34,164 research outputs found
An Einstein@home search for continuous gravitational waves from Cassiopeia A
We report the results of a directed search for continuous gravitational-wave
emission in a broad frequency range (between 50 and 1000 Hz) from the central
compact object of the supernova remnant Cassiopeia A (Cas A). The data comes
from the sixth science run of LIGO and the search is performed on the volunteer
distributed computing network Einstein@Home. We find no significant signal
candidate, and set the most constraining upper limits to date on the
gravitational-wave emission from Cas A, which beat the indirect age-based upper
limit across the entire search range. At around 170 Hz (the most sensitive
frequency range), we set 90% confidence upper limits on the gravitational wave
amplitude of , roughly twice as constraining
as the upper limits from previous searches on Cas A. The upper limits can also
be expressed as constraints on the ellipticity of Cas A; with a few reasonable
assumptions, we show that at gravitational-wave frequencies greater than
300~Hz, we can exclude an ellipticity of .Comment: 29 pages, 7 figures, 3 table
The host galaxies of luminous radio-quiet quasars
We present the results of a deep K-band imaging study which reveals the host
galaxies around a sample of luminous radio-quiet quasars. The K-band images,
obtained at UKIRT, are of sufficient quality to allow accurate modelling of the
underlying host galaxy. Initially, the basic structure of the hosts is revealed
using a modified Clean deconvolution routine optimised for this analysis. 2 of
the 14 quasars are shown to have host galaxies with violently disturbed
morphologies which cannot be modelled by smooth elliptical profiles. For the
remainder of our sample, 2D models of the host and nuclear component are fitted
to the images using the chi-squared statistic to determine goodness of fit.
Host galaxies are detected around all of the quasars. The reliability of the
modelling is extensively tested, and we find the host luminosity to be well
constrained for 9 quasars. The derived average K-band absolute K-corrected host
galaxy magnitude for these luminous radio-quiet quasars is =-25.15+/-0.04,
slightly more luminous than an L* galaxy. The spread of derived host galaxy
luminosities is small, although the spread of nuclear-to-host ratios is not.
These host luminosities are shown to be comparable to those derived from
samples of quasars of lower total luminosity and we conclude that there is no
correlation between host and nuclear luminosity for these quasars.
Nuclear-to-host ratios break the lower limit previously suggested from studies
of lower nuclear luminosity quasars and Seyfert galaxies. Morphologies are less
certain but, on the scales probed by these images, some hosts appear to be
dominated by spheroids but others appear to have disk-dominated profiles.Comment: 16 pages, 8 figures, revised version to be published in MNRA
Stellar Populations and Star Cluster Formation in Interacting Galaxies with the Advanced Camera for Surveys
Pixel-by-pixel colour-magnitude and colour-colour diagrams - based on a
subset of the Hubble Space Telescope Advanced Camera for Surveys Early Release
Observations - provide a powerful technique to explore and deduce the star and
star cluster formation histories of the Mice and the Tadpole interacting
galaxies. In each interacting system we find some 40 bright young star clusters
(20 <= F606W (mag) <= 25, with a characteristic mass of ~3 x 10^6 Msun), which
are spatially coincident with blue regions of active star formation in their
tidal tails and spiral arms. We estimate that the main events triggering the
formation of these clusters occurred ~(1.5-2.0) x 10^8 yr ago. We show that
star cluster formation is a major mode of star formation in galaxy
interactions, with >= 35% of the active star formation in encounters occurring
in star clusters. This is the first time that young star clusters have been
detected along the tidal tails in interacting galaxies. The tidal tail of the
Tadpole system is dominated by blue star forming regions, which occupy some 60%
of the total area covered by the tail and contribute ~70% of the total flux in
the F475W filter (decreasing to ~40% in F814W). The remaining pixels in the
tail have colours consistent with those of the main disk. The tidally triggered
burst of star formation in the Mice is of similar strength in both interacting
galaxies, but it has affected only relatively small, spatially coherent areas.Comment: 23 pages in preprint form, 6 (encapsulated) postscript figures;
accepted for publication in New Astronomy; ALL figures (even the grey-scale
ones) need to be printed on a colour printer style files included; for
full-resolution paper, see http://www.ast.cam.ac.uk/STELLARPOPS/ACSpaper
CHILES: HI morphology and galaxy environment at z=0.12 and z=0.17
We present a study of 16 HI-detected galaxies found in 178 hours of
observations from Epoch 1 of the COSMOS HI Large Extragalactic Survey (CHILES).
