203 research outputs found
X-rays across the galaxy population - III. The incidence of AGN as a function of star formation rate
We map the co-eval growth of galaxies and their central supermassive black
holes in detail by measuring the incidence of Active Galactic Nuclei (AGN) in
galaxies as a function of star formation rate (SFR) and redshift (to z~4). We
combine large galaxy samples with deep Chandra X-ray imaging to measure the
probability distribution of specific black hole accretion rates (LX relative to
stellar mass) and derive robust AGN fractions and average specific accretion
rates. First, we consider galaxies along the main sequence of star formation.
We find a linear correlation between the average SFR and both the AGN fraction
and average specific accretion rate across a wide range in stellar mass () and to at least z~2.5, indicating that AGN in
main-sequence galaxies are driven by the stochastic accretion of cold gas. We
also consider quiescent galaxies and find significantly higher AGN fractions
than predicted, given their low SFRs, indicating that AGN in quiescent galaxies
are fuelled by additional mechanisms (e.g. stellar winds). Next, we bin
galaxies according to their SFRs relative to the main sequence. We find that
the AGN fraction is significantly elevated for galaxies that are still
star-forming but with SFRs below the main sequence, indicating further
triggering mechanisms enhance AGN activity within these sub-main-sequence
galaxies. We also find that the incidence of high-accretion-rate AGN is
enhanced in starburst galaxies and evolves more mildly with redshift than
within the rest of the galaxy population, suggesting mergers play a role in
driving AGN activity in such high-SFR galaxies.Comment: 19 pages, 15 figures. This is a pre-copyedited, author-produced
version of an article accepted for publication in MNRAS following peer revie
The AGN-galaxy-halo connection::The distribution of AGN host halo masses to z=2.5
It is widely reported, based on clustering measurements of observed active
galactic nuclei (AGN) samples, that AGN reside in similar mass host dark matter
halos across the bulk of cosmic time, with log ~12.5-13.0 to z~2.5.
We show that this is due in part to the AGN fraction in galaxies rising with
increasing stellar mass, combined with AGN observational selection effects that
exacerbate this trend. Here, we use AGN specific accretion rate distribution
functions determined as a function of stellar mass and redshift for
star-forming and quiescent galaxies separately, combined with the latest
galaxy-halo connection models, to determine the parent and sub-halo mass
distribution function of AGN to various observational limits. We find that
while the median (sub-)halo mass of AGN, , is fairly
constant with luminosity, specific accretion rate, and redshift, the full halo
mass distribution function is broad, spanning several orders of magnitude. We
show that widely used methods to infer a typical dark matter halo mass based on
an observed AGN clustering amplitude can result in biased, systematically high
host halo masses. While the AGN satellite fraction rises with increasing parent
halo mass, we find that the central galaxy is often not an AGN. Our results
elucidate the physical causes for the apparent uniformity of AGN host halos
across cosmic time and underscore the importance of accounting for AGN
selection biases when interpreting observational AGN clustering results. We
further show that AGN clustering is most easily interpreted in terms of the
relative bias to galaxy samples, not from absolute bias measurements alone.Comment: 19 pages, 20 figures, accepted for publication in MNRAS. Updated to
show median halo masses following referee's helpful comment
AGN accretion and black hole growth across compact and extended galaxy evolution phases
The extent of black hole growth during different galaxy evolution phases and
the connection between galaxy compactness and AGN activity remain poorly
understood. We use Hubble Space Telescope imaging of the CANDELS fields to
identify star-forming and quiescent galaxies at z=0.5-3 in both compact and
extended phases and use Chandra X-ray imaging to measure the distribution of
AGN accretion rates and track black hole growth within these galaxies.
