Population III Star Formation During and After the Reionization Epoch

Population III star formation during the dark ages shifted from minihalos (~10^6 Msun) cooled via molecular hydrogen to more massive halos (~10^8 Msun) cooled via Ly-alpha as Lyman-Werner backgrounds progressively quenched molecular hydrogen cooling. Eventually, both modes of primordial star formation were suppressed by the chemical enrichment of the IGM. We present a comprehensive model for following the modes of Population III star formation that is based on a combination of analytical calculations and cosmological simulations. We characterize the properties of the transition from metal-free star formation to the first Population II clusters for an average region of the Universe and for the progenitors of the Milky Way. Finally, we highlight the possibility of observing the explosion of Population III stars within Ly-alpha cooled halos at redshift z~6 in future deep all sky surveys such as LSST.Comment: 4 pages, 2 figures; Proceedings of 'The First Stars and Galaxies: Challenges for the Next Decade", Austin, TX, March 8-11, 201

The Brightest of Reionizing Galaxies (BoRG) survey

Until now, investigating the early stages of galaxy formation has been primarily the realm of theoretical modeling and computer simulations, which require many physical ingredients and are challenging to test observationally. However, the latest Hubble Space Telescope observations in the near infrared are shedding new light on the properties of galaxies within the first billion years after the Big Bang, including our recent discovery of the most distant proto-cluster of galaxies at redshift z~8. Here, I compare predictions from models of primordial and metal-enriched star formation during the dark ages with the latest Hubble observations of galaxies during the epoch of reionization. I focus in particular on the luminosity function and on galaxy clustering as measured from our Hubble Space Telescope Brightest of Reionizing Galaxies (BoRG) survey. BoRG has the largest area coverage to find luminous and rare z~8 sources that are among the first galaxies to have formed in the Universe.Comment: 6 pages, 3 figures, to appear in proceedings of First Stars IV meeting (Kyoto, Japan; 2012

Gamma-Ray-Burst Host Galaxy Surveys at Redshift z>4: Probes of Star Formation Rate and Cosmic Reionization

Measuring the star formation rate (SFR) at high redshift is crucial for understanding cosmic reionization and galaxy formation. Two common complementary approaches are Lyman-Break-Galaxy (LBG) surveys for large samples and Gamma-Ray-Burst (GRB) observations for sensitivity to SFR in small galaxies. The z>4 GRB-inferred SFR is higher than the LBG rate, but this difference is difficult to understand, as both methods rely on several modeling assumptions. Using a physically motivated galaxy luminosity function model, with star formation in dark-matter halos with virial temperature Tvir>2e4 K (M_DM>2e8 M_sun), we show that GRB and LBG-derived SFRs are consistent if GRBs extend to faint galaxies (M_AB<-11). To test star formation below the detection limit L_lim~0.05L^*_{z=3} of LBG surveys, we propose to measure the fraction f_det(L>L_lim,z) of GRB hosts with L>L_lim. This fraction quantifies the missing star formation fraction in LBG surveys, constraining the mass-suppression scale for galaxy formation, with weak dependence on modeling assumptions. Because f_det(L>L_lim,z) corresponds to the ratio of star formation rates derived from LBG and GRB surveys, if these estimators are unbiased, measuring f_det(L>L_lim,z) also constrains the redshift evolution of the GRB production rate per unit mass of star formation. Our analysis predicts significant success for GRB host detections at z~5 with f_det(L>L_lim,z)~0.4, but rarer detections at z>6. By analyzing the upper limits on host-galaxy luminosities of six z>5 GRBs from literature data, we infer that galaxies with M_AB>-15 were present at z>5 at 95% confidence, demonstrating the key role played by very faint galaxies during reionization.Comment: Minor revision: ApJL accepted [7 pages, 4 figures

The very first Pop III stars and their relation to bright z~6 quasars

We discuss the link between dark matter halos hosting the first PopIII stars formed at redshift z > 40 and the rare, massive, halos that are generally considered to host bright z~6 quasars. We show that within the typical volume occupied by one bright high-z QSO the remnants of the first several thousands PopIII stars formed do not end up in the most massive halos at z~6, but rather live in a large variety of environments. The black hole seeds planted by these very first PopIII stars can easily grow to M > 10^{9.5} Msun by z=6 assuming Eddington accretion with radiative efficiency epsilon~0.1. Therefore quenching of the accretion is crucial to avoid an overabundance of supermassive black holes. We implement a simple feedback model for the growth of the seeds planted by PopIII stars and obtain a z~6 BH mass function consistent with the observed QSO luminosity function.Comment: 5 pages, to appear in the proceedings of "First Stars III", AIP Conference Series, T. Abel, A. Heger and B. O'Shea ed

Dynamical models and numerical simulations of incomplete violent relaxation

N-body simulations of collisionless collapse have offered important clues to the construction of realistic stellar dynamical models of elliptical galaxies. Such simulations confirm and quantify the qualitative expectation that rapid collapse of a self-gravitating collisionless system, initially cool and significantly far from equilibrium, leads to incomplete relaxation. In this paper we revisit the problem, by comparing the detailed properties of a family of distribution functions derived from statistical mechanics arguments to those of the products of collisionless collapse found in N-body simulations.Comment: 4 pages, 2 figures, to appear in the proceedings of the 39th Rencontres de Moriond, "Exploring the Universe," La Thuile, Italy, March 28 - April 4, 200

Dynamical properties of a family of collisionless models of elliptical galaxies

N-body simulations of collisionless collapse have offered important clues to the construction of realistic stellar dynamical models of elliptical galaxies. Such simulations confirm and quantify the qualitative expectation that rapid collapse of a self-gravitating collisionless system, initially cool and significantly far from equilibrium, leads to incomplete relaxation, that is to a quasi-equilibrium configuration characterized by isotropic, quasi-Maxwellian distribution of stellar orbits in the inner regions and by radially biased anisotropic pressure in the outer parts. In earlier studies, as illustrated in a number of papers several years ago (see Bertin et al. 1993 and references therein), the attention was largely focused on the successful comparison between the models (constructed under the qualitative clues offered by the N-body simulations mentioned above) and the observations. In this paper we revisit the problem of incomplete violent relaxation, by making a direct comparison between the detailed properties of a family of distribution functions and those of the products of collisionless collapse found in N-body simulations.Comment: to appear in "Plasmas in the Laboratory and in the Universe: new insights and new challenges", G. Bertin, D. Farina, R. Pozzoli eds., AIP Conference Proceedings, Vol. XXX, pp. YY