We focus on two redshift ranges between 0.108 <= z <= 0.127 and 0.162 <= z <=
0.183 which are among the worst affected by radio frequency interference (RFI).
While this represents only 10% of the total frequency coverage and 18% of the
total expected time on source compared to what will be the full CHILES survey,
we demonstrate that our data reduction pipeline recovers high quality data even
in regions severely impacted by RFI. We report on our in-depth testing of an
automated spectral line source finder to produce HI total intensity maps which
we present side-by-side with significance maps to evaluate the reliability of
the morphology recovered by the source finder. We recommend that this become a
common place manner of presenting data from upcoming HI surveys of resolved
objects. We use the COSMOS 20k group catalogue, and we extract filamentary
structure using the topological DisPerSE algorithm to evaluate the \hi\
morphology in the context of both local and large-scale environments and we
discuss the shortcomings of both methods. Many of the detections show disturbed
HI morphologies suggesting they have undergone a recent interaction which is
not evident from deep optical imaging alone. Overall, the sample showcases the
broad range of ways in which galaxies interact with their environment. This is
a first look at the population of galaxies and their local and large-scale
environments observed in HI by CHILES at redshifts beyond the z=0.1 Universe.Comment: 23 pages, 12 figures, 1 interactive 3D figure, accepted to MNRA
Abell 1033: birth of a radio phoenix
Extended steep-spectrum radio emission in a galaxy cluster is usually
associated with a recent merger. However, given the complex scenario of galaxy
cluster mergers, many of the discovered sources hardly fit into the strict
boundaries of a precise taxonomy. This is especially true for radio phoenixes
that do not have very well defined observational criteria. Radio phoenixes are
aged radio galaxy lobes whose emission is reactivated by compression or other
mechanisms. Here, we present the detection of a radio phoenix close to the
moment of its formation. The source is located in Abell 1033, a peculiar galaxy
cluster which underwent a recent merger. To support our claim, we present
unpublished Westerbork Synthesis Radio Telescope and Chandra observations
together with archival data from the Very Large Array and the Sloan Digital Sky
Survey. We discover the presence of two sub-clusters displaced along the N-S
direction. The two sub-clusters probably underwent a recent merger which is the
cause of a moderately perturbed X-ray brightness distribution. A steep-spectrum
extended radio source very close to an AGN is proposed to be a newly born radio
phoenix: the AGN lobes have been displaced/compressed by shocks formed during
the merger event. This scenario explains the source location, morphology,
spectral index, and brightness. Finally, we show evidence of a density
discontinuity close to the radio phoenix and discuss the consequences of its
presence.Comment: accepted MNRA
Near Infrared Adaptive Optics Imaging of QSO Host Galaxies
We report near-infrared (primarily H-band) adaptive optics (AO) imaging with
the Gemini-N and Subaru Telescopes, of a representative sample of 32 nearby
(z<0.3) QSOs selected from the Palomar-Green (PG) Bright Quasar Survey (BQS),
in order to investigate the properties of the host galaxies. 2D modeling and
visual inspection of the images shows that ~36% of the hosts are ellipticals,
\~39% contain a prominent disk component, and ~25% are of undetermined type.
30% show obvious signs of disturbance. The mean M_H(host) = -24.82 (2.1L_H*),
with a range -23.5 to -26.5 (~0.63 to 10 L_H*). At <L_H*, all hosts have a
dominant disk component, while at >2 L_H* most are ellipticals. "Disturbed"
hosts are found at all M_H(host), while "strongly disturbed" hosts appear to
favor the more luminous hosts. Hosts with prominent disks have less luminous
QSOs, while the most luminous QSOs are almost exclusively in ellipticals or in
mergers (which presumably shortly will be ellipticals). At z<0.13, where our
sample is complete at B-band, we find no clear correlation between M_B(QSO) and
M_H(host). However, at z>0.15, the more luminous QSOs (M_B<-24.7), and 4/5 of
the radio-loud QSOs, have the most luminous H-band hosts (>7L_H*), most of
which are ellipticals. Finally, we find a strong correlation between the
"infrared-excess", L_IR/L_BB, of QSOs with host type and degree of disturbance.
Disturbed and strongly disturbed hosts and hosts with dominant disks have
L_IR/L_BB twice that of non-disturbed and elliptical hosts, respectively. QSOs
with "disturbed" and "strongly-disturbed" hosts are also found to have
morphologies and mid/far-infrared colors that are similar to what is found for
"warm" ultraluminous infrared galaxies, providing further evidence for a
possible evolutionary connection between both classes of objects.Comment: 80 pages, accepted for publication in ApJ Supp
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