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.

0 0
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: We present the Spectroscopic Imaging survey in the near-infrared (near-IR) with SINFONI (SINS) of high-redshift galaxies. With 80 objects observed and 63 detected in at least one rest-frame optical nebular emission line, mainly Hα, SINS represents the largest survey of spatially resolved gas kinematics, morphologies, and physical properties of star-forming galaxies at z ~ 1-3. We describe the selection of the targets, the observations, and the data reduction. We then focus on the "SINS Hα sample," consisting of 62 rest-UV/optically selected sources at 1.3 < z < 2.6 for which we targeted primarily the Hα and [N II] emission lines. Only 30% of this sample had previous near-IR spectroscopic observations. The galaxies were drawn from various imaging surveys with different photometric criteria; as a whole, the SINS Hα sample covers a reasonable representation of massive M 1010 M ☉star-forming galaxies at z 1.5-2.5, with some bias toward bluer systems compared to pure K-selected samples due to the requirement of secure optical redshift. The sample spans 2 orders of magnitude in stellar mass and in absolute and specific star formation rates, with median values 3 × 1010 M ☉, 70 M ☉ yr–1, and 3 Gyr–1. The ionized gas distribution and kinematics are spatially resolved on scales ranging from 1.5 kpc for adaptive optics assisted observations to typically 4-5 kpc for seeing-limited data. The Hα morphologies tend to be irregular and/or clumpy. About one-third of the SINS Hα sample galaxies are rotation-dominated yet turbulent disks, another one-third comprises compact and velocity dispersion-dominated objects, and the remaining galaxies are clear interacting/merging systems; the fraction of rotation-dominated systems increases among the more massive part of the sample. The Hα luminosities and equivalent widths suggest on average roughly twice higher dust attenuation toward the H II regions relative to the bulk of the stars, and comparable current and past-averaged star formation rates.
    The Astrophysical Journal 11/2009; 706(2):1364. · 6.73 Impact Factor
  • Source
    [show abstract] [hide abstract]
    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
    Monthly Notices of the Royal Astronomical Society 09/2009; · 5.52 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: [Abridged] This work concerns very faint (R_lim=28 AB mag; M_(stars, lim) ~ 10^8 Msun), UV-selected sub-L* BX galaxies at z~2.3. Stellar masses, dust content, and dust-corrected SFRs are constrained using broadband SED fitting, giving insights into the nature of these low-mass systems. First, a correlation found between rest-frame UV luminosity and galaxy stellar mass suggests that many sub-L* galaxies at z~2.3 may have approximately constant star formation histories. A nearly-linear relation between stellar mass and star formation rate is also found, hinting that the rate at which a sub-L* BX galaxy forms its stars is directly related to the mass of stars that it has already formed. A possible explanation is that new gas that falls onto the galaxy's host halo along with accreting dark matter is the source of fuel for ongoing star formation. The instantaneous efficiency of star formation is low in this scenario, of order 1%. The low-mass end of the stellar mass function is steeper than expected from extrapolations of shallower surveys, resulting in a SMD at z~2.3 that's ~25% of the present-day value; this value is z~1.5-2x higher than that given by extrapolations of shallower surveys, suggesting that the build-up of stellar mass in the universe has proceeded more rapidly than previously thought. An update to the KDF z~2 UV LF finds a steeper faint-end slope than previously reported, though not as steep as that found by Reddy & Steidel (2009). Finally, sub-L* galaxies at z~2 carry very small amounts of dust compared to their more luminous cousins, so that while only ~20% of 1700A photons escape from a typical M* galaxy, more than half make it out of an M*+3 one. This paucity of dust highlights the fact that sub-L* galaxies are not simple scaled copies of their more luminous cousins. It also suggests that sub-L* are important contributors to keeping the Universe ionized at z~2.
    Monthly Notices of the Royal Astronomical Society 08/2011; · 5.52 Impact Factor

Full-text (2 Sources)

Available from

N. M. Förster Schreiber