720 research outputs found
The CoNFIG Catalogue - II. Comparison of Space Densities in the FR Dichotomy
This paper focuses on a comparison of the space densities of FRI and FRII
sources at different epochs, with a particular focus on FRI sources. First, we
present the concluding steps in constructing the Combined NVSS-FIRST Galaxy
catalogue (CoNFIG), including new VLA observations, optical identifications and
redshift estimates. The final catalogue consists of 859 sources over 4 samples
(CoNFIG-1, 2, 3 and 4 with flux density limits of S_1.4GHz = 1.3, 0.8, 0.2 and
0.05 Jy respectively). It is 95.7% complete in radio morphology classification
and 74.3% of the sources have redshift data. Combining CoNFIG with
complementary samples, the distribution and evolution of FRI and FRII sources
are investigated. We find that FRI sources undergo mild evolution and that, at
the same radio luminosity, FRI and FRII sources show similar space density
enhancements in various redshift ranges, possibly implying a common evolution.Comment: 15 pages, 15 figures, 6 tables + appendix (80 pages). accepted in
M.N.R.A.
Radio Jets in Galaxies with Actively Accreting Black Holes: new insights from the SDSS
The majority of nearby radio-loud AGN are found in massive, old elliptical
galaxies with weak emission lines. At high redshifts,however, most known radio
AGN have strong emission lines. In this paper, we examine a subset of radio AGN
with emission lines selected from the Sloan Digital Sky Survey. The probability
for a nearby radio AGN to have emission lines is a strongly decreasing function
of galaxy mass and an increasing function of radio luminosity above 10^25 W/Hz.
Emission line and radio luminosities are correlated, but with large dispersion.
At a given radio power, AGN with small black holes have higher [OIII]
luminosities (which we interpret as higher accretion rates) than AGN with big
black holes. However, if we scale both radio and emission line luminosities by
the black hole mass, we find a correlation between normalized radio power and
accretion rate in Eddington units that is independent of black hole mass. There
is also a clear correlation between normalized radio power and the age of the
stellar population in the galaxy. Present-day AGN with the highest normalized
radio powers are confined to galaxies with small black holes. High-redshift,
high radio-luminosity AGN could be explained if big black holes were similarly
active at earlier cosmic epochs. To investigate why only a small fraction of
emission line AGN become radio loud, we create matched samples of radio-loud
and radio-quiet AGN and compare their host galaxy properties and environments.
The main difference lies in their environments; our local density estimates are
a factor 2 larger around the radio-loud AGN. We propose a scenario in which
radio-loud AGN with emission lines are located in galaxies where accretion of
both cold and hot gas can occur simultaneously. (Abridged)Comment: 18 figures, submitted to MNRA
Feast and Famine: Regulation of Black Hole Growth in Low Redshift Galaxies
We analyze the observed distribution of Eddington ratios as a function of
supermassive black hole mass for a large sample of nearby galaxies drawn from
the Sloan Digital Sky Survey. We demonstrate that there are two distinct
regimes of black hole growth in nearby galaxies. The first is associated with
galaxies with significant star formation in their central kiloparsec regions,
and is characterized by a broad log-normal distribution of accretion rates
peaked at about one percent of the Eddington limit. In this regime, the
Eddington ratio distribution is independent of the mass of the black hole and
shows no further dependence on the central stellar population of the galaxy.
The second regime is associated with galaxies with old central stellar
populations, and is characterized by a power-law distribution function of
Eddington ratios. In this regime, the time-averaged mass accretion rate onto
black holes is proportional to the mass of stars in the galaxy bulge, with a
constant of proportionality that depends on the mean stellar age of the stars.
