arXiv subjects

Yu Zhang

Publications and source records attributed to Yu Zhang.

1,717 records · Page 96Linked to original sources

arXiv1997

Spectral Analysis of the Lyman Alpha Forest in a Cold Dark Matter Cosmology

We simulate the Lya forest in a standard CDM universe using a 2-level hierarchical grid code to evolve the dark and baryonic matter components self-consistently. We solve the time-dependent ionization equations for hydrogen and helium, adopting the Haardt & Madau (1996) estimate for the metagalactic UV radiation background. We compare our simulation results with the measured properties of the Lya forest by constructing synthetic spectra which we analyze in a manner similar to the method used by observers. The HI column density and Doppler parameter distributions we obtain agree closely with those measured by the Keck HIRES and earlier high spectral resolution observations over the column density range 10^{12} < NHI < 10^{16}. In particular, we find a power-law column density distribution persists to the lowest values reported from the HIRES measurements, in agreement with the incompleteness-corrected observations. We find evolution in the cloud number density and opacity comparable to the observed evolution. We also compare with measured values of the intergalactic HeII opacity. Our results require a HeII ionizing background lower than the Haardt & Madau estimate by a factor of 4, corresponding to a soft intrinsic QSO spectrum of alpha_Q = 1.8 - 2.

astro-ph↗

arXiv1997

The Column Density Distribution of the Lyman-Alpha Forest: A Measure of Small Scale Power

Absorption lines in the Lyman-alpha forest can be thought of as peaks in neutral hydrogen density along lines of sight. The column density distribution (the number density of absorption lines as a function of column density) is then a statistic of density peaks, which contains information about the underlying power spectrum. In particular, we show that the slope of the distribution provides a measure of power on scales smaller than those probed by studies of present-day large scale structure.

astro-ph↗

arXiv1996

Primordial Star Forming Regions in a CDM Universe

We developed a three-dimensional 2-level hierarchical cosmological code with a realistic and robust treatment of multi-species non-equilibrium ionization and radiative cooling processes, and use it to investigate primordial star forming regions that originate from high-σperturbations in a standard CDM dominated cosmology. We find it is possible to produce gravitationally bound and cooled structures at very high redshift (z ~ 40) with baryonic masses as small as ~1000Ms. The molecular hydrogen formation in these small scale structures follows very well the analytical predictions of Abel (1995) and Tegmark et al. (1996). We also discuss the minimum mass that cosmological structures must have in order to be able to cool and collapse.

astro-ph↗

arXiv1996

Cosmological Hydrodynamics with Multi-Species Chemistry and Nonequilibrium Ionization and Cooling

We have developed a method of solving for multi-species chemical reaction flows in non--equilibrium and self--consistently with the hydrodynamic equations in an expanding FLRW universe. The method is based on a backward differencing scheme for the required stability when solving stiff sets of equations and is designed to be efficient for three-dimensional calculations without sacrificing accuracy. In all, 28 kinetic reactions are solved including both collisional and radiative processes for the following nine separate species: H, H+, He, He+, He++, H-, H2+, H2, and e-. The method identifies those reactions (involving H- and H2+) ocurring on the shortest time scales, decoupling them from the rest of the network and imposing equilibrium concentrations to good accuracy over typical cosmological dynamical times. Several tests of our code are presented, including radiative shock waves, cosmological sheets, conservation constraints, and fully three-dimensional simulations of CDM cosmological evolutions in which we compare our method to results obtained when the packaged routine LSODAR is substituted for our algorithms.

astro-ph↗

arXiv1996

Probing Lyman-alpha Absorbers in Cosmological Simulations with Double Lines of Sight

