54 research outputs found
New Associations of Gamma-Ray Sources from the Fermi Second Source Catalog
We present the results of an all-sky radio survey between 5 and 9 GHz of the
fields surrounding all unassociated gamma-ray objects listed in the Fermi Large
Area Telescope Second Source Catalog (2FGL). The goal of these observations is
to find all new gamma-ray AGN associations with radio sources >10 mJy at 8 GHz.
We observed with the Very Large Array and the Australia Telescope Compact Array
the areas around unassociated sources, providing localizations of weak radio
point sources found in 2FGL fields at arcmin scales. Then we followed-up a
subset of those with the Very Long Baseline and the Long Baseline Arrays to
confirm detections of radio emission on parsec-scales. We quantified
association probabilities based on known statistics of source counts and
assuming a uniform distribution of background sources. In total we found 865
radio sources at arcsec scales as candidates for association and detected 95 of
170 selected for follow-up observations at milliarcsecond resolution. Based on
this we obtained firm associations for 76 previously unknown gamma-ray AGN.
Comparison of these new AGN associations with the predictions from using the
WISE color-color diagram shows that half of the associations are missed. We
found that 129 out of 588 observed gamma-ray sources at arcmin scales not a
single radio continuum source was detected above our sensitivity limit within
the 3-sigma gamma-ray localization. These "empty" fields were found to be
particularly concentrated at low Galactic latitudes. The nature of these
Galactic gamma-ray emitters is not yet determined.Comment: accepted for publication by ApJS, 18 pages, 10 figures, 12 tables;
full electronic versions of tables 2-8 are available as ancillary file
Optical Properties of High-Frequency Radio Sources from the Australia Telescope 20 GHz (AT20G) Survey
Our current understanding of radio-loud AGN comes predominantly from studies
at frequencies of 5 GHz and below. With the recent completion of the Australia
Telescope 20 GHz (AT20G) survey, we can now gain insight into the
high-frequency radio properties of AGN. This paper presents supplementary
information on the AT20G sources in the form of optical counterparts and
redshifts. Optical counterparts were identified using the SuperCOSMOS database
and redshifts were found from either the 6dF Galaxy survey or the literature.
We also report 144 new redshifts. For AT20G sources outside the Galactic plane,
78.5% have optical identifications and 30.9% have redshift information. The
optical identification rate also increases with increasing flux density.
Targets which had optical spectra available were examined to obtain a spectral
classification.
There appear to be two distinct AT20G populations; the high luminosity
quasars that are generally associated with point-source optical counterparts
and exhibit strong emission lines in the optical spectrum, and the lower
luminosity radio galaxies that are generally associated with passive galaxies
in both the optical images and spectroscopic properties. It is suggested that
these different populations can be associated with different accretion modes
(cold-mode or hot-mode). We find that the cold-mode sources have a steeper
spectral index and produce more luminous radio lobes, but generally reside in
smaller host galaxies than their hot-mode counterparts. This can be attributed
to the fact that they are accreting material more efficiently. Lastly, we
compare the AT20G survey with the S-cubed semi-empirical (S3-SEX) models and
conclude that the S3-SEX models need refining to correctly model the compact
cores of AGN. The AT20G survey provides the ideal sample to do this.Comment: Accepted for publication in MNRA
Is the observed high-frequency radio luminosity distribution of QSOs bimodal?
The distribution of QSO radio luminosities has long been debated in the
literature. Some argue that it is a bimodal distribution, implying that there
are two separate QSO populations (normally referred to as 'radio-loud' and
'radio-quiet'), while others claim it forms a more continuous distribution
characteristic of a single population. We use deep observations at 20 GHz to
investigate whether the distribution is bimodal at high radio frequencies.
Carrying out this study at high radio frequencies has an advantage over
previous studies as the radio emission comes predominantly from the core of the
AGN, hence probes the most recent activity. Studies carried out at lower
frequencies are dominated by the large scale lobes where the emission is built
up over longer timescales (10^7-10^8 yrs), thereby confusing the sample. Our
sample comprises 874 X-ray selected QSOs that were observed as part of the 6dF
Galaxy Survey. Of these, 40% were detected down to a 3 sigma detection limit of
0.2-0.5 mJy.
