940 research outputs found
High-Redshift Galaxies in Cold Dark Matter Models
We use hydrodynamic cosmological simulations to predict the star formation
properties of high-redshift galaxies (z=2-6) in five variants of the
inflationary cold dark matter scenario, paying particular attention to z=3, the
redshift of the largest "Lyman-break galaxy" (LBG) samples. Because we link the
star formation timescale to the local gas density, the rate at which a galaxy
forms stars is governed mainly by the rate at which it accretes cooled gas from
the surrounding medium. At z=3, star formation in most of the simulated
galaxies is steady on 200 Myr timescales, and the instantaneous star formation
rate (SFR) is correlated with total stellar mass. However, there is enough
scatter in this correlation that a sample selected above a given SFR threshold
may contain galaxies with a fairly wide range of masses. The redshift history
and global density of star formation in the simulations depend mainly on the
amplitude of mass fluctuations in the underlying cosmological model. The three
models whose mass fluctuation amplitudes agree with recent analyses of the
Lyman-alpha forest also reproduce the observed luminosity function of LBGs
reasonably well, though the dynamic range of the comparison is small and the
theoretical and observational uncertainties are large. The models with higher
and lower amplitudes appear to predict too much and too little star formation,
respectively, though they are not clearly ruled out. The intermediate amplitude
models predict SFR ~ 30-40 Msun/yr for galaxies with a surface density 1 per
arcmin^2 per unit redshift at z=3. They predict much higher surface densities
at lower SFR, and significant numbers of galaxies with SFR > 10 Msun/yr at z >=
5.Comment: Submitted to ApJ. 31 pages including 10 ps figures. Full resolution
version of Fig 2 available at
http://www.astronomy.ohio-state.edu/~dhw/Sph/zgal.fig2.ps.g
A spectroscopically confirmed z=1.327 galaxy-scale deflector magnifying a z~8 Lyman-Break galaxy in the Brightest of Reionizing Galaxies survey
We present a detailed analysis of an individual case of gravitational lensing
of a Lyman-Break galaxy (LBG) in a blank field, identified in Hubble
Space Telescope imaging obtained as part of the Brightest of Reionizing
Galaxies survey. To investigate the close proximity of the bright
() -dropout to a small group of foreground galaxies, we
obtained deep spectroscopy of the dropout and two foreground galaxies using
VLT/X-Shooter. We detect H-, H-, [OIII] and [OII] emission in
the brightest two foreground galaxies (unresolved at the natural seeing of
arcsec), placing the pair at . We can rule out emission lines
contributing all of the observed broadband flux in band at
, allowing us to exclude the candidate as a low redshift
interloper with broadband photometry dominated by strong emission lines. The
foreground galaxy pair lies at the peak of the luminosity, redshift and
separation distributions for deflectors of strongly lensed objects,
and we make a marginal detection of a demagnified secondary image in the
deepest () filter. We show that the configuration can be accurately
modelled by a singular isothermal ellipsoidal deflector and a S\'{e}rsic source
magnified by a factor of . The reconstructed source in the
best-fitting model is consistent with luminosities and morphologies of
LBGs in the literature. The lens model yields a group mass of
and a stellar mass-to-light ratio for the
brightest deflector galaxy of within its effective radius. The foreground galaxies'
redshifts would make this one of the few strong lensing deflectors discovered
at .Comment: Accepted for publication in MNRAS. 16 pages, 11 figures, 3 table
A progressive data compression scheme based upon adaptive transform coding: Mixture block coding of natural images
A method for efficiently coding natural images using a vector-quantized variable-blocksized transform source coder is presented. The method, mixture block coding (MBC), incorporates variable-rate coding by using a mixture of discrete cosine transform (DCT) source coders. Which coders are selected to code any given image region is made through a threshold driven distortion criterion. In this paper, MBC is used in two different applications. The base method is concerned with single-pass low-rate image data compression. The second is a natural extension of the base method which allows for low-rate progressive transmission (PT). Since the base method adapts easily to progressive coding, it offers the aesthetic advantage of progressive coding without incorporating extensive channel overhead. Image compression rates of approximately 0.5 bit/pel are demonstrated for both monochrome and color images
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