Accounting for the impact of AGN light changes ~20% of the X-ray sources from
compact to extended galaxy classifications. We find that ~10-25% of compact
star-forming galaxies host an AGN, a mild enhancement (by a factor ~2) compared
to extended star-forming galaxies or compact quiescent galaxies of equivalent
stellar mass and redshift. However, AGN are not ubiquitous in compact
star-forming galaxies and this is not the evolutionary phase, given its
relatively short timescale, where the bulk of black hole mass growth takes
place. Conversely, we measure the highest AGN fractions (~10-30%) within the
relatively rare population of extended quiescent galaxies. For massive galaxies
that quench at early cosmic epochs, substantial black hole growth in this
extended phase is crucial to produce the elevated black hole mass-to-galaxy
stellar mass scaling relation observed for quiescent galaxies at z~0. We also
show that AGN fraction increases with compactness in star-forming galaxies and
decreases in quiescent galaxies within both the compact and extended
sub-populations, demonstrating that AGN activity depends closely on the
structural properties of galaxies.Comment: 29 pages, 18 figures, submitted to MNRAS. Primary results are shown
in Fig 7 and summarised by Fig 12. See Fig 16 and 17 for key
interpretation/conclusion
The intrinsic X-ray luminosity distribution of an optically-selected SDSS quasar population
In active galactic nuclei, the relationship between UV and X-ray luminosity
is well studied (often characterised by ) but often with
heterogeneous samples. We have parametrized the intrinsic distribution of X-ray
luminosity, , for the optically-selected sample of SDSS quasars in
the Stripe 82 and XXL fields across redshifts 0.5-3.5. We make use of the
available XMM observations and a custom pipeline to produce Bayesian
sensitivity curves that are used to derive the intrinsic X-ray distribution in
a hierarchical Bayesian framework. We find that the X-ray luminosity
distribution is well described by a Gaussian function in
space with a mean that is dependent on the
monochromatic 2500A UV luminosity, . We also observe some redshift
dependence of the distribution. The mean of the distribution
increases with redshift while the width decreases. This weak but significant
redshift dependence leads to - and
- relations that evolve with redshift, and we
produce a redshift- and -dependent equation.
Neither black hole mass nor Eddington ratio appear to be potential drivers of
the redshift evolution.Comment: 18 pages, 16 figures (including appendices). Accepted for publication
in MNRA
PRIMUS: The relationship between Star formation and AGN accretion
We study the evidence for a connection between active galactic nuclei (AGN)
fueling and star formation by investigating the relationship between the X-ray
luminosities of AGN and the star formation rates (SFRs) of their host galaxies.
We identify a sample of 309 AGN with erg
s at in the PRIMUS redshift survey. We find AGN in
galaxies with a wide range of SFR at a given . We do not find a
significant correlation between SFR and the observed instantaneous for
star forming AGN host galaxies. However, there is a weak but significant
correlation between the mean and SFR of detected AGN in star
forming galaxies, which likely reflects that varies on shorter
timescales than SFR. We find no correlation between stellar mass and
within the AGN population. Within both populations of star
forming and quiescent galaxies, we find a similar power-law distribution in the
probability of hosting an AGN as a function of specific accretion rate.
