28 research outputs found
Deep radio observations of the radio halo of the bullet cluster 1E 0657-55.8
We present deep 1.1-3.1 GHz Australia Telescope Compact Array observations of
the radio halo of the bullet cluster, 1E 0657-55.8. In comparison to existing
images of this radio halo the detection in our images is at higher
significance. The radio halo is as extended as the X-ray emission in the
direction of cluster merger but is significantly less extended than the X-ray
emission in the perpendicular direction. At low significance we detect a faint
second peak in the radio halo close to the X-ray centroid of the smaller
sub-cluster (the bullet) suggesting that, similarly to the X-ray emission, the
radio halo may consist of two components. Finally, we find that the distinctive
shape of the western edge of the radio halo traces out the X-ray detected bow
shock. The radio halo morphology and the lack of strong point-to-point
correlations between radio, X-ray and weak-lensing properties suggests that the
radio halo is still being formed. The colocation of the X-ray shock with a
distinctive radio brightness edge illustrates that the shock is influencing the
structure of the radio halo. These observations support the theory that shocks
and turbulence influence the formation and evolution of radio halo synchrotron
emission.Comment: 15 pages, 16 figures, 3 tables. Accepted by MNRA
Star-Formation in the Ultraluminous Infrared Galaxy F00183-7111
We report the detection of molecular CO(1-0) gas in F00183-7111, one of the
most extreme Ultra-Luminous Infrared Galaxies known, with the Australia
Telescope Compact Array. We measure a redshift of 0.3292 for F00183-7111 from
the CO(1-0) line and estimate the mass of the molecular gas in 00183 to be 1
10 M. We find that F00183-7111 is predominately
powered by the AGN and only 14 per cent of the total luminosity is
contributed by star-formation (SFR 220 M yr). We also
present an optical image of F00183-7111, which shows an extension to the East.
We searched for star-formation in this extension using radio continuum
observations but do not detect any. This suggests that the star-formation is
likely to be predominately nuclear. These observations provide additional
support for a model in which the radio emission from ULIRGs is powered by an
intense burst of star-formation and by a radio-loud AGN embedded in its
nucleus, both triggered by a merger of gas-rich galaxies.Comment: 5 pages, 2 figures, Accepted for publication in MNRAS Letters
Accepted 2014 January 19. Received 2013 December 30; in original form 2013
November 2
A new twist to an old story: HE 0450-2958, and the ULIRG(optically bright QSO) transition hypothesis
We report on interferometric imaging of the CO J=1--0 and J=3--2 line
emission from the controversial QSO/galaxy pair HE 0450--2958. {\it The
detected CO J=1--0 line emission is found associated with the disturbed
companion galaxy not the luminous QSO,} and implies , which is \ga 30% of the dynamical mass in
its CO-luminous region. Fueled by this large gas reservoir this galaxy is the
site of an intense starburst with , placing
it firmly on the upper gas-rich/star-forming end of Ultra Luminous Infrared
Galaxies (ULIRGs, ). This makes HE 0450--2958 the
first case of extreme starburst and powerful QSO activity, intimately linked
(triggered by a strong interaction) but not coincident. The lack of CO emission
towards the QSO itself renews the controversy regarding its host galaxy by
making a gas-rich spiral (the typical host of Narrow Line Seyfert~1 AGNs) less
likely. Finally, given that HE 0450--2958 and similar IR-warm QSOs are
considered typical ULIRG(optically bright QSO) transition candidates, our
results raise the possibility that some may simply be {\it gas-rich/gas-poor
(e.g. spiral/elliptical) galaxy interactions} which ``activate'' an optically
bright unobscured QSO in the gas-poor galaxy, and a starburst in the gas-rich
one. We argue that such interactions may have gone largely unnoticed even in
the local Universe because the combination of tools necessary to disentagle the
progenitors (high resolution and S/N optical {\it and} CO imaging) became
available only recently.Comment: 25 pages, 5 figures, accepted for publication by The Astrophysical
Journa
CO survey of high-z radio galaxies, revisited with ALMA: Jet-cloud Alignments and Synchrotron Brightening by Molecular Gas in the Circumgalactic Environment
Powerful radio sources associated with super-massive black holes are among
the most luminous objects in the Universe, and are frequently recognized both
as cosmological probes and active constituents in the evolution of galaxies. We
present alignments between radio jets and cold molecular gas in the environment
of distant radio galaxies, and show that the brightness of the radio
synchrotron source can be enhanced by its interplay with the molecular gas. Our
work is based on CO J>1 observations with the Atacama Large
Millimeter/submillimeter Array (ALMA) of three radio galaxies with redshifts in
the range 1.4 < z < 2.1, namely MRC 0114-211 (z = 1.41), MRC 0156-252 (z =
2.02), and MRC 2048-272 (z = 2.05). These ALMA observations support previous
work that found molecular gas out to 50 kpc in the circumgalactic environment,
based on a CO(1-0) survey performed with the Australia Telescope Compact Array
(ATCA). The CO emission is found along the radio axes but beyond the main radio
lobes. When compared to a large sample of high-z radio galaxies from the
literature, we find that the presence of this cold molecular medium correlates
with an increased flux-density ratio of the main vs. counter lobe. This suggest
that the radio lobe brightens when encountering cold molecular gas in the
environment. While part of the molecular gas is likely related to the
interstellar medium (ISM) from either the host or a companion galaxy, a
significant fraction of the molecular gas in these systems shows very low
excitation, with r and r values 0.2. This
could be part of the circumgalactic medium (CGM).Comment: Accepted for publication in ApJ (19 pages, 6 figures
ALMA Observations of the Physical and Chemical Conditions in Centaurus A
Centaurus A, with its gas-rich elliptical host galaxy, NGC 5128, is the
nearest radio galaxy at a distance of 3.8 Mpc. Its proximity allows us to study
the interaction between an active galactic nucleus, radio jets, and molecular
gas in great detail. We present ALMA observations of low J transitions of three
CO isotopologues, HCN, HCO, HNC, CN, and CCH toward the inner projected
500 pc of NGC 5128. Our observations resolve physical sizes down to 40 pc. By
observing multiple chemical probes, we determine the physical and chemical
conditions of the nuclear interstellar medium of NGC 5128. This region contains
molecular arms associated with the dust lanes and a circumnuclear disk (CND)
interior to the molecular arms. The CND is approximately 400 pc by 200 pc and
appears to be chemically distinct from the molecular arms. It is dominated by
dense gas tracers while the molecular arms are dominated by CO and its
rare isotopologues. The CND has a higher temperature, elevated CN/HCN and
HCN/HNC intensity ratios, and much weaker CO and CO emission than
the molecular arms. This suggests an influence from the AGN on the CND
molecular gas. There is also absorption against the AGN with a low velocity
complex near the systemic velocity and a high velocity complex shifted by about
60 km s. We find similar chemical properties between the CND in emission
and both the low and high velocity absorption complexes implying that both
likely originate from the CND. If the HV complex does originate in the CND,
then that gas would correspond to gas falling toward the supermassive black
hole
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