1,712 research outputs found
Obscuration in extremely luminous quasars
The spectral energy distributions and infrared (IR) spectra of a sample of
obscured AGNs selected in the mid-IR are modeled with recent clumpy torus
models to investigate the nature of the sources, the properties of the
obscuring matter, and dependencies on luminosity. The sample contains 21
obscured AGNs at z=1.3-3 discovered in the largest Spitzer surveys (SWIRE,
NDWFS, & FLS) by means of their extremely red IR to optical colors. All sources
show the 9.7micron silicate feature in absorption and have extreme mid-IR
luminosities (L(6micron)~10^46 erg/s). The IR SEDs and spectra of 12 sources
are well reproduced with a simple torus model, while the remaining 9 sources
require foreground extinction from a cold dust component to reproduce both the
depth of the silicate feature and the near-IR emission from hot dust. The
best-fit torus models show a broad range of inclinations, with no preference
for the edge-on torus expected in obscured AGNs. Based on the unobscured QSO
mid-IR luminosity function, and on a color-selected sample of obscured and
unobscured IR sources, we estimate the surface densities of obscured and
unobscured QSOs at L(6micron)>10^12 Lsun, and z=1.3-3.0 to be about 17-22
deg^-2, and 11.7 deg^-2, respectively. Overall we find that ~35-41% of luminous
QSOs are unobscured, 37-40% are obscured by the torus, and 23-25% are obscured
by a cold absorber detached from the torus. These fractions constrain the torus
half opening angle to be ~67 deg. This value is significantly larger than found
for FIR selected samples of AGN at lower luminosity (~46 deg), supporting the
receding torus scenario. A far-IR component is observed in 8 objects. The
estimated far-IR luminosities associated with this component all exceed
3.3x10^12 Lsun, implying SFRs of 600-3000 Msun/yr. (Abridged)Comment: ApJ accepte
VLBI Images of 49 Radio Supernovae in Arp 220
We have used a Very Long Baseline Interferometry (VLBI) array at 18cm
wavelength to image the nucleus of the luminous IR galaxy Arp 220 at ~1 pc
linear resolution, and with very high sensitivity. The resulting map has an rms
of 5.5 microJy/beam, and careful image analysis results in 49 confirmed point
sources ranging in flux density from 1.2 mJy down to ~60 microJy. Comparison
with high sensitivity data from 12 months earlier reveals at least four new
sources. The favored interpretation of these sources is that they are radio
supernovae, and if all new supernovae are detectable at this sensitivity, a
resulting estimate of the supernova rate in the Arp 220 system is 4 +/- 2 per
year. The implied star formation rate is sufficient to power the entire
observed far-infrared luminosity of the galaxy. The two nuclei of Arp 220
exhibit striking similarities in their radio properties, though the western
nucleus is more compact, and appears to be ~3 times more luminous than the
eastern nucleus. There are also some puzzling differences, and differential
free-free absorption, synchrotron aging and expansion losses may all be playing
a role. Comparison with the nearby starburst galaxy M82 supports the hypothesis
that the activity in Arp 220 is essentially a scaled-up version of that in M82.Comment: 24 pages, 3 figures, 1 table. Accepted for publication in Ap.
The Motivation for Active Travel to School Survey (MATSS): Instrument development and initial validity evidence
Abstract presented at Be Active 2014, 15-18 October 2014, Canberra, Australi
The first VLBI image of an Infrared-Faint Radio Source
Context: To investigate the joint evolution of active galactic nuclei and
star formation in the Universe. Aims: In the 1.4 GHz survey with the Australia
Telescope Compact Array of the Chandra Deep Field South and the European Large
Area ISO Survey - S1 we have identified a class of objects which are strong in
the radio but have no detectable infrared and optical counterparts. This class
has been called Infrared-Faint Radio Sources, or IFRS. 53 sources out of 2002
have been classified as IFRS. It is not known what these objects are. Methods:
To address the many possible explanations as to what the nature of these
objects is we have observed four sources with the Australian Long Baseline
Array. Results: We have detected and imaged one of the four sources observed.
