Stellar Masses of High-Redshift Galaxies

We present constraints on the stellar-mass distribution of distant galaxies. These stellar-mass estimates derive from fitting population-synthesis models to the galaxies' observed multi-band spectrophotometry. We discuss the complex uncertainties (both statistical and systematic) that are inherent to this method, and offer future prospects to improve the constraints. Typical uncertainties for galaxies at z ~ 2.5 are ~ 0.3 dex (statistical), and factors of ~ 3 (systematic). By applying this method to a catalog of NICMOS-selected galaxies in the Hubble Deep Field North, we generally find a lack of high-redshift galaxies (z > 2) with masses comparable to those of present-day ``L*'' galaxies. At z < 1.8, galaxies with L*-sized masses do emerge, but with a number density below that at the present epoch. Thus, it seems massive, present-day galaxies were not fully assembled by z ~ 2.5, and that further star formation and/or merging are required to assemble them from these high-redshift progenitors. Future progress on this subject will greatly benefit from upcoming surveys such as those planned with HST/ACS and SIRTF.Comment: 7 pages, 5 figures. To appear in The Mass of Galaxies at Low and High Redshift, eds. R. Bender & A. Renzini (ESO Astrophysics Symposia, Springer-Verlag), Venice, 24-26 Oct 200

Lyman Break Galaxies in the NGST Era

With SIRTF and NGST in the offing, it is interesting to examine what the stellar populations of z~3 galaxies models imply for the existence and nature of Lyman-break galaxies at higher redshift. To this end, we ``turn back the clock'' on the stellar population models that have been fit to optical and infrared data of Lyman-break galaxies at z~3. The generally young ages (typically 10^8 +- 0.5 yr) of these galaxies imply that their stars were not present much beyond z=4. For smooth star-formation histories SFR(t) and Salpeter IMFs, the ionizing radiation from early star-formation in these galaxies would be insufficient to reionize the intergalactic medium at z~6, and the luminosity density at z~4 would be significantly lower than observed. We examine possible ways to increase the global star-formation rate at higher redshift without violating the stellar-population constraints at z~3.Comment: To appear in "The Mass of Galaxies at Low and High Redshift", ed. R. Bender and A. Renzini, ESO Astrophysics Symposia, Springer-Verlag 7 Pages, 2 figure

The Star Formation History and Stellar Assembly of High Redshift Galaxies

I discuss current observational constraints on the star-formation and stellar-assembly histories of galaxies at high redshifts. The data on massive galaxies at z2, and that their morphological configuration was in place soon thereafter. Spitzer Space Telescope 24 micron observations indicate that a substantial fraction of massive galaxies at z ~ 1.5-3 have high IR luminosities, suggesting they are rapidly forming stars, accreting material onto supermassive black holes, or both. I compare how observations of these IR-active phases in the histories of massive galaxies constrain current galaxy-formation models.Comment: 4 pages, Invited Review Talk for IAU Symposium 235, Galaxies Across the Hubble Time, J. Palous & F. Combes, eds. Uses iaus.cls, include

Using Cumulative Number Densities to Compare Galaxies across Cosmic Time

Comparing galaxies across redshifts at fixed cumulative number density is a popular way to estimate the evolution of specific galaxy populations. This method ignores scatter in mass accretion histories and galaxy-galaxy mergers, which can lead to errors when comparing galaxies over large redshift ranges (Delta z > 1). We use abundance matching in the LCDM paradigm to estimate the median change in number density with redshift and provide a simple fit (+0.16 dex per unit Delta z) for progenitors of z = 0 galaxies. We find that galaxy descendants do not evolve in the same way as galaxy progenitors, largely due to scatter in mass accretion histories. We also provide estimates for the 1-sigma range of number densities corresponding to galaxy progenitors and descendants. Finally, we discuss some limits on number density comparisons, which arise due to difficulties measuring physical quantities (e.g., stellar mass) consistently across redshifts. A public tool to calculate number density evolution for galaxies, as well as approximate halo masses, is available online.Comment: 5 pages, minor revisions to match ApJL accepted version. Code available at: http://code.google.com/p/nd-redshif

Searching for Star Formation Beyond Reionization

The goal of searching back in cosmic time to find star formation during the epoch of reionization will soon be within reach. We assess the detectability of high-redshift galaxies by combining cosmological hydrodynamic simulations of galaxy formation, stellar evolution models appropriate for the first generations of stars, and estimates of the efficiency for Lyman alpha to escape from forming galaxies into the intergalactic medium. Our simulated observations show that Lyman alpha emission at z ~ 8 may be observable in the near-infrared with 8-meter class telescopes and present-day technology. Not only is the detection of early star-forming objects vital to understanding the underlying cause of the reionization of the universe, but the timely discovery of a z > 7 star-forming population -- or even an interesting upper limit on the emergent flux from these objects -- will have implications for the design of the next generation of ground- and space-based facilities.Comment: 4 pages, submitted to ApJ Letter

Spectroscopic Confirmation of a z=2.79 Multiply Imaged Luminous Infrared Galaxy Behind the Bullet Cluster

We report spectroscopic confirmation and high-resolution infrared imaging of a z=2.79 triply-imaged galaxy behind the Bullet Cluster. This source, a Spitzer-selected luminous infrared galaxy (LIRG), is confirmed via polycyclic aromatic hydrocarbon (PAH) features using the Spitzer Infrared Spectrograph (IRS) and resolved with HST WFC3 imaging. In this galaxy, which with a stellar mass of M*=4e9 Msun is one of the two least massive ones studied with IRS at z>2, we also detect H_2 S(4) and H_2 S(5) pure rotational lines (at 3.1 sigma and 2.1 sigma) - the first detection of these molecular hydrogen lines in a high-redshift galaxy. From the molecular hydrogen lines we infer an excitation temperature T=377+68-84 K. The detection of these lines indicates that the warm molecular gas mass is 6(+36-4)% of the stellar mass and implies the likely existence of a substantial reservoir of cold molecular gas in the galaxy. Future spectral observations at longer wavelengths with facilities like the Herschel Space Observatory, the Large Millimeter Telescope, and the Atacama Pathfinder EXperiment (APEX) thus hold the promise of precisely determining the total molecular gas mass. Given the redshift, and using refined astrometric positions from the high resolution imaging, we also update the magnification estimate and derived fundamental physical properties of this system. The previously published values for total infrared luminosity, star formation rate, and dust temperature are confirmed modulo the revised magnification; however we find that PAH emission is roughly a factor of five stronger than would be predicted by the relations between the total infrared and PAH luminosity reported for SMGs and starbursts in Pope et al. (2008).Comment: 8 pages, 4 figures, accepted to Ap