No evidence of bimodality is seen in either the 20 GHz luminosity
distribution or in the distribution of the R_20 parameter: the ratio of the
radio to optical luminosities traditionally used to classify objects as being
either radio-loud or radio-quiet. Previous results have claimed that at low
radio luminosities, star formation processes can dominate the radio emission
observed in QSOs. We attempt to investigate these claims by stacking the
undetected sources at 20 GHz and discuss the limitations in carrying out this
analysis. However, if the radio emission was solely due to star formation
processes, we calculate that this corresponds to star formation rates ranging
from ~10 solar masses/yr to ~2300 solar masses/yr.Comment: 13 pages, 11 figures. Accepted for publication in Ap
Observations of cold extragalactic gas clouds at towards PKS 1610-771
We present results from MUSE observations of a 21-cm HI absorption system
detected with the Australian Square Kilometre Array Pathfinder radio telescope
at redshift towards the quasar PKS 1610-771. We
identify four galaxies (A, B, X and Y) at the same redshift as the 21-cm H I
Damped Lyman-{\alpha} (DLA) absorption system, with impact parameters ranging
from less than 10 kpc to almost 200 kpc from the quasar sightline. Ca II and Na
I absorption is seen in the MUSE spectrum of the background QSO, with
velocities coinciding with the initial HI 21-cm detection, but tracing less
dense and warmer gas. This metal-line component aligns with the rotating
ionised disc of galaxy B (impact parameter 18 kpc from the QSO) and appears to
be co-rotating with the galaxy disc. In contrast, the 21-cm HI absorber is
blueshifted relative to the galaxies nearest the absorber and has the opposite
sign to the velocity field of galaxy B. Since galaxies A and B are separated by
only 17 kpc on the sky and km s in velocity, it appears likely that
the 21-cm detection traces extragalactic clouds of gas formed from their
interaction. This system reveals that the cold 100 K neutral gas critical for
star formation can be associated with complex structures beyond the galaxy
disc, and is a first case study made in preparation for future large 21-cm
absorption surveys like the ASKAP First Large Absorption Survey in HI.Comment: Accepted. 13 pages, 7 figure
A successful search for intervening 21 cm HI absorption in galaxies at 0.4 < z <1.0 with the Australian Square Kilometre Array Pathfinder (ASKAP)
We have used the Australian Square Kilometre Array Pathfinder (ASKAP) radio
telescope to search for intervening 21 cm neutral hydrogen (HI) absorption
along the line of sight to 53 bright radio continuum sources. Our observations
are sensitive to HI column densities typical of Damped Lyman Alpha absorbers
(DLAs) in cool gas with an HI spin temperature below about 300-500 K. The
six-dish Boolardy Engineering Test Array (BETA) and twelve-antenna Early
Science array (ASKAP-12) covered a frequency range corresponding to redshift
and respectively for the HI line. Fifty of the 53
radio sources observed have reliable optical redshifts, giving a total redshift
path = 21.37. This was a spectroscopically-untargeted survey, with
no prior assumptions about the location of the lines in redshift space. Four
intervening HI lines were detected, two of them new. In each case, the
estimated HI column density lies above the DLA limit for HI spin temperatures
above 50-80 K, and we estimate a DLA number density at redshift of
. This value lies somewhat above the
general trend of with redshift seen in optical DLA studies. Although the
current sample is small, it represents an important proof of concept for the
much larger 21cm First Large Absorption Survey in HI (FLASH) project to be
carried out with the full 36-antenna ASKAP telescope, probing a total redshift
path .Comment: 21 pages, 11 figures. Accepted for publication in MNRA
Observations and properties of candidate high frequency GPS radio sources in the AT20G survey
We used the Australia Telescope Compact Array (ATCA) to obtain 40 GHz and 95
GHz observations of a number of sources that were selected from the Australia
Telescope Compact Array 20 GHz (AT20G) survey . The aim of the observations was
to improve the spectral coverage for sources with spectral peaks near 20 GHz or
inverted (rising) radio spectra between 8.6 GHz and 20 GHz. We present the
radio observations of a sample of 21 such sources along with optical spectra
taken from the ANU Siding Spring Observatory 2.3m telescope and the ESO-New
Technology Telescope (NTT). We find that as a group the sources show the same
level of variability as typical GPS sources, and that of the 21 candidate GPS
sources roughly 60% appear to be genuinely young radio galaxies. Three of the
21 sources studied show evidence of being restarted radio galaxies. If these
numbers are indicative of the larger population of AT20G radio sources then as
many as 400 genuine GPS sources could be contained within the AT20G with up to
25% of them being restarted radio galaxies.Comment: 21 pages, 24 figures, Table 1 truncated at 11 column
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