9,943 research outputs found
Old high-redshift galaxies and primordial density fluctuation spectra
We have discovered a population of extremely red galaxies at
which have apparent stellar ages of \gs 3 Gyr, based on detailed spectroscopy
in the rest-frame ultraviolet. In order for galaxies to have existed at the
high collapse redshifts indicated by these ages, there must be a minimum level
of power in the density fluctuation spectrum on galaxy scales. This paper
compares the required power with that inferred from other high-redshift
populations. If the collapse redshifts for the old red galaxies are in the
range -- 8, there is general agreement between the various
tracers on the required inhomogeneity on 1-Mpc scales. This level of
small-scale power requires the Lyman-limit galaxies to be approximately
fluctuations, implying a very large bias parameter .
The high collapse redshifts of the red galaxies as deduced from gravitational
collapse provides independent support for the ages estimated from their stellar
populations. Such early-forming galaxies are rare, and their contribution to
the cosmological stellar density is consistent with an extrapolation to higher
redshifts of the star-formation rate measured at ; there is no evidence
for a general era of spheroid formation at extreme redshifts.Comment: 9 Pages MNRAS in press. Uses MNRAS Plain TeX macro
Sub and Super-Luminal Propagation of Intense Pulses in Media with Saturated and Reverse Absorption
We develop models for the propagation of intense pulses in solid state media
which can have either saturated absorption or exhibit reverse absorption . We
show that the experiments of Bigelow {\it et al.}[Phys. Rev. Lett. {\bf 90},
113903 (2003); Science {\bf 301}, 200 (2003).] on subluminal propagation in
Ruby and superluminal propagation in Alexandrite are well explained by
modelling them as three level and four level systems coupled to Maxwell
equations. We present results well beyond the traditional pump-probe approach.Comment: 4 pages, 6 figure
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