168 research outputs found
OMEGA AND BIASING FROM OPTICAL GALAXIES VERSUS POTENT MASS
The mass density field in the local universe, recovered by the POTENT method
from peculiar velocities of 3000 galaxies, is compared with the density
field of optically-selected galaxies. Both density fields are smoothed with a
Gaussian filter of radius 12 Mpc. Under the assumptions of
gravitational instability and a linear biasing parameter b\sbo between
optical galaxies and mass, we obtain \beta\sbo \equiv \om^{0.6}/b\sbo = 0.74
\pm 0.13. This result is obtained from a regression of POTENT mass density on
optical density after correcting the mass density field for systematic biases
in the velocity data and POTENT method. The error quoted is just the
formal error estimated from the observed scatter in the density--density
scatterplot; it does not include the uncertainty due to cosmic scatter in the
mean density or in the biasing relation. We do not attempt a formal analysis of
the goodness of fit, but the scatter about the fit is consistent with our
estimates of the uncertainties.Comment: Final revised version (minor typos corrected). 13 pages, gzipped tar
file containing LaTeX and figures. The Postscript file is available at
ftp://dust0.dur.ac.uk/pub/mjh/potopt/potopt.ps.Z or (gzipped) at
ftp://xxx.lanl.gov/astro-ph/ps/9501/9501074.ps.gz or via WWW at
http://xxx.lanl.gov/ps/astro-ph/9501074 or as separate LaTeX text and
encapsulated Postscript figures in a compressed tar'd file at
ftp://dust0.dur.ac.uk/pub/mjh/potopt/latex/potopt.tar.
Shellflow. I. The Convergence of the Velocity Field at 6000 km/s
We present the first results from the Shellflow program, an all-sky
Tully-Fisher (TF) peculiar velocity survey of 276 Sb-Sc galaxies with redshifts
between 4500 and 7000 km/s. Shellflow was designed to minimize systematic
errors between observing runs and between telescopes, thereby removing the
possibility of a spurious bulk flow caused by data inhomogeneity. A fit to the
data yields a bulk flow amplitude V_bulk = 70{+100}{-70} km/s (1 sigma error)
with respect to the Cosmic Microwave Background, i.e., consistent with being at
rest. At the 95% confidence level, the flow amplitude is < 300 km/s. Our
results are insensitive to which Galactic extinction maps we use, and to the
parameterization of the TF relation. The larger bulk motion found in analyses
of the Mark III peculiar velocity catalog are thus likely to be due to
non-uniformities between the subsamples making up Mark III. The absence of bulk
flow is consistent with the study of Giovanelli and collaborators and flow
field predictions from the observed distribution of IRAS galaxies.Comment: Accepted version for publication in ApJ. Includes an epitaph for
Jeffrey Alan Willick (Oct 8, 1959 - Jun 18, 2000
Constraints on Primordial Nongaussiantiy from the High-Redshift Cluster MS1054--03
The implications of the massive, X-ray selected cluster of galaxies
MS1054--03 at are discussed in light of the hypothesis that the
primordial density fluctuations may be nongaussian. We generalize the
Press-Schechter (PS) formalism to the nongaussian case, and calculate the
likelihood that a cluster as massive as MS1054 would appear in the EMSS. The
probability of finding an MS1054-like cluster depends only on \omegam and the
extent of primordial nongaussianity. We quantify the latter by adopting a
specific functional form for the PDF, denoted which tends to
Gaussianity for and show how is related to the more
familiar statistic the probability of fluctuations for a
given PDF relative to a Gaussian. We find that Gaussian initial density
fluctuations are consistent with the data on MS1054 only if \omegam\simlt
0.2. For \omegam\ge 0.25 a significant degree of nongaussianity is required,
unless the mass of MS1054 has been substantially overestimated by X-ray and
weak lensing data. The required amount of nongaussianity is a rapidly
increasing function of \omegam for 0.25 \le \omegam \le 0.45, with (T \simgt 7) at the upper end of this range. For a fiducial
\omegam=0.3, \omegal=0.7 universe, favored by several lines of evidence we
obtain an upper limit corresponding to a This
finding is consistent with the conclusions of Koyama, Soda, & Taruya (1999),
