70 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
Cold gas and star formation in a merging galaxy sequence
We explore the evolution of the cold gas and star-formation activity during
galaxy interactions, using a merging galaxy sequence comprising both pre- and
post-mergers. Data for this study come from the literature but supplemented by
new radio observations presented here. Firstly, we confirm that the
star-formation efficiency (SFE) increases close to nuclear coalescence. At
post-merger stages there is evidence that the SFE declines to values typical of
ellipticals. This trend can be attributed to M(H_2) depletion due to
interaction induced star-formation. However, there is significant scatter,
likely to arise from differences in the interaction details of individual
systems. Secondly, we find that the central molecular hydrogen surface density,
increases close to the final stages of the merging of the two nuclei. Such a
trend is also predicted by numerical simulations. Furthermore, there is
evidence for a decreasing fraction of cold gas mass from early interacting
systems to merger remnants, attributed to gas conversion into other forms. The
evolution of the total-radio to blue-band luminosity ratio, reflecting the
disk+nucleus star-formation activity, is also investigated. Although this ratio
is on average higher than that of isolated spirals, we find a marginal increase
along the merging sequence, attributed to the relative insensitivity of disk
star-formation to interactions. However, a similar result is also obtained for
the nuclear radio emission, although galaxy interactions are believed to
significantly affect the activity in the central galaxy regions. Finally, we
find that the FIR--radio flux ratio distribution of interacting galaxies is
consistent with star-formation being the main energising source.Comment: 18 pages, 17 figures, accepted for publication in MNRA
Keck Imaging of the Globular Cluster Systems in the Early--type Galaxies NGC 1052 and NGC 7332
The presence of two globular cluster subpopulations in early-type galaxies is
now the norm rather than the exception. Here we present two more examples for
which the host galaxy appears to have undergone a recent merger. Using
multi-colour Keck imaging of NGC 1052 and NGC 7332 we find evidence for a
bimodal globular cluster colour distribution in both galaxies, with roughly
equal numbers of blue and red globular clusters. The blue ones have similar
colours to those in the Milky Way halo and are thus probably very old and
metal-poor. If the red GC subpopulations are at least solar metallicity, then
stellar population models indicate young ages. We discuss the origin of
globular clusters within the framework of formation models. We conclude that
recent merger events in these two galaxies have had little effect on their
overall GC systems. We also derive globular cluster density profiles, global
specific frequencies and in the case of NGC 1052, radial colour gradients and
azimuthal distribution. In general these globular cluster properties are normal
for early-type galaxies.Comment: 11 pages, Latex, 15 figures, 2 tables, accepted by MNRA
Seeking for the leading actor on the cosmic stage: Galaxies versus Supermassive Black Holes
We present a Special Issue on the interplay of galaxies and Supermassive
Black Holes (SMBHs) recently published in Advances in Astronomy. This is the
introductory paper containing the motivation for this Special Issue together
with a brief description of the articles which are part of the manuscript and
the link to the entire book (http://www.hindawi.com/journals/aa/si/610485/). We
hope this Special Issue will be useful for many astronomers who want to get an
update on the current status of the AGN-Galaxy coevolution topic.Comment: 4 pages, published in Advances in Astronomy as Introductory paper to
the Special Issue "Seeking for the leading actor on the cosmic stage:
Galaxies versus SMBHs". The entire book can be downloaded as PDF here:
http://www.hindawi.com/journals/aa/si/610485
A Serendipitous XMM Survey of the SDSS: the evolution of the colour-magnitude diagram of X-ray AGN from z=0.8 to z=0.1
A new serendipitous XMM survey in the area of the Sloan Digital Sky Survey is
described (XMM/SDSS), which includes features such as the merging of
overlapping fields to increase the sensitivity to faint sources, the use of a
new parametrisation of the XMM point spread function for the source detection
and photometry, the accurate estimation of the survey sensitivity. About 40,000
X-ray point sources are detected over a total area of 122deg2. A subsample of
209 sources detected in the 2-8keV spectral band with SDSS spectroscopic
redshifts in the range 0.03<z<0.2, optical magnitudes r<17.77mag and
logLx(2-10keV)>41.5 (erg/s) are selected to explore their distribution on the
colour magnitude diagram. This is compared with the colour-magnitude diagram of
X-ray AGN in the AEGIS field at z~0.8. We find no evidence for evolution of the
rest-frame colours of X-ray AGN hosts from z=0.8 to z=0.1. This suggests that
the dominant accretion mode of the AGN population, which is expected to imprint
on the properties of their host galaxies, does not change since z=0.8. This
argues against scenarios which attribute the rapid decline of the accretion
power of the Universe with time (1dex since z=0.8) to changes in the AGN
fueling/triggering mode.Comment: To appear in MNRAS. Data available at
http://www.astro.noa.gr/~age/xmmsdss.htm
The evolution of the X-ray luminosity functions of unabsorbed and absorbed AGNs out to z~5
We present new measurements of the evolution of the X-ray luminosity
functions (XLFs) of unabsorbed and absorbed Active Galactic Nuclei (AGNs) out
to z~5. We construct samples containing 2957 sources detected at hard (2-7 keV)
X-ray energies and 4351 sources detected at soft (0.5-2 keV) energies from a
compilation of Chandra surveys supplemented by wide-area surveys from ASCA} and
ROSAT. We consider the hard and soft X-ray samples separately and find that the
XLF based on either (initially neglecting absorption effects) is best described
by a new flexible model parametrization where the break luminosity,
normalization and faint-end slope all evolve with redshift. We then incorporate
absorption effects, separately modelling the evolution of the XLFs of
unabsorbed () and absorbed ()
AGNs, seeking a model that can reconcile both the hard- and soft-band samples.
We find that the absorbed AGN XLF has a lower break luminosity, a higher
normalization, and a steeper faint-end slope than the unabsorbed AGN XLF out to
z~2. Hence, absorbed AGNs dominate at low luminosities, with the absorbed
fraction falling rapidly as luminosity increases. Both XLFs undergo strong
luminosity evolution which shifts the transition in the absorbed fraction to
higher luminosities at higher redshifts. The evolution in the shape of the
total XLF is primarily driven by the changing mix of unabsorbed and absorbed
populations.Comment: 36 pages, 20 figures, 11 tables. A casual reader is directed to
figures 7, 8, 9 and 20. Updated to version accepted for publication in MNRA
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
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