47 research outputs found
Proper Motions Of VLBI Lenses, Inertial Frames and The Evolution of Peculiar Velocities
Precise determinations of the image positions in quad gravitational lenses
using VLBI can be used to measure the transverse velocity of the lens galaxy
and the observer. The typical proper motions are as yr, so the time
scale to measure the motion is ten years. By measuring the dipole of the proper
motions in an ensemble of lenses we can set limits on the deviation of the
inertial frame defined by the lenses from that defined by the CMB dipole and
estimate the Hubble constant. The residual proper motions after subtracting the
dipole probe the evolution of peculiar velocities with redshift and can be used
to estimate the density parameter . For lenses, VLBI measurement
accuracies of , and a baseline of years, we estimate that
the 2 limit on the rms peculiar velocity of the lens galaxies is 3100
(\sigma_\theta/10\mu\{as})({yrs}/T)/N^{1/2} \kms, and that the time required
for the 2-- limit to reach the level of the local rms peculiar velocity
is approximately 10 N^{-1/2}
(v_{0,rms}/600\kms)(\sigma_\theta/10\mu as) years. For a ten year baseline and
lenses we expect the 1 limit on the misalignment with the CMB
dipole to be or equivalently to obtain an upper
limit of .Comment: 23 pages, figures included uuencoded gzipped ps-file, submitted to
the ApJ. One correction made from the original versio
Wiener Reconstruction of Large-Scale Structure from Peculiar Velocities
We present an alternative, Bayesian method for large-scale reconstruction
from observed peculiar velocity data. The method stresses a rigorous treatment
of the random errors and it allows extrapolation into poorly sampled regions in
real space or in k-space. A likelihood analysis is used to determine the
fluctuation power spectrum, followed by a Wiener Filter (WF) analysis to obtain
the minimum-variance mean fields of velocity and mass density. Constrained
Realizations (CR) are then used to sample the statistical scatter about the WF
mean field. The WF/CR method is applied as a demonstration to the Mark III data
with 1200 km/s, 900 km/s, and 500 km/s resolutions. The main reconstructed
structures are consistent with those extracted by the POTENT method. A
comparison with the structures in the distribution of IRAS 1.2Jy galaxies
yields a general agreement. The reconstructed velocity field is decomposed into
its divergent and tidal components relative to a cube of +/-8000 km/s centered
on the Local Group. The divergent component is very similar to the velocity
field predicted from the distribution of IRAS galaxies. The tidal component is
dominated by a bulk flow of 194 +/- 32 km/s towards the general direction of
the Shapley concentration, and it also indicates a significant quadrupole.Comment: 28 pages and 8 GIF figures, Latex (aasms4.sty), submitted to ApJ.
Postscript version of the figures can be obtained by anonymous ftp from:
ftp://alf.huji.ac.il/pub/saleem
Is the Lambda CDM Model Consistent with Observations of Large-Scale Structure?
The claim that large-scale structure data independently prefers the Lambda
Cold Dark Matter model is a myth. However, an updated compilation of
large-scale structure observations cannot rule out Lambda CDM at 95%
confidence. We explore the possibility of improving the model by adding Hot
Dark Matter but the fit becomes worse; this allows us to set limits on the
neutrino mass.Comment: To appear in Proceedings of "Sources and Detection of Dark
Matter/Energy in the Universe", ed. D. B. Cline. 6 pages, including 2 color
figure
The Velocity Function of Galaxies
We present a galaxy circular velocity function, Psi(log v), derived from
existing luminosity functions and luminosity-velocity relations. Such a
velocity function is desirable for several reasons. First, it enables an
objective comparison of luminosity functions obtained in different bands and
for different galaxy morphologies, with a statistical correction for dust
extinction. In addition, the velocity function simplifies comparison of
observations with predictions from high-resolution cosmological N-body
simulations.
We derive velocity functions from five different data sets and find rough
agreement among them, but about a factor of 2 variation in amplitude. These
velocity functions are then compared with N-body simulations of a LCDM model
(corrected for baryonic infall) in order to demonstrate both the utility and
current limitations of this approach. The number density of dark matter halos
and the slope of the velocity function near v_*, the circular velocity
corresponding to an ~L_* spiral galaxy, are found to be comparable to that of
observed galaxies. The primary sources of uncertainty in construction of
Psi(log v) from observations and N-body simulations are discussed and
explanations are suggected to account for these discrepancies.Comment: Latex. 28 pages, 4 figures. Accepted by Ap
Discovery of Highly Obscured Galaxies in the Zone of Avoidance
We report the discovery of twenty-five previously unknown galaxies in the
Zone of Avoidance. Our systematic search for extended extra-galactic sources in
the GLIMPSE and MIPSGAL mid-infrared surveys of the Galactic plane has revealed
two overdensities of these sources, located around l ~ 47 and 55 degrees and
|b| less than 1 degree in the Sagitta-Aquila region. These overdensities are
consistent with the local large-scale structure found at similar Galactic
longitude and extending from |b| ~ 4 to 40 degrees. We show that the infrared
spectral energy distribution of these sources is indeed consistent with those
of normal galaxies. Photometric estimates of their redshift indicate that the
majority of these galaxies are found in the redshift range z = 0.01 - 0.05,
with one source located at z = 0.07. Comparison with known sources in the local
Universe reveals that these galaxies are located at similar overdensities in
redshift space. These new galaxies are the first evidence of a bridge linking
the large-scale structure between both sides of the Galactic plane at very low
Galactic latitude and clearly demonstrate the feasibility of detecting galaxies
in the Zone of Avoidance using mid-to-far infrared surveys.Comment: Accepted for publication in the Astronomical Journal, 28 pages, 5
tables, 11 figure