850 research outputs found
The very bright SCUBA galaxy count: looking for SCUBA galaxies with the Mexican Hat Wavelet
We present the results of a search for bright high-redshift galaxies in two
large SCUBA scan-maps of Galactic regions. A Mexican Hat Wavelet technique was
used to locate point sources in these maps, which suffer high foreground
contamination as well as typical scan-map noise signatures. A catalogue of
point source objects was selected and observed again in the submillimetre
continuum, and in HCO+ (3->2) at zero redshift to rule out Galactic sources. No
extragalactic sources were found. Simulations show that the survey was
sensitive to sources with fluxes > 50 mJy, depending on the local background.
These simulations result in upper limits on the 850-micron counts of SCUBA
galaxies of 53 per square degree at 50 mJy and 2.9 per square degree at 100
mJy.Comment: Accepted by MNRA
Lensing-Induced Structure of Submillimeter Sources: Implications for the Microwave Background
We consider the effect of lensing by galaxy clusters on the angular
distribution of submillimeter wavelength objects. While lensing does not change
the total flux and number counts of submillimeter sources, it can affect the
number counts and fluxes of flux-limited samples. Therefore imposing a flux cut
on point sources not only reduces the overall Poisson noise, but imprints the
correlations between lensing clusters on the unresolved flux distribution.
Using a simple model, we quantify the lensing anisotropy induced in
flux-limited samples and compare this to Poisson noise. We find that while the
level of induced anisotropies on the scale of the cluster angular correlation
length is comparable to Poisson noise for a slowly evolving cluster model, it
is negligible for more realistic models of cluster evolution. Thus the removal
of point sources is not expected to induce measurable structure in the
microwave or far-infrared backgrounds.Comment: 22 pages, 9 figures, accepted to Astrophysical Journa
Submillimetre and far-infrared spectral energy distributions of galaxies: the luminosity-temperature relation and consequences for photometric redshifts
The spectral energy distributions (SEDs) of dusty high-redshift galaxies are
poorly sampled in frequency and spatially unresolved. Their form is crucially
important for estimating the large luminosities of these galaxies accurately,
for providing circumstantial evidence concerning their power sources, and for
estimating their redshifts in the absence of spectroscopic information. We
discuss the suite of parameters necessary to describe their SEDs adequately
without introducing unnecessary complexity. We compare directly four popular
descriptions, explain the key degeneracies between the parameters in each when
confronted with data, and highlight the differences in their best-fitting
values. Using one representative SED model, we show that fitting to even a
large number of radio, submillimetre and far-infrared (far-IR) continuum
colours provides almost no power to discriminate between the redshift and dust
temperature of an observed galaxy, unless an accurate relationship with a tight
scatter exists between luminosity and temperature for the whole galaxy
population. We review our knowledge of this luminosity-dust temperature
relation derived from three galaxy samples, to better understand the size of
these uncertainties. Contrary to recent claims, we stress that far-IR-based
photometric redshifts are unlikely to be sufficiently accurate to impose useful
constraints on models of galaxy evolution: finding spectroscopic redshifts for
distant dusty galaxies will remain essential.Comment: 14 pages, 14 figures, in press at MNRAS. Replaced with final updated
proof versio
Automated, high accuracy classification of Parkinsonian disorders: a pattern recognition approach
Progressive supranuclear palsy (PSP), multiple system atrophy (MSA) and idiopathic Parkinson’s disease (IPD) can be clinically indistinguishable, especially in the early stages, despite distinct patterns of molecular pathology. Structural neuroimaging holds promise for providing objective biomarkers for discriminating these diseases at the single subject level but all studies to date have reported incomplete separation of disease groups. In this study, we employed multi-class pattern recognition to assess the value of anatomical patterns derived from a widely available structural neuroimaging sequence for automated classification of these disorders. To achieve this, 17 patients with PSP, 14 with IPD and 19 with MSA were scanned using structural MRI along with 19 