86,346 research outputs found
Radio faint AGN: a tale of two populations
We study the Extended Chandra Deep Field South (E-CDFS) Very Large Array
sample, which reaches a flux density limit at 1.4 GHz of 32.5 microJy at the
field centre and redshift ~ 4, and covers ~ 0.3 deg^2. Number counts are
presented for the whole sample while the evolutionary properties and luminosity
functions are derived for active galactic nuclei (AGN). The faint radio sky
contains two totally distinct AGN populations, characterised by very different
evolutions, luminosity functions, and Eddington ratios: radio-quiet
(RQ)/radiative-mode, and radio-loud/jet-mode AGN. The radio power of RQ AGN
evolves ~ (1+z)^2.5, similarly to star-forming galaxies, while the number
density of radio-loud ones has a peak at ~ 0.5 and then declines at higher
redshifts. The number density of radio-selected RQ AGN is consistent with that
of X-ray selected AGN, which shows that we are sampling the same population.
The unbiased fraction of radiative-mode RL AGN, derived from our own and
previously published data, is a strong function of radio power, decreasing from
~ 0.5 at P_1.4GHz ~ 10^24 W/Hz to ~ 0.04$ at P_1.4GHz ~ 10^22 W/Hz. Thanks to
our enlarged sample, which now includes ~ 700 radio sources, we also confirm
and strengthen our previous results on the source population of the faint radio
sky: star-forming galaxies start to dominate the radio sky only below ~ 0.1
mJy, which is also where radio-quiet AGN overtake radio-loud ones.Comment: 19 pages, 13 figures, accepted for publication in MNRA
A Chandra Survey of the X-ray Properties of Broad Absorption Line Radio-Loud Quasars
This work presents the results of a Chandra study of 21 broad absorption line
(BAL) radio-loud quasars (RLQs). We conducted a Chandra snapshot survey of 12
bright BAL RLQs selected from SDSS/FIRST data and possessing a wide range of
radio and CIV absorption properties. Optical spectra were obtained nearly
contemporaneously with the Hobby-Eberly Telescope; no strong flux or BAL
variability was seen between epochs. We also include in our sample 9 additional
BAL RLQs possessing archival Chandra coverage. We compare the properties of
(predominantly high-ionization) BAL RLQs to those of non-BAL RLQs as well as to
BAL radio-quiet quasars (RQQs) and non-BAL RQQs for context.
All 12 snapshot and 8/9 archival BAL RLQs are detected, with observed X-ray
luminosities less than those of non-BAL RLQs having comparable optical/UV
luminosities by typical factors of 4.1-8.5. (BAL RLQs are also X-ray weak by
typical factors of 2.0-4.5 relative to non-BAL RLQs having both comparable
optical/UV and radio luminosities.) However, BAL RLQs are not as X-ray weak
relative to non-BAL RLQs as are BAL RQQs relative to non-BAL RQQs. While some
BAL RLQs have harder X-ray spectra than typical non-BAL RLQs, some have
hardness ratios consistent with those of non-BAL RLQs, and there does not
appear to be a correlation between X-ray weakness and spectral hardness, in
contrast to the situation for BAL RQQs. RLQs are expected to have X-ray
continuum contributions from both disk-corona and small-scale jet emission.
While the entire X-ray continuum in BAL RLQs cannot be obscured to the same
degree as in BAL RQQs, we calculate that the jet is likely partially covered in
many BAL RLQs. We comment briefly on implications for geometries and source
ages in BAL RLQs.Comment: 48 pages, 5 tables, 14 figures, accepted by Ap
The micro-Jy Radio Source Population: the VLA-CDFS View
We analyse the 267 radio sources from our deep (flux limit of 42 microJy at
the field center at 1.4 GHz) Chandra Deep Field South 1.4 and 5 GHz VLA survey.
The radio population is studied by using a wealth of multi-wavelength
information, including morphology and spectral types, in the radio, optical,
and X-ray bands. The availability of redshifts for ~ 70% of our sources allows
us to derive reliable luminosity estimates for the majority of the objects.
Contrary to some previous results, we find that star-forming galaxies make up
only a minority (~ 1/3) of sub-mJy sources, the bulk of which are faint radio
galaxies, mostly of the Fanaroff-Riley I type.Comment: 6 pages, 3 figures, to appear in the proceedings of "At the Edge of
the Universe", Sintra, Portugal, Oct. 9 - 13, 200
The sub-mJy radio sky in the Extended Chandra Deep Field South: source population
The sub-mJy radio population is a mixture of active systems, that is star
forming galaxies (SFGs) and active galactic nuclei (AGNs). We study a sample of
883 radio sources detected at 1.4 GHz in a deep Very Large Array survey of the
Extended Chandra Deep Field South (E-CDFS) that reaches a best rms sensitivity
of 6 microJy. We have used a simple scheme to disentangle SFGs, radio-quiet
(RQ), and radio-loud (RL) AGNs based on the combination of radio data with
Chandra X-ray data and mid-infrared observations from Spitzer. We find that at
flux densities between about 30 and 100 microJy the radio population is
dominated by SFGs (~60%) and that RQ AGNs become increasingly important over RL
ones below 100 microJy. We also compare the host galaxy properties of the three
classes in terms of morphology, optical colours and stellar masses. Our results
show that both SFG and RQ AGN host galaxies have blue colours and late type
morphology while RL AGNs tend to be hosted in massive red galaxies with early
type morphology. This supports the hypothesis that radio emission in SFGs and
RQ AGNs mainly comes from the same physical process: star formation in the host
galaxy.Comment: 13 pages, 11 figures, 1 table, accepted for publication in MNRA
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