who applied the generalized PS formalism to low (z\simlt 0.1) and
intermediate (z\simlt 0.6) redshift cluster data sets.Comment: 15 pages, 11 figures, submitted to the Astrophysical Journal, uses
emulateapj.st
Cosmological parameter estimation with large scale structure and supernovae data
Most cosmological parameter estimations are based on the same set of
observations and are therefore not independent. Here, we test the consistency
of parameter estimations using a combination of large-scale structure and
supernovae data, without cosmic microwave background (CMB) data. We combine
observations from the IRAS 1.2 Jy and Las Campanas redshift surveys, galaxy
peculiar velocities and measurements of type Ia supernovae to obtain
h=0.57_{-0.14}^{+0.15}, Omega_m=0.28+/-0.05 and sigma_8=0.87_{-0.05}^{+0.04} in
agreement with the constraints from observations of the CMB anisotropies by the
WMAP satellite. We also compare results from different subsets of data in order
to investigate the effect of priors and residual errors in the data. We find
that some parameters are consistently well constrained whereas others are
consistently ill-determined, or even yield poorly consistent results, thereby
illustrating the importance of priors and data contributions.Comment: (1) Astrophysics Group, Cavendish Laboratory, Cambridge Unviersity,
UK (2) Dipartimento di Fisica, Universita di Roma "La Sapienza", Ital
A Cepheid Distance to NGC 4603 in Centaurus
In an attempt to use Cepheid variables to determine the distance to the
Centaurus cluster, we have obtained images of NGC 4603 with the Hubble Space
Telescope on 9 epochs using WFPC2 and the F555W and F814W filters. This galaxy
has been suggested to lie within the ``Cen30'' portion of the cluster and is
the most distant object for which this method has been attempted. Previous
distance estimates for Cen30 have varied significantly and some have presented
disagreements with the peculiar velocity predicted from redshift surveys,
motivating this investigation. Using our observations, we have found 61
candidate Cepheid variable stars; however, a significant fraction of these
candidates are likely to be nonvariable stars whose magnitude measurement
errors happen to fit a Cepheid light curve of significant amplitude for some
choice of period and phase. Through a maximum likelihood technique, we
determine that we have observed 43 +/- 7 real Cepheids and that NGC 4603 has a
distance modulus of 32.61 +0.11/-0.10 (random, 1 sigma) +0.24/-0.25
(systematic, adding in quadrature), corresponding to a distance of 33.3 Mpc.
This is consistent with a number of recent estimates of the distance to NGC
4603 or Cen30 and implies a small peculiar velocity consistent with predictions
from the IRAS 1.2 Jy redshift survey if the galaxy lies in the foreground of
the cluster.Comment: Accepted for publication in the Astrophysical Journal. 17 pages with
17 embedded figures and 3 tables using emulateapj.sty. Additional figures and
images may be obtained from http://astro.berkeley.edu/~marc/n4603
Avoiding selection bias in gravitational wave astronomy
When searching for gravitational waves in the data from ground-based
gravitational wave detectors it is common to use a detection threshold to
reduce the number of background events which are unlikely to be the signals of
interest. However, imposing such a threshold will also discard some real
signals with low amplitude, which can potentially bias any inferences drawn
from the population of detected signals. We show how this selection bias is
naturally avoided by using the full information from the search, considering
both the selected data and our ignorance of the data that are thrown away, and
considering all relevant signal and noise models. This approach produces
unbiased estimates of parameters even in the presence of false alarms and
incomplete data. This can be seen as an extension of previous methods into the
high false rate regime where we are able to show that the quality of parameter
inference can be optimised by lowering thresholds and increasing the false
alarm rate.Comment: 13 pages, 2 figure
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