Michael R. Santos

Probing reionization with Lyman α emission lines

Lyman a emission from high-redshift galaxies may be a powerful probe of the ionization history of the intergalactic medium (IGM) at z > 6; the observed Lyman a emission line is sensitive to the IGM hydrogen neutral fraction over the range 0.1-1. We present calculations of observed Lyman a emission lines from z > 6 galaxies, illustrating the effect of varying the many free parameters associated with the emitting galaxy, its halo and the IGM around the galaxy. In particular, we use a dynamic model of the IGM that includes the effect of IGM infall toward the emitting galaxy. Galactic winds may play a crucial role in determining observed Lyman a line fluxes. We compare our model predictions with observations of two z = 6.5 galaxies and conclude that, if galactic winds are allowed for, existing observations place no constraint on the neutral fraction of the IGM at z = 6.5. Future infrared observations will constrain the importance of galactic winds; if winds are unimportant for the observed z = 6.5 galaxies, our models suggest that the IGM neutral fraction at z = 6.5 is ≤0.1.

The contribution of the first stars to the cosmic infrared background

We calculate the contribution to the cosmic infrared background from very massive metal-free stars at high redshift. We explore two plausible star formation models and two limiting cases for the reprocessing of the ionizing stellar emission. We find that Population III stars may contribute significantly to the cosmic near-infrared background if the following conditions are met. (i) The first stars were massive, with M 100 M� . (ii) Molecular hydrogen can cool baryons in low-mass haloes. (iii) Population III star formation is ongoing, and not shut off through negative feedback effects. (iv) Virialized haloes form stars at ∼40 per cent efficiency up to the redshift of reionization, z ∼ 7. (v) The escape fraction of the ionizing radiation into the intergalactic medium is small. (vi) Nearly all of the stars end up in massive black holes without contributing to the metal enrichment of the Universe.

The Abundance of Low-Luminosity Lyα Emitters at High Redshift*

We derive the luminosity function of high-redshift Lyα-emitting sources from a deep, blind, spectroscopic survey that utilized strong-lensing magnification by intermediate-redshift clusters of galaxies. We observed carefully selected regions near nine clusters, consistent with magnification factors generally greater than 10 for the redshift range 4.5 L) ∝ L-1 over 1041-1042.5 ergs s-1. When combined with the results of other surveys, limited at higher luminosities, our results suggest evidence for the suppression of star formation in low-mass halos, as predicted in popular models of galaxy formation.

The Varied Fates of z ~ 2 Star-forming Galaxies

Star-forming galaxies constitute the majority of galaxies with stellar masses 1010 h?2 M? at -->z ~ 2. It is thus critical to understand their origins, evolution, and connection to the underlying dark matter distribution. To this end, we identify the dark matter halos (including subhalos) that are likely to contain star-forming galaxies at -->z ~ 2 (z2SFGs) within a large dissipationless cosmological simulation and then use halo merger histories to follow the evolution of z2SFG descendants to -->z ~ 1 and -->z ~ 0. The evolved halos at these epochs are then confronted with an array of observational data in order to uncover the likely descendants of z2SFGs. Although the evolved halos have clustering strengths comparable to red galaxies at -->z ~ 1 and -->z ~ 0, we find that the bulk of z2SFGs do not evolve into red galaxies, at either epoch. This conclusion is based primarily on the fact that the space density of z2SFGs is much higher than that of lower redshift red galaxies, even when accounting for the merging of z2SFG descendants, which decreases the number density of z2SFG descendants by at most a factor of two by -->z ~ 0. Of the ~50% of z2SFGs that survive to -->z ~ 0, ~70% reside at the center of -->z ~ 0 dark matter halos with -->M > 1012 h?1 M?. Halo occupation modeling of -->z ~ 0 galaxies suggests that such halos are occupied by galaxies with -->Mr ? 20.5, implying that these z2SFGs evolve into typical ~L* galaxies today, including our own Galaxy. The remaining ~30% become satellite galaxies by -->z ~ 0, and comparison to halo occupation modeling suggests that they are rather faint, with -->Mr ? 19.5. These conclusions are qualitatively generic in the sense that any halo-mass-selected sample of galaxies at one epoch will evolve into a more complex and heterogeneous sample of galaxies at a later epoch.

