arXiv subjects

Lam Hui

Publications and source records attributed to Lam Hui.

147 records · Page 9Linked to original sources

arXiv1996

The Low Column Density Lyman-alpha Forest

We develop an analytical method based on the lognormal approximation to compute the column density distribution of the Lyman-alpha forest in the low column density limit. We compute the column density distributions for six different cosmological models and found that the standard, COBE-normalized CDM model cannot fit the observations of the Lyman-alpha forest at z=3. The amplitude of the fluctuations in that model has to be lowered by a factor of almost 3 to match observations. However, the currently viable cosmological models like the lightly tilted COBE-normalized CDM+Lambda model, the CHDM model with 20% neutrinos, and the low-amplitude Standard CDM model are all in agreement with observations, to within the accuracy of our approximation, for the value of the cosmological baryon density at or higher than the old Standard Bing Bang Nucleosynthesis value of 0.0125 for the currently favored value of the ionizing radiation intensity. With the low value for the baryon density inferred by Hogan & Rugers (1996), the models can only marginally match observations.

astro-ph↗

arXiv1995

Standard Cosmology and the BATSE Number vs. Peak Flux Distribution

The observed 2B BATSE distribution is consistent with the faintest GRBs in our sample originating from a redshift of Zmax ~ 0.8-3.0 (90\%), with the most likely values in the range of 1.0-2.2, and is largely insensitive to Omega for models with no evolution. To constrain the model parameter Omega to the range 0.1-1.0 using only Log N -- Log P distributions, more than 4000 GRBs, with a most likely value of ~ 9,000 GRBs to BATSE sensitivity. This requires a live integration time of >6 years with BATSE. Detectors sensitive to much lower limits (~ 70-400 in sensitivity) require ~ 200 GRBs, with <0.03 year 4pi ster coverage. We place limits on the amount of frequency density and, separately, peak luminosity evolution in the sample of GRBs. We find that frequency density evolution models can place the faintest GRBs at Zmax ~ 10-200, without conflicting with the observations of relative time dilation of ~ 2.

astro-ph↗

arXiv1995

Local Approximations to the Gravitational Collapse of Cold Matter

We investigate three different local approximations for nonlinear gravitational instability in the framework of cosmological Lagrangian fluid dynamics of cold dust. They include the Zel'dovich approximation (ZA), the ``non-magnetic'' approximation of Bertschinger \& Jain (1994, NMA), and a new ``local tidal'' approximation (LTA). The LTA is exact for any perturbations whose gravitational and velocity equipotentials have the same constant shape with time, including spherical, cylindrical, and plane-parallel perturbations. We tested all three local approximations with the collapse of a homogeneous triaxial ellipsoid, for which an exact solution exists for an ellipsoid embedded in empty space and an excellent approximation is known in the cosmological context. We find that the LTA is significantly more accurate in general than the ZA and the NMA. Like the ZA, but unlike the NMA, the LTA generically leads to pancake collapse. For a randomly chosen mass element in an Einstein-de Sitter universe, assuming a Gaussian random field of initial density fluctuations, the LTA predicts that at least 78\% of initially underdense regions collapse owing to nonlinear effects of shear and tides.

astro-ph↗