178 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
Do Spirals and Ellipticals Trace the Same Velocity Field?
We test the hypothesis that the velocity field derived from Tully-Fisher
measurements of spiral galaxies, and that derived independently from Dn-sigma
measurements of ellipticals and S0s, are noisy versions of the same underlying
velocity field. The radial velocity fields are derived using tensor Gaussian
smoothing of radius 1200 km/s. They are compared at grid points near which the
sampling by both types of galaxies is proper. This requirement defines a volume
of ~(50 Mpc/h)^3, containing ~10 independent subvolumes, mostly limited by the
available ellipticals. The two fields are compared using a correlation
statistic, whose distribution is determined via Monte-Carlo simulations. We
find that the data is consistent with the hypothesis, at the 10% level. We
demonstrate that the failure to reject the correlation is not just a result of
the errors being big, by using the same method to rule out complete
independence between the fields at the 99.8% level. The zero points of the two
distance indicators are matched by maximizing the correlation between the two
velocity fields. There is a marginal hint that the ellipticals tend to stream
slower than the spirals by ~8%. The correlation reinforced here is consistent
with the common working hypotheses that (a) the derived large-scale velocity
field is real, (b) it has a gravitational origin, and (c) the large-scale
velocities of spirals and ellipticals are hardly biased relative to each other.
On the other hand, it does not rule out any alternative to gravity where
objects of all types obtain similar large-scale velocities.Comment: 16 pages, compressed and uuencoded PostScript 0.6Mbyte, (Also
anonymous ftp venus.huji.ac.il pub/dekel/es/es.ps.Z of 0.43Mbyte
Gravitational lensing of type Ia supernovae by galaxy clusters
We propose a method to remove the mass sheet degeneracy that arises when the
mass of galaxy clusters is inferred from gravitational shear. The method
utilizes high-redshift standard candles that undergo weak lensing. Natural
candidates for such standard candles are type Ia supernovae (SN Ia). When
corrected with the light-curve shape (LCS), the peak magnitude of SN Ia
provides a standard candle with an uncertainty in apparent magnitude of . Gravitational magnification of a background SN Ia by an
intervening cluster would cause a mismatch between the observed SN Ia peak
magnitude compared to that expected from its LCS and redshift. The average
detection rate for SN Ia with a significant mismatch of behind a
cluster at is about supernovae per cluster per year at
. Since SNe are point-like sources for a limited period,
they can experience significant microlensing by MACHOs in the intracluster
medium. Microlensing events caused by MACHOs of are
expected to have time scales similar to that of the SN light curve. Both the
magnification curve by a MACHO and the light curve of a SN Ia have
characteristic shapes that allow to separate them. Microlensing events due to
MACHOs of smaller mass can unambiguously be identified in the SN light curve if
the latter is continuously monitored. The average number of identifiable
microlensing events per nearby cluster () per year is , where is the fraction of the cluster mass in MACHOs of masses
.Comment: Accepted for publication in the MNRA
How to Plant a Merger Tree
We investigate several approaches for constructing Monte Carlo realizations
of the merging history of virialized dark matter halos (``merger trees'') using
the extended Press-Schechter formalism. We describe several unsuccessful
methods in order to illustrate some of the difficult aspects of this problem.
We develop a practical method that leads to the reconstruction of mean
quantities such as the conditional and overall mass functions as given by the
Press-Schechter model. This method is convenient, computationally efficient,
and works for any power spectrum or background cosmology. In addition, we
investigate statistics that describe the distribution of the number of
progenitors and their masses as a function of redshift.Comment: 13 pages, LaTeX, 10 postscript figures. To appear in MNRAS. Changed
to MNRAS format with inlined figures. Minor changes in text and figures to
match published version. No significant changes in conten
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