Critical Metallicities for Second-Generation Stars

The first massive stars may influence the formation of second-generation stars, in part by their metal enrichment of the surrounding gas. We investigate the "critical metallicity", defined as the the value, Z_crit, at which primordial gas cools more efficiently by fine-structure lines of O I (63.18 microns, Si II 34.8 microns, Fe II (25.99 and 35.35 microns), and C II (157.74 microns) than by either H I or H2 line emission. We explore the time-dependent thermodynamics and fragmentation of cooling gas at redshifts z = 10-30, seeded by trace heavy elements expelled from early supernovae. Because different modes of nucleosynthesis (alpha-process, Fe-group) produce abundance ratios far from solar values, these early stellar populations are likely to be influenced by O, Si, and Fe cooling. Our models also include radiative coupling of the fine structure lines and H2 to the cosmic microwave background (CMB), which sets a temperature floor (70-80K at z = 25-30) that may increase the Jeans mass. The H2 forms from catalytic effects of electrons left over from the recombination epoch or produced during virialization. These electrons form the H^- ion (H + e -> H- + gamma), which in turn forms H2 through associative detachment (H- + H -> H2 + e). In virialized halos at z = 10-30, the gas densities (n = 1-100 cm^{-3}) are well below the critical densities, n_cr = 10^{5-6} cm^{-3}, at which (O, Si, Fe) fine-structure lines reach LTE populations and produce their most efficient cooling. Thus, Z_crit may initially exceed 0.01 Z_sun at n = 1-100 cm^{-3}, and then drop to 10^{-3.5} Z_sun at n = 10^6 cm^{-3}, where the Jeans mass may be imprinted on the stellar mass function. Primordial clouds of 10^8 M_sun at 0.01 Z_sun and 200K will produce redshifted fine structure lines, with fluxes between 10^{-22} and 10^{-21} Watts/m^2 at z = 4.Comment: From "First Stars III" Conference (6 pages incl 4 figures

HST/COS Observations of the Quasar Q0302-003: Probing the He II Reionization Epoch and QSO Proximity Effects

Q0302-003 ($z=3.2860 \pm 0.0005$) was the first quasar discovered that showed a He II Gunn-Peterson trough, a sign of incomplete helium reionization at $z > 2.9$. We present its HST/Cosmic Origins Spectrograph far-UV medium-resolution spectrum, which resolves many spectral features for the first time, allowing study of the quasar itself, the intergalactic medium, and quasar proximity effects. Q0302-003 has a harder intrinsic extreme-UV spectral index than previously claimed, as determined from both a direct fit to the spectrum (yielding $\alpha_{\nu} = -0.8$) and the helium-to-hydrogen ion ratio in the quasar's line-of-sight proximity zone. Intergalactic absorption along this sightline shows that the helium Gunn-Peterson trough is largely black in the range $2.87 < z < 3.20$, apart from ionization due to local sources, indicating that helium reionization has not completed at these redshifts. However, we tentatively report a detection of nonzero flux in the high-redshift trough when looking at low-density regions, but zero flux in higher-density regions. This constrains the He II fraction to be a few percent, suggesting helium reionization has progressed substantially by $z \sim 3.1$. The Gunn-Peterson trough recovers to a He II Ly$\alpha$ forest at $z < 2.87$. We confirm a transmission feature due to the ionization zone around a $z = 3.05$ quasar just off the sightline, and resolve the feature for the first time. We discover a similar such feature possibly caused by a luminous $z = 3.23$ quasar further from the sightline, which suggests that this quasar has been luminous for >34 Myr.Comment: ApJ accepted version; 20 pages, 16 figure

Cosmic Metal Production and the Contribution of QSO Absorption Systems to the Ionizing Background

The recent discovery by Cowie \etal and Tytler \etal of metals in the \Lya clouds shows that the intergalactic medium (IGM) at high redshift is contaminated by the products of stars, and suggests that ionizing photons from massive star formation may be a significant contributor to the UV background radiation at early epochs. We assess the validity of the stellar photoionization hypothesis. Based on recent computations of metal yields and O-star Lyman continuum (LyC) fluxes, we find that 0.2\% of the rest-mass energy of the metals produced is radiated as LyC. By modeling the transfer of ionizing radiation through the IGM and the rate of chemical enrichment, we demonstrate that the background intensity of photons at 1 Ryd that accompanies the production of metals in the \Lya forest clouds may be significant, approaching 0.5\times 10^{-21}\uvunits at $z\approx 3$ if the LyC escape fraction is \gta 0.25. Together with quasars, massive stars could then, in principle, provide the hydrogen and helium LyC photons required to ionize the universe at high redshifts. We propose that observations of the \HeII Gunn-Peterson effect and of the metal ionization states of the \Lya forest and Lyman-limit absorbers should show the signature of a stellar spectrum. We also note that the stellar photoionization model fails if a large fraction of the UV radiation emitted from stars cannot escape into the IGM, as suggested by the recent {\it Hopkins Ultraviolet Telescope} observations by Leitherer \etal of low-redshift starburst galaxies, or if most of the metals observed at $z\approx 3$ were produced at much earlier epochs.Comment: ApJ, in press; uuencoded, compressed, PS fil