1,442 research outputs found
Composite infrared bolometers with Si_3N_4 micromesh absorbers
We report the design and performance of 300-mK composite bolometers that use micromesh absorbers and support structures patterned from thin films of low-stress silicon nitride. The small geometrical filling factor of the micromesh absorber provides 20Ă reduction in heat capacity and cosmic ray cross section relative to a solid absorber with no loss in IR-absorption efficiency. The support structure is mechanically robust and has a thermal conductance, G < 2 Ă 10^(â11) W/K, which is four times smaller than previously achieved at 300 mK. The temperature rise of the bolometer is measured with a neutron transmutation doped germanium thermistor attached to the absorbing mesh. The dispersion in electrical and thermal parameters of a sample of 12 bolometers optimized for the SunyaevâZelâdovich Infrared Experiment is ±7% in R (T), ±5% in optical efficiency, and ±4% in G
Limits on Arcminute Scale Cosmic Microwave Background Anisotropy with the BIMA Array
We have used the Berkeley-Illinois-Maryland-Association (BIMA) millimeter
array outfitted with sensitive cm-wave receivers to search for Cosmic Microwave
Background (CMB) anisotropies on arcminute scales. The interferometer was
placed in a compact configuration which produces high brightness sensitivity,
while providing discrimination against point sources. Operating at a frequency
of 28.5 GHz, the FWHM primary beam of the instrument is 6.6 arcminutes. We have
made sensitive images of seven fields, five of which where chosen specifically
to have low IR dust contrast and be free of bright radio sources. Additional
observations with the Owens Valley Radio Observatory (OVRO) millimeter array
were used to assist in the location and removal of radio point sources.
Applying a Bayesian analysis to the raw visibility data, we place limits on CMB
anisotropy flat-band power Q_flat = 5.6 (+3.0 -5.6) uK and Q_flat < 14.1 uK at
68% and 95% confidence. The sensitivity of this experiment to flat band power
peaks at a multipole of l = 5470, which corresponds to an angular scale of
approximately 2 arcminutes. The most likely value of Q_flat is similar to the
level of the expected secondary anisotropies.Comment: 15 pages, 5 figures, LaTex, aas2pp4.sty, ApJ submitte
Cosmological Parameter Extraction from the First Season of Observations with DASI
The Degree Angular Scale Interferometer (\dasi) has measured the power
spectrum of the Cosmic Microwave Background anisotropy over the range of
spherical harmonic multipoles 100<l<900. We compare this data, in combination
with the COBE-DMR results, to a seven dimensional grid of adiabatic CDM models.
Adopting the priors h>0.45 and 0.0<=tau_c<=0.4, we find that the total density
of the Universe Omega_tot=1.04+/-0.06, and the spectral index of the initial
scalar fluctuations n_s=1.01+0.08-0.06, in accordance with the predictions of
inflationary theory. In addition we find that the physical density of baryons
Omega_b.h^2=0.022+0.004-0.003, and the physical density of cold dark matter
Omega_cdm.h^2=0.14+/-0.04. This value of Omega_b.h^2 is consistent with that
derived from measurements of the primordial abundance ratios of the light
elements combined with big bang nucleosynthesis theory. Using the result of the
HST Key Project h=0.72+/-0.08 we find that Omega_t=1.00+/-0.04, the matter
density Omega_m=0.40+/-0.15, and the vacuum energy density
Omega_lambda=0.60+/-0.15. (All 68% confidence limits.)Comment: 7 pages, 4 figures, minor changes in response to referee comment
Imaging the Sunyaev-Zel'dovich Effect
We report on results of interferometric imaging of the Sunyaev-Zel'dovich
Effect (SZE) with the OVRO and BIMA mm-arrays. Using low-noise cm-wave
receivers on the arrays, we have obtained high quality images for 27 distant
galaxy clusters. We review the use of the SZE as a cosmological tool. Gas mass
fractions derived from the SZE data are given for 18 of the clusters, as well
as the implied constraint on the matter density of the universe, . We
find . A best guess for the matter
density obtained by assuming a reasonable value for the Hubble constant and
also by attempting to account for the baryons contained in the galaxies as well
as those lost during the cluster formation process gives .
