25 research outputs found
The Time Development of a Resonance Line in the Expanding Universe
The time-dependent spectral profile of a resonance line in a homogeneous
expanding medium is studied by numerically solving an improved Fokker-Planck
diffusion equation. The solutions are used to determine the time required to
reach a quasi-static solution near the line center. A simple scaling law for
this relaxation time is derived and is fitted to the numerical results. The
results are applied to the case of Lyman alpha scattering during primordial
recombination of hydrogen. For a wide range of cosmological models it is found
that the relaxation times are smaller than the recombination timescale,
although not by a very large factor. Thus the standard assumption of a
quasi-static solution in cosmological recombination calculations is reasonably
valid, and should not cause substantial errors in the solutions.Comment: 20 pages text and 10 figures, in 30 pages of uuencoded, compressed
postscript. CFA preprint no. 375
--PhotoZ: Photometric Redshifts by Inverting the Tolman Surface Brightness Test
Surface brightness is a fundamental observational parameter of galaxies. We
show, for the first time in detail, how it can be used to obtain photometric
redshifts for galaxies, the -PhotoZ method.
We demonstrate that the Tolman surface brightness relation, , is a powerful tool for determining galaxy redshifts from
photometric data.
We develop a model using and a color percentile (ranking) measure to
demonstrate the -PhotoZ method. We apply our method to a set of galaxies
from the SHELS survey, and demonstrate that the photometric redshift accuracy
achieved using the surface brightness method alone is comparable with the best
color-based methods.
We show that the -PhotoZ method is very effective in determining the
redshift for red galaxies using only two photometric bands. We discuss the
properties of the small, skewed, non-gaussian component of the error
distribution.
We calibrate from the SDSS to redshift, and tabulate the
result, providing a simple, but accurate look up table to estimate the redshift
of distant red galaxies.Comment: Accepted for publication in the Astronomical Journa
Detailed Mass Map of CL0024+1654 from Strong Lensing
We construct a high resolution mass map of the z=0.39 cluster 0024+1654,
based on parametric inversion of the associated gravitational lens. The lens
creates eight well-resolved sub-images of a background galaxy, seen in deep
imaging with HST. Excluding mass concentrations centered on visible galaxies,
more than 98% of the remaining mass is represented by a smooth concentration of
dark matter centered near the brightest cluster galaxies, with a 35 h^{-1} kpc
soft core. The asymmetry in the mass distribution is <3% inside 107 ~h^{-1} kpc
radius. The dark matter distribution we observe in CL0024 is far more smooth,
symmetric, and nonsingular than in typical simulated clusters in either Omega=1
or Omega=0.3 CDM cosmologies. Integrated to 107 h^{-1} kpc radius, the
rest-frame mass to light ratio is M/L_V = 276\pm 40 h (M/L_V)_solar.Comment: 16 pages, 4 figures (3 .jpg, 1 .ps), minor changes to make consistent
with the final ApJL article. To appear in ApJL, May 8 199