IMPLICATIONS OF NONTHERMAL MOTIONS FOR STATISTICS OF LYMAN-ALPHA FOREST CLOUDS

We investigate the effects of nonthermal bulk motions within Ly alpha forest clouds on some observationally derived statistics of these clou ds. We calculate the absorption-line profiles and curves of growth for three illustrative cases of single-peaked, non-Maxwellian velocity di stributions. We then calculate the distributions in H I column density f(N) and velocity dispersion f(sigma) for Ly alpha forest clouds, usi ng the corresponding distributions derived under the Maxwellian assump tion, while also maintaining consistency with measurable line properti es (Ly alpha central intensity and Ly alpha equivalent width, or Ly al pha and Ly beta equivalent widths). We fund that, for systems with sat urated Ly alpha lines, in the case of velocity distributions with shor ter tails than the Maxwellian, f(N) is steeper at log N greater than o r similar to 15.2 than f(N) in the Maxwellian case. On the other hand, in the case of velocity distributions with longer tails than the Maxw ellian,f(N) is shallower at log N greater than or similar to 15.9 than f(N) in the Maxwellian case. The effect found here is not expected to be relevant for log N greater than or similar to 17.2, where f(N) is d erived from quantities independent of the velocity distribution. The f (sigma) also depends on the velocity distribution, but less so than f( N). Finally, using some published high-resolution observations, we fin d that, even for samples that show no N-sigma correlation in the case of the Maxwellian velocity distribution, a significant positive correl ation exists in the cases of the velocity distributions with shorter t ails than the Maxwellian, while a significant negative correlation exi sts in the cases of the velocity distributions with longer tails than the Maxwellian. Our results imply that various properties derived from f(N), such as the effective photoelectric opacity of the Ly alpha for est clouds to the ionizing background at high redshifts, could differ from previous estimates if the velocity distribution within Ly alpha f orest clouds were non-Maxwellian.