Article

A Substantial Population of Red Galaxies at z > 2: Modeling of the Spectral Energy Distributions of an Extended Sample

The Astrophysical Journal (Impact Factor: 6.73). 12/2008; 616(1):40. DOI:10.1086/424838
Source: OAI

ABSTRACT We investigate the nature of the substantial population of high-redshift galaxies with Js - Ks ≥ 2.3 colors recently discovered as part of our Faint Infrared Extragalactic Survey (FIRES). This color cut efficiently isolates galaxies at z > 2 with red rest-frame optical colors ("distant red galaxies" [DRGs]). We select Js - Ks ≥ 2.3 objects in both FIRES fields, the ≈25 × 25 Hubble Deep Field-South (HDF-S) and the ≈5' × 5' field around the MS 1054-03 cluster at z = 0.83; the surface densities at Ks,Vega < 21 mag are 1.6 ± 0.6 and 1.0 ± 0.2 arcmin-2, respectively. We here discuss a subsample of 34 DRGs at 2 ≤ z ≤ 3.5: 11 at Ks,Vega < 22.5 mag in HDF-S and 23 at Ks,Vega < 21.7 mag in the MS 1054-03 field. This sample enables for the first time a robust assessment of the population properties of DRGs. We analyze the λ = 0.3-2.2 μm spectral energy distributions (SEDs) constructed from our very deep near-infrared (NIR) and optical imaging collected at the ESO Very Large Telescope and from the Hubble Space Telescope. We develop diagnostics involving the I814 - Js, Js - H, and H - Ks colors to argue that the red NIR colors of our DRG sample cannot be attributed solely to interstellar dust extinction and require for many the presence of an evolved stellar population with a prominent Balmer/4000 Å break. In the rest frame, the optical colors of DRGs fall within the envelope of normal nearby galaxies and the ultraviolet colors suggest a wide range in star formation activity and/or extinction. This is in stark contrast with the much bluer and more uniform SEDs of Lyman break galaxies (LBGs). From evolutionary synthesis modeling assuming constant star formation (CSF), we derive for the DRGs old ages, large extinctions, and high stellar masses, mass-to-light ratios, and star formation rates (SFRs). For solar metallicity, a Salpeter initial mass function (IMF) between 0.1 and 100 M☉, and the Calzetti et al. extinction law, the median values for the HDF-S (MS 1054-03 field) sample are 1.7 (2.0) Gyr, AV = 2.7 (2.4) mag, M* = 0.8 (1.6) × 1011 M☉, M*/LV,* = 1.2 (2.3) M☉ L, and SFR = 120 (170) M☉ yr-1. Models assuming exponentially declining SFRs with e-folding timescales in the range from 10 Myr to 1 Gyr generally imply younger ages, lower extinction, and lower SFRs, but similar stellar masses within a factor of 2. Compared to LBGs at similar redshifts and rest-frame V-band luminosities, DRGs are older, more massive, and more obscured for any given star formation history. For the entire sample of Ks-band selected galaxies in the FIRES fields at 2 ≤ z ≤ 3.5 and to the above magnitude limits, we find that the derived ages, extinctions, and stellar masses increase with redder Js - Ks colors. Although the rest-frame optical colors of the DRGs are similar to those of local normal galaxies, the derived properties are quite different; detailed studies of this new z > 2 population may significantly enhance our understanding of how massive galaxies assembled their stellar mass.

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Article: Lyman Break Galaxies at z~1 and the evolution of the dust attenuation in star-forming galaxies with the redshift
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ABSTRACT: Ultraviolet (UV) galaxies have been selected from GALEX. The presence of a FUV-dropout in their spectral energy distributions proved to be a very complete (83.3%) but not very efficient (21.4%) tool for identifying Lyman Break Galaxies (LBGs) at z~1. We divide the LBG sample into two sub-classes: red LBGs (RLBGs) detected at 24 micron which are mainly Luminous IR Galaxies (LIRGs) and blue LBGs (BLBGs) undetected at 24 microns down to 83 microJy. Two of the RLBGs are also detected at 70 micron. The median SED of the RLBGs is similar (above lambda~1 micron) to the dusty starburst HR10. However, unlike local (U)LIRGs, RLBGs are UV bright objects. We suggest that these objects contain a large amount of dust but that some bare stellar populations are also directly visible. The median SED of the BLBGs is consistent with their containing the same stellar population as the RLBGs but with a lower dust content. The luminosity function of our LBG sample at z~1 is similar to the luminosity function of NUV-selected galaxies at the same redshift. The integrated luminosity densities of z~1 LBGs and NUV-selected galaxies are very consistent. We show that star formation rates (SFRs) estimated from UV measurements and corrected using the IRX-beta method provide average total SFR_TOT in agreement with SFR_UV + SFR_dust. However, IRX-beta-based SFR_TOT shows a large dispersion. Summing up the detected UV (1150A rest-frame) and IR-based star formation rates of the detected objects, we find that only one third of the total (i.e. UV + dust) LBG SFR resides in BLBGs and two thirds in RLBGs, even though most LBGs at z~1 are BLBGs. On the other hand, the total SFR of LBGs accounts for only 11% of the total SFR at z~1. Finally, we observe a regular decrease of L_TIR / L_FUV from z=0 to z~2 for UV-selected samples. Comment: Table 1 and Table 3 are not included in the paper (too large) and could be obtained by sending an email to: denis.burgarella(at)oamp.fr
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Article: On the Stellar Masses of IRAC detected Lyman Break Galaxies at z~3
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ABSTRACT: We present results of a large survey of the mid--IR properties of 248 Lyman Break Galaxies with confirmed spectroscopic redshift using deep Spitzer/IRAC observations in six cosmological fields. We model the Spectral Energy Distributions (SEDs) employing a revised version of the Bruzual and Charlot synthesis population code that incorporates a new treatment of the TP--AGB phase (CB07). Our primary aim is to investigate the impact of the AGB phase in the stellar masses of the LBGs, and compare our new results with previous stellar mass estimates. Based on the new CB07 code we find that the stellar masses of LBGs are smaller on average by a factor of ~1.4 compared to previous estimates. LBGs with 8um and/or 24um detections show higher masses (M~10^11 Mo) than LBGs faint in the IRAC bands (M~10^9 Mo). The ages of these massive LBGs are considerably higher than the rest of the population, indicating that they have been star-forming for at least ~1 Gyr. We also show how the addition of the IRAC bands, improves the accuracy of the estimated stellar masses and reduced the scatter on the derived M/L ratios. In particular, we present a tight correlation between the 8um IRAC band (rest-frame K for galaxies at z~3) and the stellar mass. We calculate the number density of massive (M > 10^11 Mo) LBGs and find it to be $\Phi$= (1.12 $\pm$ 0.4) x 10^(-5) Mpc^(-3), ~1.5 times lower than that found by previous studies. Finally, based on UV-corrected SFRs we investigate the SFR-stellar mass correlation at z~3, find it similar to the one observed at other redshifts and show that our data place the peak of the evolution of the specific star formation rate at z~3. Comment: Accepted for publication in MNRAS
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