Spitzer and Hubble Space Telescope Constraints on the Physical Properties of the z ~ 7 Galaxy Strongly Lensed by A2218* **

We report the detection of a z ~ 7 galaxy strongly lensed by the massive galaxy cluster A2218 (z = 0.175) at 3.6 and 4.5 μm using the Spitzer Space Telescope and at 1.1 μm using the Hubble Space Telescope. The new data indicate a refined photometric redshift in the range of 6.6-6.8 depending on the presence of Lyα emission. The spectral energy distribution is consistent with having a significant Balmer break, suggesting that the galaxy is in the poststarburst stage with an age of at least ~50 Myr and quite possibly a few hundred million years. This suggests the possibility that a mature stellar population is already in place at such a high redshift. Compared with typical Lyman break galaxies at z ~ 3-4, the stellar mass is an order of magnitude smaller (~109 M☉), but the specific star formation rate (star formation rate/Mstar) is similarly large (>10-9 yr-1), indicating equally vigorous star-forming activity.

A Faint Star-forming System Viewed through the Lensing Cluster Abell 2218: First Light at [CLC][ITAL]z[/ITAL][/CLC] ≃ 5.6?

Published in: Astrophys. J. 560 (2001) L119-L122 citations recorded in [Science Citation Index] Abstract: We discuss the physical nature of a remarkably faint pair of Lyman alpha-emitting images discovered close to the giant cD galaxy in the lensing cluster Abell 2218 (z=0.18) during a systematic survey for highly-magnified star-forming galaxies beyond z=5. A well-constrained mass model suggests the pair arises via a gravitationally-lensed source viewed at high magnification. Keck spectroscopy confirms the lensing hypothesis and implies the unlensed source is a very faint (I~30) compact (

Where Can We Really Find the First Stars’ Remnants Today?

A number of recent numerical investigations concluded that the remnants of rare structures formed at very high redshift, such as the very first stars and bright redshift z~6 QSOs, are preferentially located at the center of the most massive galaxy clusters at redshift z=0. In this paper we readdress this question using a combination of cosmological simulations of structure formation and extended Press-Schechter formalism and we show that the typical remnants of Population III stars are instead more likely to be found in a group environment, that is in dark matter halos of mass ~2x10^{13} h^{-1}M_sun. Similarly, the descendants of the brightest z~6 QSOs are expected to be in medium-sized clusters (mass of a few 10^{14} h^{-1}M_sun), rather than in the most massive superclusters (M>10^{15} h^{-1}M_sun) found within the typical 1 Gpc^3 cosmic volume where a bright z~6 QSO lives. The origin of past claims that the most massive clusters preferentially host these remnants is rooted in the numerical method used to initialize their numerical simulations: Only a small region of the cosmological volume of interest was simulated with sufficient resolution to identify low-mass halos at early times, and this region was chosen to host the most massive halo in the cosmological volume at late times. The conclusion that the earliest structures formed in the entire cosmological volume evolve into the most massive halo at late times was thus arrived at by construction. We demonstrate that, to the contrary, the first structures to form in a cosmological region evolve into relatively typical objects at later times. We propose alternative numerical methods for simulating the earliest structures in cosmological volumes.

A faint star-forming system viewed through the lensing cluster abell 2218: first light at Z 5.6?

Published in: Astrophys. J. 560 (2001) L119-L122 citations recorded in [Science Citation Index] Abstract: We discuss the physical nature of a remarkably faint pair of Lyman alpha-emitting images discovered close to the giant cD galaxy in the lensing cluster Abell 2218 (z=0.18) during a systematic survey for highly-magnified star-forming galaxies beyond z=5. A well-constrained mass model suggests the pair arises via a gravitationally-lensed source viewed at high magnification. Keck spectroscopy confirms the lensing hypothesis and implies the unlensed source is a very faint (I~30) compact (

Searches for High Redshift Galaxies Using Gravitational Lensing

We present different methods used to identify high redshift (z > 5) objects in the high-magnification regions of lensing galaxy clusters, taking advantage of very well constrained lensing models. The research procedures are explained and discussed. The detection of emission lines in the optical/NIR spectra, such as Lyman-alpha, allows us to determine the redshift of these sources. Thanks to the lensing magnification, it is possible to identify and to study more distant or intrinsically fainter objects with respect to standard field surveys.