This result is once again independent of black hole mass. We show that both the
slope of the power-law and the decrease in the accretion rate onto black holes
in old galaxies are consistent with population synthesis model predictions of
the decline in stellar mass loss rates as a function of mean stellar age. Our
results lead to a very simple picture of black hole growth in the local
Universe. If the supply of cold gas in a galaxy bulge is plentiful, the black
hole regulates its own growth at a rate that does not further depend on the
properties of the interstellar medium. Once the gas runs out, black hole growth
is regulated by the rate at which evolved stars lose their mass.Comment: 22 pages, 11 figures, accepted by MNRAS (revised version incorporates
an improved correction for star formation contribution to L[OIII]
Star formation in high-redshift quasars: excess [O II] emission in the radio-loud population
We investigate the [O II] emission line properties of 18,508 quasars at z<1.6
drawn from the Sloan Digital Sky Survey (SDSS) quasar sample. The quasar sample
has been separated into 1,692 radio-loud and 16,816 radio-quiet quasars (RLQs
and RQQs hereafter) matched in both redshift and i'-band absolute magnitude.
We use the [O II]\lambda3726+3729 line as an indicator of star formation.
Based on these measurements we find evidence that star-formation activity is
higher in the RLQ population. The mean equivalent widths (EW) for [O II] are
EW([O II])_RL=7.80\pm0.30 \AA, and EW([O II])_RQ=4.77\pm0.06 \AA, for the RLQ
and RQQ samples respectively. The mean [O II] luminosities are \log[L([O
II])_RL/W]=34.31\pm0.01 and \log[L([O II])_RQ/W]=34.192\pm0.004 for the samples
of RLQs and RQQs respectively. Finally, to overcome possible biases in the EW
measurements due to the continuum emission below the [O II] line being
contaminated by young stars in the host galaxy, we use the ratio of the [O II]
luminosity to rest-frame i'-band luminosity, in this case, we find for the RLQs
\log[L([O II])_RL/L_opt]=-3.89\pm0.01 and \log[L([O
II])_RQ/L_opt]=-4.011\pm0.004 for RQQs. However the results depend upon the
optical luminosity of the quasar. RLQs and RQQs with the same high optical
luminosity \log(L_opt/W)>38.6, tend to have the same level of [O II] emission.
On the other hand, at lower optical luminosities \log(L_opt/W)<38.6, there is a
clear [O II] emission excess for the RLQs. As an additional check of our
results we use the [O III] emission line as a tracer of the bolometric
accretion luminosity, instead of the i'-band absolute magnitude, and we obtain
similar results.
Radio jets appear to be the main reason for the [O II] emission excess in the
case of RLQs. In contrast, we suggest AGN feedback ensures that the two
populations acquire the same [O II] emission at higher optical luminosities.Comment: 12 pages, 9 figures, accepted for publication in MNRA
Mergers as triggers for nuclear activity : A near-IR study of the close environment of AGN in the VISTA-VIDEO survey
copyright 2014 The Authors; Published by Oxford University Press on behalf of the Royal Astronomical SocietyThere is an ongoing debate concerning the driver of nuclear activity in galaxies, with active galactic nuclei (AGN) either being triggered by major or minor galactic mergers or, alternatively, through secular processes like cold gas accretion and/or formation of bars. We investigate the close environment of active galaxies selected in the X-ray, the radio and the mid-IR. We utilize the first data release of the new near-IR VISTA Deep Extragalactic Observations (VIDEO) survey of the XMM-Large Scale Structure field. We use two measures of environment density, namely counts within a given aperture and a finite redshift slice (pseudo- 3D density) and closest neighbour density measures ∑2 and ∑5. We select both AGN and control samples, matching them in redshift and apparent Ks-band magnitude. We find that AGN are found in a range of environments, with a subset of the AGN samples residing in overdense environments. Seyfert-like X-ray AGN and flat-spectrum radio-AGN are found to inhabit significantly overdense environments compared to their control sample. The relation between overdensities and AGN luminosity does not however reveal any positive correlation. Given the absence of an environment density-AGN luminosity relation, we find no support for a scheme where high-luminosity AGN are preferentially triggered by mergers. On the contrary, we find that AGN likely trace over dense environments at high redshift due to the fact that they inhabit the most massive galaxies, rather than being an AGN.Peer reviewe
Micro-Jy radio sources in the z=0.83 cluster MS1054-03
An extremely deep 5 GHz radio observation is presented of the rich cluster
MS1054-03 at redshift z=0.83. 34 radio sources are detected down to a 32
micro-Jy (6 sigma), compared to about 25 expected from previous blank field
