1,825 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
The Parkes quarter-Jansky flat-spectrum sample 3. Space density and evolution of QSOs
We analyze the Parkes quarter-Jansky flat-spectrum sample of QSOs in terms of
space density, including the redshift distribution, the radio luminosity
function, and the evidence for a redshift cutoff. With regard to the luminosity
function, we note the strong evolution in space density from the present day to
epochs corresponding to redshifts ~ 1. We draw attention to a selection effect
due to spread in spectral shape that may have misled other investigators to
consider the apparent similarities in shape of luminosity functions in
different redshift shells as evidence for luminosity evolution. To examine the
evolution at redshifts beyond 3, we develop a model-independent method based on
the V_max test using each object to predict expectation densities beyond z=3.
With this we show that a diminution in space density at z > 3 is present at a
significance level >4 sigma. We identify a severe bias in such determinations
from using flux-density measurements at epochs significantly later than that of
the finding survey. The form of the diminution is estimated, and is shown to be
very similar to that found for QSOs selected in X-ray and optical wavebands.
The diminution is also compared with the current estimates of star-formation
evolution, with less conclusive results. In summary we suggest that the
reionization epoch is little influenced by powerful flat-spectrum QSOs, and
that dust obscuration does not play a major role in our view of the QSO
population selected at radio, optical or X-ray wavelengths.Comment: 18 pages, 11 figures, accepted 18 Dec 2004, Astron. & Astrophys. The
accepted version is expanded to include an analysis of the form of the
decline in radio-QSO space density at high redshifts. This is compared with
the forms of epoch dependence derived for optically-selected QSOs, for
X-ray-selected QSOs, and for star formation rat
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
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 VLA 1.4GHz Survey of the Extended Chandra Deep Field South: Second Data Release
Deep radio observations at 1.4GHz for the Extended Chandra Deep Field South
were performed in June through September of 2007 and presented in a first data
release (Miller et al. 2008). The survey was made using six separate pointings
of the Very Large Array (VLA) with over 40 hours of observation per pointing.
In the current paper, we improve on the data reduction to produce a second data
release (DR2) mosaic image. This DR2 image covers an area of about a third of a
square degree and reaches a best rms sensitivity of 6 uJy and has a typical
sensitivity of 7.4 uJy per 2.8" by 1.6" beam. We also present a more
comprehensive catalog, including sources down to peak flux densities of five or
more times the local rms noise along with information on source sizes and
relevant pointing data. We discuss in some detail the consideration of whether
sources are resolved under the complication of a radio image created as a
mosaic of separate pointings each suffering some degree of bandwidth smearing,
and the accurate evaluation of the flux densities of such sources. Finally, the
radio morphologies and optical/near-IR counterpart identifications (Bonzini et
al. 2012) are used to identify 17 likely multiple-component sources and arrive
at a catalog of 883 radio sources, which is roughly double the number of
sources contained in the first data release.Comment: to appear in the Astrophysical Journal Supplement Series; 41 page
Metabolic capabilities of microorganisms involved in and associated with the anaerobic oxidation of methane
In marine sediments the anaerobic oxidation of methane with sulfate as electron acceptor (AOM) is responsible for the removal of a major part of the greenhouse gas methane. AOM is performed by consortia of anaerobic methane-oxidizing archaea (ANME) and their specific partner bacteria. The physiology of these organisms is poorly understood, which is due to their slow growth with doubling times in the order of months and the phylogenetic diversity in natural and in vitro AOM enrichments. Here we study sediment-free long-term AOM enrichments that were cultivated from seep sediments sampled off the Italian Island Elba (20◦C; hereon called E20) and from hot vents of the Guaymas Basin, Gulf of California, cultivated at 37◦C (G37) or at 50◦C (G50). These enrichments were dominated by consortia of ANME-2 archaea and Seep-SRB2 partner bacteria (E20) or by ANME-1, forming consortia with Seep-SRB2 bacteria (G37) or with bacteria of the HotSeep-1 cluster (G50). We investigate lipid membrane compositions as possible factors for the different temperature affinities of the different ANME clades and show autotrophy as characteristic feature for both ANME clades and their partner bacteria. Although in the absence of additional substrates methane formation was not observed, methanogenesis from methylated substrates (methanol and methylamine) could be quickly stimulated in the E20 and the G37 enrichment. Responsible for methanogenesis are archaea from the genus Methanohalophilus and Methanococcoides, which are minor community members during AOM (1–7h of archaeal 16S rRNA gene amplicons). In the same two cultures also sulfur disproportionation could be quickly stimulated by addition of zero-valent colloidal sulfur. The isolated partner bacteria are likewise minor community members (1–9h of bacterial 16S rRNA gene amplicons), whereas the dominant partner bacteria (Seep-SRB1a, Seep-SRB2, or HotSeep-1) did not grow on elemental sulfur. Our results support a functioning of AOM as syntrophic interaction of obligate methanotrophic archaea that transfer non-molecular reducing equivalents (i.e., via direct interspecies electron transfer) to obligate sulfate-reducing partner bacteria. Additional katabolic processes in these enrichments but also in sulfate methane interfaces are likely performed by minor community members
HCO+ and HCN J=3-2 absorption toward the center of Centaurus A
We have investigated the presence of dense gas toward the radio source Cen A
by looking at the absorption of the HCO+ and HCN (3-2) lines in front of the
bright continuum source with the Submillimeter Array. We detect narrow HCO+
(3-2) absorption, and tentatively HCN (3-2), close to the systemic velocity.
For both molecules, the J=3-2 absorption is much weaker than for the J=1-0
line. From simple excitation analysis, we conclude that the gas density is on
the order of a few 10^4 cm^-3 for a column density N(HCO+)/dV of 3x10^12 cm^-2
km^-1 s and a kinetic temperature of 10 K. In particular, we find no evidence
for molecular gas density higher than a few 10^4 cm^-3 on the line of sight to
the continuum source. We discuss the implications of our finding on the nature
of the molecular gas responsible for the absorption toward Cen A.Comment: Accepted for publication in Ap
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