SIZES, SHAPES, AND CORRELATIONS OF LYMAN-ALPHA CLOUDS AND THEIR EVOLUTION IN THE LAMBDA-CDM UNIVERSE

This study analyzes the sizes, shapes, and correlations of Ly alpha cl ouds produced by a hydrodynamic simulation of a spatially flat CDM uni verse with a nonzero cosmological constant (Ohm(0) = 0.4, Lambda(0) = 0.6, sigma(8) = 0.79) over the redshift range 2 less than or equal to z less than or equal to 4. The Ly alpha clouds range in size from seve ral kiloparsecs to about a hundred kiloparsecs in proper units, and th ey range in shape from roundish, high column density regions with N-HI greater than or equal to 10(15) cm(-2) to low column density sheet-li ke structures with N-HI less than or equal to 10(13) cm(-2) at z = 3. The most common shape found in the simulation resembles that of a Batt ened cigar. The physical size of a typical cloud grows with time rough ly as (1 + 2)(-3/2), while its shape hardly evolves (except for the mo st dense regions with rho(cut) > 30). Collectively, the clouds form la rge networks of filaments and sheets. Our result indicates that any si mple model with a population of spheres (or other shapes) of a uniform size is oversimplified; if such a model agrees with observational evi dence, it is probably only by coincidence. We also illustrate why the use of pairs of quasar sight lines to set lower limits on cloud sizes is useful only when the perpendicular sight line separation is small ( Delta r less than or equal to 50 h(-1) kpc). Finally, we conjecture th at high column density Ly alpha clouds (N-HI greater than or equal to 10(15) cm(-2)) may be the progenitors of the lower redshift faint blue galaxies, based on consideration of their correlation, number density , and mass.