We perform a double line of sight (DLOS) analysis of the Lyman-alpha forest structures that form and evolve in cosmological N-body/hydrodynamic simulations. Pairs of simulated spectra, extracted from lines of sight separated by distances from D=12.5kpc up to 800kpc, and a ``control sample'' of unrelated lines of sight, are analyzed at redshifts 3, 2, and 1. Coincident line samples are defined for HI column density thresholds of Nco = 10^{12.5}, 10^{13}, and 10^{14} per square cm. We find that: 1) Under the assumption of a single structure size, a Bayesian analysis yields sizes that are larger for smaller Nco, and at fixed Nco the size decreases with decreasing redshift. However, these derived sizes are found to increase with increasing D indicating that the assumption of a single structure size is invalid. 2) The column densities of coincident pairs are highly correlated for small D, with increasing scatter as D is increased, consistent with structures that have a centrally peaked N(HI) that decreases gradually with radius. 3) The velocity difference distribution for coincident lines is very narrow for small D, and widens as D is increased to meet the expectation for chance coincidences in unrelated lines of sight. This behavior is indicative of organized motion within the structures. 4) For small D, the distribution of anticoincident line column densities, Nac, falls steeply as Nac increases from the cutoff value, but has a significant tail at large values which is inconsistent with a population of spherical absorbers with sharp edges, and consistent with a flattened geometry. The conclusions reached on the basis of the DLOS analysis are validated by an examination of the three-dimensional structures and velocity flows in the simulation data.

astro-ph↗

arXiv1996

Cosmological Filaments and Minivoids: The Origin of Intergalactic Absorption

Soon after the first QSO was identified, Gunn & Peterson searched for the expected characteristic absorption trough on the blueward side of Lya in the spectrum of the QSO due to an Intergalactic Medium (IGM). They failed to find it, placing a constraint on the density of neutral hydrogen in the IGM that was less than 1 part in 10^5 of that residing in galaxies, and concluded that the IGM must be highly ionized. Soon afterwards absorption by Lya was detected in the IGM; not by a diffuse component, but by a clumpy component of intergalactic gas clouds, the Lya forest. The goal then became to search for `excess' absorption beyond that expected from the forest. No clear absorption by a diffuse HI component, however, was ever detected. The results of recent numerical hydrodynamics computations of the formation of the Lya forest appear to indicate that the division between a clumpy component and a diffuse one may be inappropriate. We find that in a CDM-dominated cosmology, no substantial diffuse medium should be expected. Instead, the medium condenses into a network of complex structures that reveal themselves as discrete absorption systems in the spectra of QSOs. The lowest column density lines arise from the fine structure in minivoids -- small regions with densities below the cosmic mean. These results suggest that the long sought for diffuse HI Gunn-Peterson effect may not exist.

astro-ph↗

arXiv1995

A Multi-Species Model for Hydrogen and Helium Absorbers in Lyman-Alpha Forest Clouds

We have performed a multi-species hydrodynamical simulation of the formation and evolution of Lyman alpha clouds in a flat CDM dominated universe with an external flux of ionizing radiation. We solve the fully coupled non-equilibrium rate equations for the following species: H, H^+, H^-, H_2, H_2^+, He, He^+, He^{++}, and e^-. The statistical properties of the distribution and evolution of both hydrogen and helium absorption lines are extracted and compared to observed data. We find excellent agreement for the following neutral hydrogen data: the distribution of column densities is fit well by a power law with exponent beta=1.55 with a possible deficiency of lines above column density 10^{15} cm^{-2}; the integrated distribution matches observed data over a broad range of column densities 10^{13} to 10^{17} cm^{-2}; a Gaussian statistical fit to the Doppler parameter distribution yields a median of 35.6 km s^{-1}; the evolution of the number of clouds with column densities larger than 10^{14} cm^{-2} follows a power law with exponent gamma=2.22. Analogous calculations are presented for HeII absorption lines and we find the ratio of Doppler parameters b_{HeII}/b_{HI} = 0.87. Our data also suggests that Ly$α$ clouds may belong to two morphologically different groups: small column density clouds which tend to reside in sheets or filamental structures and are very elongated and/or flattened, and the large column density clouds which are typically found at the intersections of these caustic structures and are quasi-spherical.

astro-ph↗