CANDELS: THE EVOLUTION OF GALAXY REST-FRAME ULTRAVIOLET COLORS FROMz= 8 TO 4

Abstract

We study the evolution of galaxy rest-frame ultraviolet (UV) colors in the epoch 4 z 8. We use new wide-field near-infrared data in the Great Observatories Origins Deep Survey -South field from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, Hubble Ultra Deep Field 2009 and Early Release Science programs to select galaxies via photometric redshift measurements. Our sample consists of 2812 candidate galaxies at z 3.5, including 113 at z ≃ 7-8. We fit the observed spectral energy distribution (SED) to a suite of synthetic stellar population models, and measure the value of the UV spectral slope (β) from the best-fit model spectrum. We run simulations to show that this measurement technique results in a smaller scatter on β than other methods, as well as a reduced number of galaxies with catastrophically incorrect β measurements (i.e., ∆β > 1). We find that the median value of β evolves significantly from −1.82 +0.00 −0.04 at z = 4, to −2.37 +0.26 −0.06 at z = 7. Additionally, we find that faint galaxies at z = 7 have β = −2.68 +0.39 −0.24 (∼ −2.4 after correcting for observational bias); this is redder than previous claims in the literature, and does not require "exotic" stellar populations (e.g., very-low metallicities or top-heavy initial mass functions) to explain their colors. This evolution can be explained by an increase in dust extinction, from low amounts at z = 7, to A V ∼ 0.5 mag at z = 4. The timescale for this increase is consistent with low-mass AGB stars forming the bulk of the dust. We find no significant (< 2σ) correlation between β and M UV when measuring M UV at a consistent restframe wavelength of 1500 Å. This is particularly true at bright magnitudes, though our results do show evidence for a weak correlation at faint magnitudes when galaxies in the HUDF are considered separately, hinting that dynamic range in sample luminosities may play a role. We do find a strong correlation between β and the stellar mass at all redshifts, in that more massive galaxies exhibit redder colors. The most massive galaxies in our sample have similarly red colors at each redshift, implying that dust can build up quickly in massive galaxies, and that feedback is likely removing dust from low-mass galaxies at z ≥ 7. Thus the stellar-massmetallicity relation, previously observed up to z ∼ 3, may extend out to z = 7 -8.

Dust Properties of C ii Detected z ∼ 5.5 Galaxies: New HST/WFC3 Near-IR Observations

“…Furthermore, the measurement of β is particularly sensitive to the signal to noise of photometry and systematics in flux measurements, such as confusion from neighboring sources, background subtraction, and surface brightness effects (e.g., Dunlop et al 2012;Finkelstein et al 2012;Rogers et al 2013;Bouwens et al 2014). High S/N data accompanied by simulations are therefore essential for correcting for these effects.…”

Essential physics of early galaxy formation

“…Ouchi et al 2010;Bowler et al 2012Bowler et al , 2014. Spectral energy distribution (SED) fitting to broad-band photometry has also been used to build galaxy stellar mass functions (SMF; González et al 2011), understand the stellar populations in these sources through their UV (β) slopes (Bouwens et al 2010b;Finkelstein et al 2012;Wilkins et al 2011;Dunlop et al 2012Dunlop et al , 2013Bouwens et al 2013;Rogers et al 2014), infer their physical properties (Oesch et al 2010a;Labbé et al 2010a;McLure et al 2011;Labbé et al 2013) and calculate the growth of stellar mass density (SMD) with redshift (Labbé et al 2010a;González et al 2011;Stark et al 2013). The Cluster Lensing And Supernova survey with Hubble (CLASH; Postman et al 2012) is now using galaxies magnified by strong-lensing to further constrain the number-density of high-z sources (Zheng et al 2012;Bradley et al 2013;Coe et al 2013) and study their nebular emission (Smit et al 2014).…”

Formation of disc galaxies in preheated media: a preventative feedback model

“…With the advent of deep surveys from both ground-based and space telescopes, the galaxy luminosity/stellar mass functions have been estimated to z ∼ 8 (e.g. Bradley et al 2012;Finkelstein et al 2012;Oesch et al 2012;Yan et al 2012;McLure et al 2013), allowing us to study the star formation and assembly histories of galaxies over the cosmic time. In addition, high quality imaging data from the HST also permits studies of the evolution of galaxy morphology out to z ∼ 2 (e.g.…”