4,339 research outputs found
Using SCUBA to place upper limits on arcsecond scale CMB anisotropies at 850 microns
The SCUBA instrument on the James Clerk Maxwell Telescope has already had an
impact on cosmology by detecting relatively large numbers of dusty galaxies at
high redshift. Apart from identifying well-detected sources, such data can also
be mined for information about fainter sources and their correlations, as
revealed through low level fluctuations in SCUBA maps. As a first step in this
direction we analyse a small SCUBA data-set as if it were obtained from a
Cosmic Microwave Background (CMB) differencing experiment. This enables us to
place limits on CMB anisotropy at 850 microns. Expressed as Q_{flat}, the
quadrupole expectation value for a flat power spectrum, the limit is 152
microKelvin at 95 per cent confidence, corresponding to C_0^{1/2} < 355
microKelvin for a Gaussian autocorrelation function, with a coherence angle of
about 20--25 arcsec; These results could easily be reinterpretted in terms of
any other fluctuating sky signal. This is currently the best limit for these
scales at high frequency, and comparable to limits at similar angular scales in
the radio. Even with such a modest data-set, it is possible to put a constraint
on the slope of the SCUBA counts at the faint end, since even randomly
distributed sources would lead to fluctuations. Future analysis of sky
correlations in more extensive data-sets ought to yield detections, and hence
additional information on source counts and clustering.Comment: 12 pages, 9 postscript figures, uses mn.st
Sub-mm counterparts to Lyman-break galaxies
We summarize the main results from our SCUBA survey of Lyman-break galaxies
(LBGs) at z~3. Analysis of our sample of LBGs reveals a mean flux of
S850=0.60.2 mJy, while simple models of emission based on the UV
properties predict a mean flux about twice as large. Known populations of LBGs
are expected to contribute flux to the weak sub-mm source portion of the far-IR
background, but are not likely to comprise the bright source (S850>5 mJy) end
of the SCUBA-detected source count. The detection of the LBG, Westphal-MM8, at
1.9 mJy suggests that deeper observations of individual LBGs in our sample
could uncover detections at similar levels, consistent with our UV-based
predictions. By the same token, many sub-mm selected sources with S850<2 mJy
could be LBGs. The data are also consistent with the FarIR/ relation
holding at z=3.Comment: 6 pages, 1 figure, contributed talk at UMass/INAOE Conference ``Deep
Millimeter Surveys'
An upper limit to [C II] emission in a z ~= 5 galaxy
Low-ionization-state far-infrared (FIR) emission lines may be useful
diagnostics of star-formation activity in young galaxies, and at high redshift
may be detectable from the ground. In practice, however, very little is known
concerning how strong such line emission might be in the early Universe. We
attempted to detect the 158 micron [C II] line from a lensed galaxy at z =
4.926 using the Caltech Submillimeter Observatory. This source is an ordinary
galaxy, in the sense that it shows high but not extreme star formation, but
lensing makes it visible. Our analysis includes a careful consideration of the
calibrations and weighting of the individual scans. We find only modest
improvement over the simpler reduction methods, however, and the final spectrum
remains dominated by systematic baseline ripple effects. We obtain a 95 per
cent confidence upper limit of 33 mJy for a 200 km/s full width at half maximum
line, corresponding to an unlensed luminosity of 1x10^9 L_sun for a standard
cosmology. Combining this with a marginal detection of the continuum emission
using the James Clerk Maxwell Telescope, we derive an upper limit of 0.4 per
cent for the ratio of L_CII/L_FIR in this object.Comment: 5 pages, 2 figures, accepted for publication in MNRA
The bias of the submillimetre galaxy population: SMGs are poor tracers of the most massive structures in the z ~ 2 Universe
It is often claimed that overdensities of (or even individual bright)
submillimetre-selected galaxies (SMGs) trace the assembly of the most-massive
dark matter structures in the Universe. We test this claim by performing a
counts-in-cells analysis of mock SMG catalogues derived from the Bolshoi
cosmological simulation to investigate how well SMG associations trace the
underlying dark matter structure. We find that SMGs exhibit a relatively
complex bias: some regions of high SMG overdensity are underdense in terms of
dark matter mass, and some regions of high dark matter overdensity contain no
SMGs. Because of their rarity, Poisson noise causes scatter in the SMG
overdensity at fixed dark matter overdensity. Consequently, rich associations
of less-luminous, more-abundant galaxies (i.e. Lyman-break galaxy analogues)
trace the highest dark matter overdensities much better than SMGs. Even on
average, SMG associations are relatively poor tracers of the most significant
dark matter overdensities because of 'downsizing': at z < ~2.5, the
most-massive galaxies that reside in the highest dark matter overdensities have
already had their star formation quenched and are thus no longer SMGs. At a
given redshift, of the 10 per cent most-massive overdensities, only ~25 per
cent contain at least one SMG, and less than a few per cent contain more than
one SMG.Comment: 6 pages, 3 figures, 1 table; accepted for publication in MNRAS; minor
revisions from previous version, conclusions unchange
A SCUBA Scanmap of the HDF: Measuring the bright end of the sub-mm source counts
Using the 850 micron SCUBA camera on the JCMT and a scanning technique
different from other sub-mm surveys, we have obtained a 125 square arcminute
map centered on the Hubble Deep Field. The one-sigma sensitivity to point
sources is roughly 3 mJy and thus our map probes the brighter end of the sub-mm
source counts. We find 6 sources with a flux greater than about 12 mJy (>4
sigma) and, after a careful accounting of incompleteness and flux bias,
estimate the integrated density of bright sources N(>12 mJy)= 164 (+77/-58) per
square degree (68 per cent confidence bounds).Comment: 5 pages, 2 figures, Accepted for publication in MNRA
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