Furthermore, at a given stellar mass, we find a star forming galaxy
more likely than a quiescent galaxy to host an AGN of a given specific
accretion rate. The probability of a galaxy hosting an AGN is constant across
the main sequence of star formation. These results indicate that there is an
underlying connection between star formation and the presence of AGN, but AGN
are often hosted by quiescent galaxies
Obscuration-dependent evolution of Active Galactic Nuclei
We aim to constrain the evolution of AGN as a function of obscuration using
an X-ray selected sample of AGN from a multi-tiered survey including
the CDFS, AEGIS-XD, COSMOS and XMM-XXL fields. The spectra of individual X-ray
sources are analysed using a Bayesian methodology with a physically realistic
model to infer the posterior distribution of the hydrogen column density and
intrinsic X-ray luminosity. We develop a novel non-parametric method which
allows us to robustly infer the distribution of the AGN population in X-ray
luminosity, redshift and obscuring column density, relying only on minimal
smoothness assumptions. Our analysis properly incorporates uncertainties from
low count spectra, photometric redshift measurements, association
incompleteness and the limited sample size. We find that obscured AGN with
account for of the number
density and luminosity density of the accretion SMBH population with , averaged over cosmic time. Compton-thick AGN account
for approximately half the number and luminosity density of the obscured
population, and of the total. We also find evidence that the
evolution is obscuration-dependent, with the strongest evolution around
. We highlight this by measuring the
obscured fraction in Compton-thin AGN, which increases towards , where
it is higher than the local value. In contrast the fraction of
Compton-thick AGN is consistent with being constant at ,
independent of redshift and accretion luminosity. We discuss our findings in
the context of existing models and conclude that the observed evolution is to
first order a side-effect of anti-hierarchical growth.Comment: Published in Ap
Higher prevalence of X-ray selected AGN in intermediate age galaxies up to z~1
We analyse the stellar populations in the host galaxies of 53 X-ray selected
optically dull active galactic nuclei (AGN) at 0.34<z<1.07 with ultra-deep
(m=26.5) optical medium-band (R~50) photometry from the Survey for High-z
Absorption Red and Dead Sources (SHARDS). The spectral resolution of SHARDS
allows us to consistently measure the strength of the 4000 AA break, Dn(4000),
a reliable age indicator for stellar populations. We confirm that most X-ray
selected moderate-luminosity AGN (L_X<10^44 erg/s) are hosted by massive
galaxies (typically M*>10^10.5 M_sun) and that the observed fraction of
galaxies hosting an AGN increases with the stellar mass. A careful selection of
random control samples of inactive galaxies allows us to remove the stellar
mass and redshift dependencies of the AGN fraction to explore trends with
several stellar age indicators. We find no significant differences in the
distribution of the rest-frame U-V colour for AGN hosts and inactive galaxies,
in agreement with previous results. However, we find significantly shallower
4000 AA breaks in AGN hosts, indicative of younger stellar populations. With
the help of a model-independent determination of the extinction, we obtain
extinction-corrected U-V colours and light-weighted average stellar ages. We
find that AGN hosts have younger stellar populations and higher extinction
compared to inactive galaxies with the same stellar mass and at the same
redshift. We find a highly significant excess of AGN hosts with Dn(4000)~1.4
and light weighted average stellar ages of 300-500 Myr, as well as a deficit of
AGN in intrinsic red galaxies. We interpret failure in recognising these trends
in previous studies as a consequence of the balancing effect in observed
colours of the age-extinction degeneracy.Comment: Accepted for publication in MNRAS, 12 pages, 8 figures, 1 tabl
X-ray Surface Brightness Profiles of Active Galactic Nuclei in the Extended Groth Strip: Implications for AGN Feedback
Using data from the All Wavelength Extended Groth Strip International Survey
(AEGIS) we statistically detect the extended X-ray emission in the interstellar
medium (ISM)/intra-cluster medium (ICM) in both active and normal galaxies at
0.3 <= z <= 1.3. For both active galactic nuclei (AGN) host galaxy and normal
galaxy samples that are matched in restframe color, luminosity, and redshift
distribution, we tentatively detect excess X-ray emission at scales of 1--10
arcsec at a few sigma significance in the surface brightness profiles. The
exact significance of this detection is sensitive to the true characterization
of Chandra's point spread function. The observed excess in the surface
brightness profiles is suggestive of lower extended emission in AGN hosts
compared to normal galaxies. This is qualitatively similar to theoretical
predictions of the X-ray surface brightness profile from AGN feedback models,
where feedback from AGN is likely to evacuate the gas from the center of the
galaxy/cluster. We propose that AGN that are intrinsically under-luminous in
X-rays, but have equivalent bolometric luminosities to our sources will be the
ideal sample to study more robustly the effect of AGN feedback on diffuse
ISM/ICM gas.Comment: Accepted in PAS
- …