healthy controls (HCs). An advanced probabilistic pattern recognition approach was employed to evaluate the diagnostic value of several pre-defined anatomical patterns for discriminating the disorders, including: (i) a subcortical motor network; (ii) each of its component regions and (iii) the whole brain. All disease groups could be discriminated simultaneously with high accuracy using the subcortical motor network. The region providing the most accurate predictions overall was the midbrain/brainstem, which discriminated all disease groups from one another and from HCs. The subcortical network also produced more accurate predictions than the whole brain and all of its constituent regions. PSP was accurately predicted from the midbrain/brainstem, cerebellum and all basal ganglia compartments; MSA from the midbrain/brainstem and cerebellum and IPD from the midbrain/brainstem only. This study demonstrates that automated analysis of structural MRI can accurately predict diagnosis in individual patients with Parkinsonian disorders, and identifies distinct patterns of regional atrophy particularly useful for this process
The evolution of submillimetre galaxies: two populations and a redshift cut-off
We explore the epoch dependence of number density and star-formation rate for
submillimetre galaxies (SMGs) found at 850 um. The study uses a sample of 38
SMG in the GOODS-N field, for which cross-waveband identifications have been
obtained for 35/38 members together with redshift measurements or estimates. A
maximum-likelihood analysis is employed, along with the `single-source-survey'
technique. We find a diminution in both space density and star formation rate
at z > 3, closely mimicking the redshift cut-offs found for QSOs selected in
different wavebands. The diminution in redshift is particularly marked, at a
significance level too small to measure. The data further suggest, at a
significance level of about 0.001, that two separately-evolving populations may
be present, with distinct luminosity functions. These results parallel the
different evolutionary behaviours of LIRGs and ULIRGs, and represent another
manifestation of `cosmic down-sizing', suggesting that differential evolution
extends to the most extreme star-forming galaxies.Comment: 12 pages, 11 figures, MNRAS accepted. The new version, as accepted
for MNRAS, is substantially revised, with more detail on sample selection as
well as extended significance tests of the result
SCUBA observations of the host galaxies of four dark gamma-ray bursts
We present the results of a search for submillimetre-luminous host galaxies
of optically dark gamma-ray bursts (GRBs) using the Submillimetre Common-User
Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). We made
photometry measurements of the 850-micron flux at the location of four `dark
bursts', which are those with no detected optical afterglow despite rapid deep
searches, and which may therefore be within galaxies containing substantial
amounts of dust. We were unable to detect any individual source significantly.
Our results are consistent with predictions for the host galaxy population as a
whole, rather than for a subset of dusty hosts. This indicates that optically
dark GRBs are not especially associated with very submillimetre-luminous
galaxies and so cannot be used as reliable indicators of dust-enshrouded
massive star-formation activity. Further observations are required to establish
the relationship between the wider GRB host galaxy population and SCUBA
galaxies.Comment: 6 pages. Accepted for publication in MNRA
SCUBA sub-millimeter observations of gamma-ray bursters IV. GRB 021004, 021211, 030115, 030226, 041006
We discuss our ongoing program of Target of Opportunity (ToO) sub-millimeter
observations of gamma-ray bursts (GRBs) using the Sub-millimetre Common-User
Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). In this
paper, we present the ToO observations of GRBs 021004, 021211, 030115, 030226,
and 041006. The observations of GRBs 021004, 021211, 030226, and 041006 all
started within ~1 day of the burst, but did not detect any significant
sub-millimeter emission from the reverse shock and/or afterglow. These
observations put some constraints on the models for the early emission,
although the generally poor observing conditions and/or the faintness of these
afterglows at other wavelengths limit the inferences that can be drawn from
these lack of detections. However, these observations demonstrate that SCUBA
can perform rapid observations of GRBs, and provide encouragement for future
observations in the Swift era. None of these GRBs had significant
