Transition From Population III to Population II Stars

The transition from Population III to Population II stars is determined by the presence of a sufficient amount of metals, in particular, oxygen and carbon. The vastly different yields of these relevant metals between different initial stellar mass functions would then cause such a transition to occur at different times. We show that the transition from Pop III to Pop II stars is likely to occur before the universe can be reionized, if the IMF is entirely very massive stars (M > 140 M_sun). A factor of about 10 more ionizing photons would be produced in the case with normal top-heavy IMF (e.g., M ~ 10-100 M_sun), when such a transition occurs. Thus, a high Thomson optical depth (tau_e >= 0.11-0.14) may be indication that the Population III stars possess a more conventional top-heavy IMF.Comment: 4 pages, 2 figures. Minor revisions, accepted by ApJ Letter

Probing the Statistics of the Temperature-Density Relation of the IGM

Gravitational instability induces a simple correlation between the large and small scale fluctuations of the Ly-alpha flux spectrum. However, non-gravitational processes involved in structure formation and evolution will alter such a correlation. In this paper we explore how scatter in the temperature-density relation of the IGM reduces the gravitationally induced scale-scale correlation. By examining whether or not observations of the correlation are close to that predicted by pure gravity, this puts constraints on the scatter in the temperature-density relation and in turn on any physical process which would lead to scatter, e.g. strong fluctuations in the UV background or radiative transfer effects. By applying this method to high resolution Keck spectra of Q 1422+231 and HS 1946+7658, we find the predicted correlation signal induced by gravity, and the diminishing of this correlation signal at small scales. This suggests extra physics affects the small-scale structure of the forest, and we can constrain the scatter in the temperature-density relation to a conservative 20% upper limit. A crude model suggests, if there is any spatial correlation of temperature, the coherence length scale must be smaller than ~ 0.3/h Mpc to be consistent with the Keck data.Comment: 4 pages, 2 figures. Minor revisions, accepted by ApJ Letter