We also give preliminary results for the Hubble constant. Lastly, the power for
investigating the high redshift universe with a non-targeted high sensitivity
SZE survey is discussed and an interferometric survey is proposed.Comment: 14 pages, 7 figures, latex, contribution to Nobel Symposium "Particle
Physics and the Universe" to appear in Physica Scripta and World Scientific,
eds L. Bergstrom, P. Carlson and C. Fransso
A Method for Individual Source Brightness Estimation in Single- and Multi-band Data
We present a method of reliably extracting the flux of individual sources
from sky maps in the presence of noise and a source population in which number
counts are a steeply falling function of flux. The method is an extension of a
standard Bayesian procedure in the millimeter/submillimeter literature. As in
the standard method, the prior applied to source flux measurements is derived
from an estimate of the source counts as a function of flux, dN/dS. The key
feature of the new method is that it enables reliable extraction of properties
of individual sources, which previous methods in the literature do not. We
first present the method for extracting individual source fluxes from data in a
single observing band, then we extend the method to multiple bands, including
prior information about the spectral behavior of the source population(s). The
multi-band estimation technique is particularly relevant for classifying
individual sources into populations according to their spectral behavior. We
find that proper treatment of the correlated prior information between
observing bands is key to avoiding significant biases in estimations of
multi-band fluxes and spectral behavior, biases which lead to significant
numbers of misclassified sources. We test the single- and multi-band versions
of the method using simulated observations with observing parameters similar to
that of the South Pole Telescope data used in Vieira, et al. (2010).Comment: 11 emulateapj pages, 3 figures, revised to match published versio
A Sunyaev-Zel'dovich Effect Survey for High Redshift Clusters
Interferometric observations of the Sunyaev-Zel'dovich Effect (SZE) toward
clusters of galaxies provide sensitive cosmological probes. We present results
from 1 cm observations (at BIMA and OVRO) of a large, intermediate redshift
cluster sample. In addition, we describe a proposed, higher sensitivity array
which will enable us to survey large portions of the sky. Simulated
observations indicate that we will be able to survey one square degree of sky
per month to sufficient depth that we will detect all galaxy clusters more
massive than 2x10^{14} h^{-1}_{50}M_\odot, regardless of their redshift. We
describe the cluster yield and resulting cosmological constraints from such a
survey.Comment: 7 pages, 6 figures, latex, contribution to VLT Opening Symposiu
Millimeter Wavelength Brightness Fluctuations of the Atmosphere Above the South Pole
We report measurements of the millimeter wavelength brightness fluctuations
produced by the atmosphere above the South Pole made with the Arcminute
Cosmology Bolometer Array Receiver (ACBAR). The data span the 2002 Austral
winter during which ACBAR was mounted on the Viper telescope at the South Pole.
We recover the atmospheric signal in the presence of instrument noise by
calculating the correlation between signals from distinct elements of the ACBAR
bolometer array. With this method, it is possible to measure atmospheric
brightness fluctuations with high SNR even under the most stable atmospheric
conditions. The observed atmospheric signal is characterized by the parameters
of the Komolgorov-Taylor (KT) model, which are the amplitude and power law
exponent describing the atmospheric power spectrum, and the two components of
the wind angular velocity at the time of the observation. The KT model is
typically a good description of the observed fluctuations, and fits to the data
produce values of the Komolgorov exponent that are consistent with theoretical
expectations. By combining the wind angular velocity results with measurements
of the wind linear velocity, we find that the altitude of the observed
atmospheric fluctuations is consistent with the distribution of water vapor
determined from radiosonde data. For data corresponding to frequency passbands
centered on 150, 219, and 274 GHz, we obtain median fluctuation power
amplitudes of [10, 38, 74] mK^{2} rad^{-5/3} in Rayleigh-Jeans temperature
units. Comparing with previous work, we find that these median amplitudes are
approximately an order of magnitude smaller than those found at the South Pole
during the Austral summer and at least 30 times lower than found at the ALMA
site in the Atacama desert.Comment: 13 pages, 15 figures, submitted to ApJ, vertical margins fixe
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