Article

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

University of California, Santa Cruz, Santa Cruz, California, United States
The Astrophysical Journal (Impact Factor: 5.99). 12/2008; 616(1):40. DOI: 10.1086/424838
Source: arXiv

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.

Full-text preview

Available from: ArXiv
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We discuss the environmental dependence of galaxy evolution based on deep panoramic imaging of two distant clusters taken with Suprime-Cam as part of the PISCES project. By combining with the SDSS data as a local counterpart for comparison, we construct a large sample of galaxies that spans wide ranges in environment, time, and stellar mass (or luminosity). We find that colours of galaxies, especially those of faint galaxies ($M_V>M_V^*+1$), change from blue to red at a break density as we go to denser regions. Based on local and global densities of galaxies, we classify three environments: field, groups, and clusters. We show that the cluster colour-magnitude relation is already built at $z=0.83$. In contrast to this, the bright-end of the field colour-magnitude relation has been vigorously built all the way down to the present-day and the build-up at the faint-end has not started yet. A possible interpretation of these results is that galaxies evolve in the 'down-sizing' fashion. That is, massive galaxies complete their star formation first and the truncation of star formation is propagated to smaller objects as time progresses. This trend is likely to depend on environment since the build-up of the colour-magnitude relation is delayed in lower-density environments. Therefore, we may suggest that the evolution of galaxies took place earliest in massive galaxies and in high density regions, and it is delayed in less massive galaxies and in lower density regions. Comment: 23pages, 19 figures, accepted for publication in MNRAS
    Full-text · Article · Jun 2005 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: To understand the evolution of galaxies, we need to know as accurately as possible how many galaxies were present in the Universe at different epochs. Galaxies in the young Universe have hitherto mainly been identified using their expected optical colours, but this leaves open the possibility that a significant population remains undetected because their colours are the result of a complex mix of stars, gas, dust or active galactic nuclei. Here we report the results of a flux-limited I-band survey of galaxies at look-back times of 9 to 12 billion years. We find 970 galaxies with spectroscopic redshifts between 1.4 and 5. This population is 1.6 to 6.2 times larger than previous estimates, with the difference increasing towards brighter magnitudes. Strong ultraviolet continua (in the rest frame of the galaxies) indicate vigorous star formation rates of more than 10-100 solar masses per year. As a consequence, the cosmic star formation rate representing the volume-averaged production of stars is higher than previously measured at redshifts of 3 to 4.
    Full-text · Article · Oct 2005 · Nature
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The study of colliding galaxies has progressed rapidly in the last few years, driven by observations with powerful new ground and space-based instruments. These instruments have used for detailed studies of specific nearby systems, statistical studies of large samples of relatively nearby systems, and increasingly large samples of high redshift systems. Following a brief summary of the historical context, this review attempts to integrate these studies to address the following key issues. What role do collisions play in galaxy evolution, and how can recently discovered processes like downsizing resolve some apparently contradictory results of high redshift studies? What is the role of environment in galaxy collisions? How is star formation and nuclear activity orchestrated by the large scale dynamics, before and during merger? Are novel modes of star formation involved? What are we to make of the association of ultraluminous X-ray sources with colliding galaxies? To what do degree do mergers and feedback trigger long-term secular effects? How far can we push the archaeology of individual systems to determine the nature of precursor systems and the precise effect of the interaction? Tentative answers to many of these questions have been suggested, and the prospects for answering most of them in the next few decades are good.
    Preview · Article · Dec 2005
Show more