We explore the effects of stellar population models on estimating star formation histories, ages and masses of high redshift galaxies. The focus is on the Thermally-Pulsing Asymptotic Giant Branch (TP-AGB) phase of stellar evolution, whose treatment is a source of major discrepancy among different evolutionary population synthesis. In particular, besides the models usually adopted in the literature, we use models (by Maraston 2005), in which the contribution of the TP-AGB phase is calibrated with local stellar populations and is the dominant source of bolometric and near-IR energy for stellar populations in the age range 0.2 to 2 Gyr. These models also have an underlying different treatment of convective overshooting and Red Giant Branch stars. For our experiment we use a sample of high-z (1.4 ∼ < z ∼ < 2.5) galaxies in the Hubble Ultra Deep Field held to be mostly in passive evolution, with low-resolution UV-spectroscopy and spectroscopic redshifts from GRAPES, and Spitzer IRAC and MIPS photometry from the Great Observatories Origins Deep Survey. We choose these galaxies because their mid-UV spectra exhibit features typical of A-or F-type stars, therefore TP-AGB stars ought to be expected in post-Main Sequence. We find that indeed the TP-AGB phase plays a key role in the interpretation of Spitzer data for high-z galaxies, when the rest-frame near-IR is sampled. When fitting without dust reddening, the models with the empirically-calibrated TP-AGB phase always reproduce better the observed spectral energy distributions (SEDs), in terms of a considerably smaller χ 2 . Allowing for dust reddening improves the fits with literature models in some cases. In both cases, the results from Maraston models imply younger ages by factors up to 6 and lower stellar masses (by ∼ 60% on average). The observed strengths of the Mg UV spectral feature compare better to the predicted ones in the case of the Maraston models, implying a better overall consistency of SED fitting. Finally, we find that photometric redshifts improve significantly using these models on the SEDs extending over the IRAC bands. These results are primarily the consequence of the treatment of the TP-AGB phase in the Maraston models, which produces models with redder rest-frame optical to near-IR colors. This work provides the first direct evidence of TP-AGB stars in the primeval Universe.
High-redshift galaxies selected on the basis of strong Ly emission tend to be young and have small physical sizes. We show this by analyzing the spectral energy distribution of nine Ly -emitting galaxies (LAEs) at 4.0 < z < 5.7 in the Hubble Ultra Deep Field. Rest-frame UV-to-optical (700 8 < k < 7500 8) luminosities, or upper limits, are used to constrain old stellar populations. We derive best-fit, as well as maximally massive and maximally old, properties of all nine objects. We show that these faint and distant objects are all very young, most likely only a few million years old, and not massive, the mass in stars being %10 6 Y10 8 M . Deep Spitzer Infrared Array Camera observations of these objects, even in cases where the object was not detected, proved crucial in constraining the masses of these objects. The space density of these objects, %1.25 ; 10 À4 Mpc À3 , is comparable to previously reported space densities of LAEs at moderate-to-high redshifts. These Ly galaxies show modest star formation rates of %8 M yr À1 , which is nevertheless strong enough to have allowed them to assemble their stellar mass in less than a few million years. These sources appear to have small physical sizes, usually smaller than 1 kpc, and are also rather concentrated. They are likely to be some of the least massive and youngest high-redshift galaxies observed to date.
Deep ACS slitless grism observations and identification of stellar sources are presented within the Great Observatories Origins Deep Survey (GOODS) North and South fields which were obtained in the Probing Evolution And Reionization Spectroscopically (PEARS) program. It is demonstrated that even low resolution spectra can be a very powerful means to identify stars in the field, especially low mass stars with stellar types M0 and later. The PEARS fields lay within the larger GOODS fields, and we used new, deeper images to further refine the selection of stars in the PEARS field, down to a magnitude of z 850 = 25 using a newly developed stellarity parameter. The total number of stars with reliable spectroscopic and morphological identification was 95 and 108 in the north and south fields respectively. The sample of spectroscopically identified stars allows constraints to be set on the thickness of the Galactic thin disk as well as contributions from a thick disk and a halo component. We derive a thin disk scale height, as traced by the population of M4 to M9 dwarfs along two independent lines of sight, of h thin = 370 +60 −65 pc. When including the more massive M0 to M4 -2dwarf population, we derive h thin = 300 ± 70pc. In both cases, we observe that we must include a combination of thick and halo components in our models in order to account for the observed numbers of faint dwarfs. The required thick disk scale height is typically h thick = 1000pc and the acceptable relative stellar densities of the thin disk to thick disk and the thin disk to halo components are in the range of 0.00025 < f halo < 0.0005 and 0.05 < f thick < 0.08 and are somewhat dependent on whether the more massive M0 to M4 dwarfs are included in our sample.Subject headings:
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