Assuming that the source is at a high redshift, we find its properties in
agreement with properties of Compact Steep Spectrum sources. However, due to
the lack of optical and infrared data the constraints are not particularly
strong.Comment: Accepted for publication in Astronomy and Astrophysics, 5 pages,
needs aa.cl
Realisation of a low frequency SKA Precursor: The Murchison Widefield Array
The Murchison Widefield Array is a low frequency (80-300 MHz) SKA Precursor, comprising 128 aperture array elements distributed over an area of 3 km diameter. The MWA is located at the extraordinarily radio quiet Murchison Radioastronomy Observatory in the mid-west of Western Australia, the selected home for the Phase 1 and Phase 2 SKA low frequency arrays. The MWA science goals include: 1) detection of fluctuations in the brightness temperature of the diffuse redshifted 21 cm line of neutral hydrogen from the epoch of reionisation; 2) studies of Galactic and extragalactic processes based on deep, confusion-limited surveys of the full sky visible to the array; 3) time domain astrophysics through exploration of the variable radio sky; and 4) solar imaging and characterisation of the heliosphere and ionosphere via propagation effects on background radio source emission. This paper will focus on a brief discussion of the as-built MWA system, highlighting several novel characteristics of the instrument, and a brief progress report (as of June 2012) on the final construction phase. Practical completion of the MWA is expected in November 2012, with commissioning commencing from approximately August 2012 and operations commencing near mid 2013. A brief description of recent science results from the MWA prototype instrument is given
The radio spectra of the compact sources in Arp 220: A mixed population of supernovae and supernova remnants
We report the first detection at multiple radio wavelengths (13, 6 and 3.6
cm) of the compact sources within both nuclei of the Ultra Luminous Infra-Red
Galaxy Arp 220. We present the radio spectra of the 18 detected sources. In
just over half of the sources we find that these spectra and other properties
are consistent with the standard model of powerful Type IIn supernovae
interacting with their pre-explosion stellar wind. The rate of appearance of
new radio sources identified with these supernova events suggests that an
unusually large fraction of core collapse supernovae in Arp 220 are highly
luminous; possibly implying a radically different stellar initial mass function
or stellar evolution compared to galactic disks. Another possible explanation
invokes very short (~3 x 10^5 year) intense (~10^3 M_Sol year^-1) star
formation episodes with a duty cycle of ~10%. A second group of our detected
sources, consisting of the brightest and longest monitored sources at 18 cm do
not easily fit the radio supernova model. These sources show a range of
spectral indexes from -0.2 to -1.9. We propose that these are young supernova
remnants which have just begun interacting with a surrounding ISM with a
density between 10^4 and 10^5 cm^-3. One of these sources is probably resolved
at 3.6 cm wavelength with a diameter 0.9 pc. In the western nucleus we estimate
that the ionized component of the ISM gives rise to foreground free-free
absorption with opacity at 18 cm of <0.6 along the majority of lines of sight.
Other sources may be affected by absorption with opacity in the range 1 to 2.
These values are consistent with previous models as fitted to the radio
recombination lines and the continuum spectrum.Comment: 44 pages, 9 figures, 2 tables. Accepted for publication in Ap
Clustering of galaxies at 3.6 microns in the Spitzer Wide-area Infrared Extragalactic legacy survey
We investigate the clustering of galaxies selected in the 3.6 micron band of
the Spitzer Wide-area Infrared Extragalactic (SWIRE) legacy survey. The angular
two-point correlation function is calculated for eleven samples with flux
limits of S_3.6 > 4-400 mujy, over an 8 square degree field. The angular
clustering strength is measured at >5-sigma significance at all flux limits,
with amplitudes of A=(0.49-29)\times10^{-3} at one degree, for a power-law
model, A\theta^{-0.8}. We estimate the redshift distributions of the samples
using phenomological models, simulations and photometric redshifts, and so
derive the spatial correlation lengths. We compare our results with the GalICS
(Galaxies In Cosmological Simulations) models of galaxy evolution and with
parameterized models of clustering evolution. The GalICS simulations are
consistent with our angular correlation functions, but fail to match the
spatial clustering inferred from the phenomological models or the photometric
redshifts. We find that the uncertainties in the redshift distributions of our
samples dominate the statistical errors in our estimates of the spatial
clustering. At low redshifts (median z<0.5) the comoving correlation length is
approximately constant, r_0=6.1\pm0.5h^{-1} Mpc, and then decreases with
increasing redshift to a value of 2.9\pm0.3h^{-1} Mpc for the faintest sample,
for which the median redshift is z=1. We suggest that this trend can be
attributed to a decrease in the average galaxy and halo mass in the fainter
flux-limited samples, corresponding to changes in the relative numbers of
early- and late-type galaxies. However, we cannot rule out strong evolution of
the correlation length over 0.5<z<1.Comment: 14 pages, 9 (colour) figures. Published in MNRA
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