sub-millimeter emission from their host galaxies. This adds to the indication
that GRBs are not closely linked to the most luminous dusty star-forming
galaxies.Comment: 10 pages, 1 color figure (no information lost if printed in black and
white
A Monte Carlo Approach to Evolution of the Far-Infrared Luminosity Function with BLAST
We constrain the evolution of the rest-frame far-infrared (FIR) luminosity
function out to high redshift, by combining several pieces of complementary
information provided by the deep Balloon-borne Large-Aperture Submillimeter
Telescope surveys at 250, 350 and 500 micron, as well as other FIR and
millimetre data. Unlike most other phenomenological models, we characterise the
uncertainties in our fitted parameters using Monte Carlo Markov Chains. We use
a bivariate local luminosity function that depends only on FIR luminosity and
60-to-100 micron colour, along with a single library of galaxy spectral energy
distributions indexed by colour, and apply simple luminosity and density
evolution. We use the surface density of sources, Cosmic Infrared Background
(CIB) measurements and redshift distributions of bright sources, for which
identifications have been made, to constrain this model. The precise evolution
of the FIR luminosity function across this crucial range has eluded studies at
longer wavelengths (e.g., using SCUBA and MAMBO) and at shorter wavelengths
(e.g., Spitzer), and should provide a key piece of information required for the
study of galaxy evolution. Our adoption of Monte Carlo methods enables us not
only to find the best-fit evolution model, but also to explore correlations
between the fitted parameters. Our model-fitting approach allows us to focus on
sources of tension coming from the combination of data-sets. We specifically
find that our choice of parameterisation has difficulty fitting the combination
of CIB measurements and redshift distribution of sources near 1 mm. Existing
and future data sets will be able to dramatically improve the fits, as well as
break strong degeneracies among the models. [abridged]Comment: 20 pages, 14 figures, accepted to MNRA
The VLA-COSMOS Survey. II. Source Catalog of the Large Project
The VLA-COSMOS Large Project is described and its scientific objective is discussed. We present a catalog of ~3600 radio sources found in the 2 deg^2 COSMOS field at 1.4 GHz. The observations in the VLA A and C configuration resulted in a resolution of 1.5" Ă— 1.4" and a mean rms noise of ~10.5 (15) ÎĽJy beam^(-1) in the central 1 (2) deg^2. Eighty radio sources are clearly extended consisting of multiple components, and most of them appear to be double-lobed radio galaxies. The astrometry of the catalog has been thoroughly tested, and the uncertainty in the relative and absolute astrometry are 130 and <55 mas, respectively
On the Association of Gamma-ray Bursts with Massive Stars: Implications for Number Counts and Lensing Statistics
Recent evidence appears to link gamma-ray bursts (GRBs) to star-forming
regions in galaxies at cosmological distances. If short-lived massive stars are
the progenitors of GRBs, the rate of events per unit cosmological volume should
be an unbiased tracer (i.e. unaffected by dust obscuration and surface
brightness limits) of the cosmic history of star formation. Here we use
realistic estimates for the evolution of the stellar birthrate in galaxies to
model the number counts, redshift distribution, and time-delay factors of GRBs.
We present luminosity function fits to the BATSE log N-log P relation for
different redshift distributions of the bursts. Our results imply about 1-2
GRBs every one million Type II supernovae, and a characteristic
`isotropic-equivalent' burst luminosity in the range 3-20 x 1e51 ergs/s (for
H_0=65 km/s/Mpc). We compute the rate of multiple imaging of background GRBs
due to foreground mass condensations in a Lambda-dominated cold dark matter
cosmology, assuming that dark halos approximate singular isothermal spheres on
galaxy scales and Navarro-Frenk-White profiles on group/cluster scales, and are
distributed in mass according to the Press-Schechter model. We show that the
expected sensivity increase of Swift relative to BATSE could result in a few
strongly lensed individual bursts detected down to a photon flux of 0.1
phot/s/cm^2 in a 3-year survey. Because of the partial sky coverage, however,
it is unlikely that the Swift satellite will observe recurrent events (lensed
pairs).Comment: emulateapj, 13 pages, 7 figures, revised version accepted for
publication in the Ap
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