radio source count determinations; the sources giving rise to these excess
counts lie within 2 arcmins (700 kpc) of the cluster centre. Existing imaging
and spectroscopy has provided optical identifications for 21 of the radio
sources and redshifts for 11, of which 8 are confirmed cluster members. 4 of
these 8 confirmed cluster sources are associated with close galaxy pairs (10-25
kpc projected offset) of similar magnitude, implying that the radio source may
be triggered by an interaction. However, although MS1054-03 has a very high
fraction (17%) of on-going mergers (separations <~ 10 kpc), no radio emission
is detected towards any of these merger events, setting a mean upper limit of
10 Msun/yr for any star formation associated with these mergers. This supports
a hypothesis that low luminosity radio sources may be onset by initial weak
interactions rather than direct mergers. The host galaxies of the other four
confirmed cluster radio sources are all isolated, and show a range of
morphologies from early-type to Sc. A comparison between the emission line and
radio luminosities suggests that two of these four radio sources are
low-luminosity AGN, whilst for at least one of the other two the radio emission
is associated with on-going star formation. All of the radio sources associated
with the galaxy pairs appear more likely AGN than starburst origin. The overall
proportion of radio sources associated with AGN in this cluster (>75%) is
higher than at these flux density levels in the field (40-50%).Comment: 19 pages LaTeX, including 8 figures. Accepted for publication in
MNRAS. Version with higher resolution figures available from
http://www.roe.ac.uk/~pnb/papers.htm
A catalogue of faint local radio AGN and the properties of their host galaxies
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. ©: 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We present a catalogue of 2210 local ( z < 0.1) galaxies that contain faint active galactic nuclei (AGN). We select these objects by identifying galaxies that exhibit a significant excess in their radio luminosities, compared to what is expected from the observed levels of star formation activity in these systems. This is achieved by comparing the optical (spectroscopic) star formation rate (SFR) to the 1.4 GHz luminosity measured from the Faint Images of the Radio Sky at Twenty centimeters survey. The majority of the AGN identified in this study are fainter than those in previous work, such as in the Best and Heckman (2012) catalogue. We show that these faint AGN make a non-negligible contribution to the radio luminosity function at low luminosities (below 1022.5 W Hz−1), and host ∼13 per cent of the local radio luminosity budget. Their host galaxies are predominantly high stellar-mass systems (with a median stellar mass of 1011 M⊙), are found across a range of environments (but typically in denser environments than star-forming galaxies) and have early-type morphologies. This study demonstrates a general technique to identify AGN in galaxy populations where reliable optical SFRs can be extracted using spectro-photometry and where radio data are also available so that a radio excess can be measured. Our results also demonstrate that it is unsafe to infer SFRs from radio emission alone, even if bright AGN have been excluded from a sample, since there is a significant population of faint radio AGN that may contaminate the radio-derived SFRs.Peer reviewedFinal Published versio
Modeling the cosmological co-evolution of supermassive black holes and galaxies: II. The clustering of quasars and their dark environment
We use semi-analytic modeling on top of the Millennium simulation to study
the joint formation of galaxies and their embedded supermassive black holes.
Our goal is to test scenarios in which black hole accretion and quasar
activity are triggered by galaxy mergers, and to constrain different models for
the lightcurves associated with individual quasar events. In the present work
we focus on studying the spatial distribution of simulated quasars. At all
luminosities, we find that the simulated quasar two-point correlation function
is fit well by a single power-law in the range 0.5 < r < 20 h^{-1} Mpc, but its
normalization is a strong function of redshift. When we select only quasars
with luminosities within the range typically accessible by today's quasar
surveys, their clustering strength depends only weakly on luminosity, in
agreement with observations. This holds independently of the assumed lightcurve
model, since bright quasars are black holes accreting close to the Eddington
limit, and are hosted by dark matter haloes with a narrow mass range of a few
10^12 h^{-1} M_sun. Therefore the clustering of bright quasars cannot be used
to disentangle lightcurve models, but such a discrimination would become
possible if the observational samples can be pushed to significantly fainter
limits.
