNs are
consistent with the models adopting SED with a strong big blue bump, i.e., a
geometrically thin, optically thick disk. These findings imply that the
formation of powerful AGN jets requires the accretion disk with harder ionizing
SED (i.e., a RIAF). We discuss the obscuring structure of YRL AGNs as a
plausible origin of the difference in flux ratios of [O{\sc iii}]5007/[O{\sc iii}]4363.Comment: 17 pages, 8 figures, accepted for publication in Ap
A celestial gamma-ray foreground due to the albedo of small solar system bodies and a remote probe of the interstellar cosmic ray spectrum
We calculate the gamma-ray albedo flux from cosmic-ray (CR) interactions with
the solid rock and ice in Main Belt asteroids (MBAs), Jovian and Neptunian
Trojan asteroids, and Kuiper Belt objects (KBOs) using the Moon as a template.
We show that the gamma-ray albedo for the Main Belt, Trojans, and Kuiper Belt
strongly depends on the small-body size distribution of each system. Based on
an analysis of the Energetic Gamma Ray Experiment Telescope (EGRET) data we
infer that the diffuse emission from the MBAs, Trojans, and KBOs has an
integrated flux of less than ~6x10^{-6} cm^{-2} s^{-1} (100-500 MeV), which
corresponds to ~12 times the Lunar albedo, and may be detectable by the
forthcoming Gamma Ray Large Area Space Telescope (GLAST). If detected by GLAST,
it can provide unique direct information about the number of small bodies in
each system that is difficult to assess by any other method. Additionally, the
KBO albedo flux can be used to probe the spectrum of CR nuclei at
close-to-interstellar conditions. The orbits of MBAs, Trojans, and KBOs are
distributed near the ecliptic, which passes through the Galactic center and
high Galactic latitudes. Therefore, the asteroid gamma-ray albedo has to be
taken into account when analyzing weak gamma-ray sources close to the ecliptic,
especially near the Galactic center and for signals at high Galactic latitudes,
such as the extragalactic gamma-ray emission. The asteroid albedo spectrum also
exhibits a 511 keV line due to secondary positrons annihilating in the rock.
This may be an important and previously unrecognized celestial foreground for
the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) observations of
the Galactic 511 keV line emission including the direction of the Galactic
center.Comment: 10 pages, 5 figures, 1 table, emulateapj.cls; ApJ in press.
Calculations extended to include Jovian and Neptunian Trojan groups, and
Centaurs, in addition to Main Belt Asteroids and Kuiper Belt Objects. Many
other considerable change
Active Galactic Nucleus Pairs from the Sloan Digital Sky Survey. II. Evidence for Tidally Enhanced Star Formation and Black Hole Accretion
Active galactic nuclei (AGNs) are occasionally seen in pairs, suggesting that
tidal encounters are responsible for the accretion of material by both central
supermassive black holes (BHs). In Paper I of this series, we selected a sample
of AGN pairs with projected separations r_p < 100 kpc and velocity offsets <
600 km/s from the SDSS DR7 and quantified their frequency. In this paper, we
address the BH accretion and recent star formation properties in their host
galaxies. AGN pairs experience stronger BH accretion, as measured by their [O
III]5007 luminosities (corrected for contribution from star formation) and
Eddington ratios, than do control samples of single AGNs matched in redshift
and host stellar mass. Their host galaxies have stronger post-starburst
activity and younger mean stellar ages, as indicated by stronger H-delta
absorption and smaller 4000 A break in their spectra. The BH accretion and
recent star formation in the host galaxies both increase with decreasing
projected separation in AGN pairs, for r_p ~< 10-30 kpc. The intensity of BH
accretion, the post-starburst strength, and the mean stellar ages are
correlated between the two AGNs in a pair. The luminosities and Eddington
ratios of AGN pairs are correlated with recent star formation in their host
galaxies, with a scaling relation consistent with that observed in single AGNs.
Our results suggest that galaxy tidal interactions enhance both BH accretion
and star formation in AGN pairs, even though the majority of low redshift AGNs
is not coincident with on-going interactions.Comment: 16 pages, 12 figures; to appear in Ap
The AMIGA sample of isolated galaxies. XI. Optical characterisation of nuclear activity
Context.- This paper is part of a series involving the AMIGA project
(Analysis of the Interstellar Medium of Isolated GAlaxies), which identifies
and studies a statistically-significant sample of the most isolated galaxies in
the northern sky. Aims.- We present a catalogue of nuclear activity, traced by
optical emission lines, in a well-defined sample of the most isolated galaxies
in the local Universe, which will be used as a basis for studying the effect of
the environment on nuclear activity. Methods.- We obtained spectral data from
the 6th Data Release of the Sloan Digital Sky Survey, which were inspected in a
semi-automatic way. We subtracted the underlying stellar populations from the
spectra (using the software Starlight) and modelled the nuclear emission
features. Standard emission-line diagnostics diagrams were applied, using a new
classification scheme that takes into account censored data, to classify the
type of nuclear emission. Results.- We provide a final catalogue of
spectroscopic data, stellar populations, emission lines and classification of
optical nuclear activity for AMIGA galaxies. The prevalence of optical active
galactic nuclei (AGN) in AMIGA galaxies is 20.4%, or 36.7% including transition
objects. The fraction of AGN increases steeply towards earlier morphological
types and higher luminosities. We compare these results with a matched analysis
of galaxies in isolated denser environments (Hickson Compact Groups). After
correcting for the effects of the morphology and luminosity, we find that there
is no evidence for a difference in the prevalence of AGN between isolated and
compact group galaxies, and we discuss the implications of this result.
Conclusions.- We find that a major interaction is not a necessary condition for
the triggering of optical AGN.Comment: 16 pages, 11 figures, 12 tables, published in Astronomy and
Astrophysics. Figure 5 corrected: [OI] diagram adde
The Rise and Fall of Galaxy Activity in Dark Matter Haloes
We use a SDSS galaxy group catalogue to study the dependence of galaxy
activity on stellar mass, halo mass, and group hierarchy (centrals vs.
satellites). We split our galaxy sample in star-forming galaxies, galaxies with
optical AGN activity and radio sources. We find a smooth transition in halo
mass as the activity of central galaxies changes from star formation to optical
AGN activity to radio emission. Star-forming centrals preferentially reside in
haloes with M<10^{12} Msun, central galaxies with optical-AGN activity
typically inhabit haloes with M \sim 10^{13} Msun, and centrals emitting in the
radio mainly reside in haloes more massive than 10^{14} Msun. Although this
seems to suggest that the environment (halo mass) determines the type of
activity of its central galaxy, we find a similar trend with stellar mass:
central star formers typically have stellar masses below 10^{10} Msun, while
optical-AGN hosts and central radio sources have characteristic stellar masses
of 10^{10.8} Msun and 10^{11.6} Msun, respectively. Since more massive haloes
typically host more massive centrals, it is unclear whether the activity of a
central galaxy is causally connected to its stellar mass or to its halo mass.
In general, satellite galaxies have their activity suppressed wrt central
galaxies of the same stellar mass. At fixed stellar mass, we find that the
activity of satellite galaxies depends only weakly on halo mass. In fact, for
satellite galaxies the dependence of galaxy activity on halo mass is more than
four times weaker than the dependence on stellar mass. As we discuss, all these
results are consistent with a picture in which low mass haloes accrete cold
gas, while massive haloes have coronae of hot gas that promote radio activity
of their central galaxies. [Abridged]Comment: 17 pages, 13 figures. Submitted for publication in MNRA
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