Sizes, Shapes, and Correlations of Lyman Alpha Clouds and Their Evolution in the CDM+Λ+\Lambda Universe

This study analyzes the sizes, shapes and correlations of \lya clouds produced by a hydrodynamic simulation of a spatially flat CDM universe with a non-zero cosmological constant ($\Omega_0=0.4$, $\Lambda_0=0.6$, $\sigma_8 =0.79$), over the redshift range $2\le z \le 4$. The \lya clouds range in size from several kiloparsecs to about a hundred kiloparsecs in proper units, and they range in shape from roundish, high column density regions with \nhi\ge 10^{15} cm^{-2} to low column density sheet-like structures with \nhi \le 10^{13} cm^{-2} at z=3. The most common shape found in the simulation resembles that of a flattened cigar. The physical size of a typical cloud grows with time roughly as $(1+z)^{-3/2}$ while its shape hardly evolves (except for the most dense regions $\rho_{cut}>30$). Our result indicates that any simple model with a population of spheres (or other shapes) of a uniform size is oversimplified; if such a model agrees with observational evidence, it is probably only by coincidence. We also illustrate why the use of double quasar sightlines to set lower limits on cloud sizes is useful only when the perpendicular sightline separation is small ($\Delta r \le 50h^{-1}$ kpc). Finally, we conjecture that high column density \lya clouds (\nhi\ge 10^{15} cm^{-2}) may be the progenitors of the lower redshift faint blue galaxies. This seems plausible because their correlation length, number density (extrapolated to lower redshift) and their masses are in fair agreement with those observed.Comment: ApJ, in press, 34 pages, 21 figures, figs (1a,b,c) can be at http://astro.princeton.edu/~cen/LYASSC/lyassc.htm

A Wide-Field CCD Survey for Centaurs and Kuiper Belt Objects

A modified Baker-Nunn camera was used to conduct a wide-field survey of 1428 square degrees of sky near the ecliptic in search of bright Kuiper Belt objects and Centaurs. This area is an order of magnitude larger than any previously published CCD survey for Centaurs and Kuiper Belt Objects. No new objects brighter than red magnitude m=18.8 and moving at a rate 1"/hr to 20"/hr were discovered, although one previously discovered Centaur 1997 CU26 Chariklo was serendipitously detected. The parameters of the survey were characterized using both visual and automated techniques. From this survey the empirical projected surface density of Centaurs was found to be SigmaCentaur(m<18.8)=7.8(+16.0 -6.6)x10^-4 per square degree and we found a projected surface density 3sigma upper confidence limit for Kuiper Belt objects of SigmaKBO(m< 18.8)<4.1x10^-3 per square degree. We discuss the current state of the cumulative luminosity functions of both Centaurs and Kuiper Belt objects. Through a Monte Carlo simulation we show that the size distribution of Centaurs is consistent with a q=4 differential power law, similar to the size distribution of the parent Kuiper Belt Objects. The Centaur population is of order 10^7 (radius > 1 km) assuming a geometric albedo of 0.04. About 100 Centaurs are larger than 50 km in radius, of which only 4 are presently known. The current total mass of the Centaurs is 10^-4 Earth Masses. No dust clouds were detected resulting from Kuiper Belt object collisions, placing a 3sigma upper limit <600 collisionally produced clouds of m<18.8 per year.Comment: 13 pages, 5 figures, Accepted for Publication in A

Approximations for modelling CO chemistry in GMCs: a comparison of approaches

We examine several different simplified approaches for modelling the chemistry of CO in three-dimensional numerical simulations of turbulent molecular clouds. We compare the different models both by looking at the behaviour of integrated quantities such as the mean CO fraction or the cloud-averaged CO-to-H2 conversion factor, and also by studying the detailed distribution of CO as a function of gas density and visual extinction. In addition, we examine the extent to which the density and temperature distributions depend on our choice of chemical model. We find that all of the models predict the same density PDF and also agree very well on the form of the temperature PDF for temperatures T > 30 K, although at lower temperatures, some differences become apparent. All of the models also predict the same CO-to-H2 conversion factor, to within a factor of a few. However, when we look more closely at the details of the CO distribution, we find larger differences. The more complex models tend to produce less CO and more atomic carbon than the simpler models, suggesting that the C/CO ratio may be a useful observational tool for determining which model best fits the observational data. Nevertheless, the fact that these chemical differences do not appear to have a strong effect on the density or temperature distributions of the gas suggests that the dynamical behaviour of the molecular clouds on large scales is not particularly sensitive to how accurately the small-scale chemistry is modelled.Comment: 18 pages, 10 figures. Minor revisions, including the addition of a comparison of simulated and observed C/CO ratios. Accepted by MNRA

On the Relationship Between Molecular Hydrogen and Carbon Monoxide Abundances in Molecular Clouds

The most usual tracer of molecular gas is line emission from CO. However, the reliability of that tracer has long been questioned in environments different from the Milky Way. We study the relationship between H2 and CO abundances using a fully dynamical model of magnetized turbulence coupled to a chemical network simplified to follow only the dominant pathways for H2 and CO formation and destruction, and including photodissociation using a six-ray approximation. We find that the abundance of H2 is primarily determined by the amount of time available for its formation, which is proportional to the product of the density and the metallicity, but insensitive to photodissociation. Photodissociation only becomes important at extinctions under a few tenths of a visual magnitude, in agreement with both observational and prior theoretical work. On the other hand, CO forms quickly, within a dynamical time, but its abundance depends primarily on photodissociation, with only a weak secondary dependence on H2 abundance. As a result, there is a sharp cutoff in CO abundance at mean visual extinctions A_V < 3. At lower values of A_V we find that the ratio of H2 column density to CO emissivity X_CO is proportional to A_V^(-3.5). This explains the discrepancy observed in low metallicity systems between cloud masses derived from CO observations and other techniques such as infrared emission. Our work predicts that CO-bright clouds in low metallicity systems should be systematically larger or denser than Milky Way clouds, or both. Our results further explain the narrow range of observed molecular cloud column densities as a threshold effect, without requiring the assumption of virial equilibrium.Comment: 16 pages, 11 figures. Updated to match version accepted by MNRA

SILCC-ZOOM: The early impact of ionizing radiation on forming molecular clouds

As part of the SILCC-ZOOM project we present our first sub-parsec resolution radiation-hydrodynamic simulations of two molecular clouds self-consistently forming from a turbulent, multi-phase ISM. The clouds have similar initial masses of few 10$^4$ M$_{\odot}$, escape velocities of ~5 km s$^{-1}$, and a similar initial energy budget. We follow the formation of star clusters with a sink based model and the impact of radiation from individual massive stars with the tree-based radiation transfer module TreeRay. Photo-ionizing radiation is coupled to a chemical network to follow gas heating, cooling and molecule formation and dissociation. For the first 3 Myr of cloud evolution we find that the overall star formation effciency is considerably reduced by a factor of ~4 to global cloud values of < 10 % as the mass accretion of sinks that host massive stars is terminated after <1 Myr. Despite the low effciency, star formation is triggered across the clouds. Therefore, a much larger region of the cloud is affected by radiation and the clouds begin to disperse. The time scale on which the clouds are dispersed sensitively depends on the cloud substructure and in particular on the amount of gas at high visual extinction. The damage of radiation done to the highly shielded cloud (MC1) is delayed. We also show that the radiation input can sustain the thermal and kinetic energy of the clouds at a constant level. Our results strongly support the importance of ionizing radiation from massive stars for explaining the low observed star formation effciency of molecular cloud