Overall, our clustering results for the simulated quasar population agree
rather well with observations, lending support to the conjecture that galaxy
mergers could be the main physical process responsible for triggering black
hole accretion and quasar activity.Comment: 17 pages, 16 figures, to be published on MNRA
Black hole masses, accretion rates and hot- and cold-mode accretion in radio galaxies at z ~ 1
Date of Acceptance: 25/11/2014Understanding the evolution of accretion activity is fundamental to our understanding of how galaxies form and evolve over the history of the Universe. We analyse a complete sample of 27 radio galaxies which includes both high-excitation galaxies (HEGs) and low-excitation galaxies (LEGs), spanning a narrow redshift range of 0.9 < z < 1.1 and covering a factor of ~1000 in radio luminosity. Using data from the Spitzer Space Telescope combined with ground-based optical and near-infrared imaging, we show that the host galaxies have masses in the range of 10.7<log10(M/M⊙)<12.0with HEGs and LEGs exhibiting no difference in their mass distributions. We also find that HEGs accrete at significantly higher rates than LEGs, with the HEG/LEG division lying at an Eddington ratio of λ~0.04, which is in excellent agreement with theoretical predictions of where the accretion rate becomes radiatively inefficient, thus supporting the idea of HEGs and LEGs being powered by different modes of accretion. Our study also shows that at least up to L151MHz ~ 3 × 1027WHz-1 sr-1, HEGs and LEGs are indistinguishable in terms of their radio properties. From this result we infer that, at least for the lower radio luminosity range, another factor besides accretion rate must play an important role in the process of triggering jet activity.Peer reviewe
Evidence for powerful AGN winds at high redshift: Dynamics of galactic outflows in radio galaxies during the "Quasar Era"
AGN feedback now appears as an attractive mechanism to resolve some of the
outstanding problems with the "standard" cosmological models, in particular
those related to massive galaxies. To directly constrain how this may influence
the formation of massive galaxies near the peak in the redshift distribution of
powerful quasars, z~2, we present an analysis of the emission-line kinematics
of 3 powerful radio galaxies at z~2-3 (HzRGs) based on rest-frame optical
integral-field spectroscopy obtained with SINFONI on the VLT. HzRGs are among
the most massive galaxies, so AGN feedback may have a particularly clear
signature. We find evidence for bipolar outflows in all HzRGs, with kinetic
energies that are equivalent to 0.2% of the rest-mass of the supermassive black
hole. Velocity offsets in the outflows are ~800-1000 km s^-1 between the
blueshifted and redshifted line emission, FWHMs ~1000 km s^-1 suggest strong
turbulence. Ionized gas masses estimated from the Ha luminosity are of order
10^10 M_s, similar to the molecular gas content of HzRGs, underlining that
these outflows may indicate a significant phase in the evolution of the host
galaxy. The total energy release of ~10^60 erg during a dynamical time of ~10^7
yrs corresponds to about the binding energy of a massive galaxy. Geometry,
timescales and energy injection rates of order 10% of the kinetic energy flux
of the jet suggest that the outflows are most likely driven by the radio
source. The global energy density release of ~10^57 erg s^-1 Mpc^-3 may also
influence the subsequent evolution of the HzRG by enhancing the entropy and
pressure in the surrounding halo and facilitating ram-pressure stripping of gas
in satellite galaxies that may contribute to the subsequent mass assembly of
the HzRG through low-dissipation "dry" mergers.Comment: A&A in press, minor edits & typo in table captions 2-
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