Subaru Deep Survey. V. A Census of Lyman Break Galaxies at z ≃ 4 and 5 in the Subaru Deep Fields: Photometric Properties

Abstract

We investigate the photometric properties of Lyman break galaxies (LBGs) at z = 3.5-5.2 based on large samples of 2600 LBGs detected in deep ( i ' ≲ 27) and wide-field (1200 arcmin 2 ) images taken in the Subaru Deep Field (SDF) and the Subaru/ XMM-Newton Deep Field (SXDF) using broadband B , V , R , i ', and z ' filters. The selection criteria for the LBG samples are examined with 85 spectroscopically identified objects, and the completeness and contamination of the samples are estimated from Monte Carlo simulations based on a photometric-redshift catalog of the Hubble Deep Field-North. We find that these LBG samples are nearly rest-frame UV magnitude-limited samples, missing systematically only 10% of red high- z galaxies (in number), which are a dusty population with E ( B - V ) ≳ 0.4. We calculate luminosity functions (LFs) of the LBGs with the estimated completeness and contamination and find (1) that the number density of bright galaxies ( M 1700 < -22 ; corresponding to SFR ≳ 100 h M ☉ yr -1 with extinction correction) decreases significantly from z = 4 to 5 and (2) that the faint-end slope of the LFs of LBGs may become steeper toward higher redshifts. We estimate the dust extinction of z ≃ 4 LBGs with M < M * (≃-21) from UV-continuum slopes and obtain E ( B - V ) = 0.15 ± 0.03 as the mean value. The dust extinction remains constant with apparent luminosity but increases with intrinsic (i.e., extinction-corrected) luminosity. We find no evolution in dust extinction between LBGs at z = 3 and 4. We investigate the evolution of UV-luminosity density by integrating the LFs of LBGs and find that the UV-luminosity density at 1700 Å, ρ UV , does not significantly change from z = 3 to 5, i.e., ρ UV ( z = 4)/ρ UV ( z = 3) = 1.0 ± 0.2 and ρ UV ( z = 5)/ρ UV ( z = 3) = 0.8 ± 0.4; thus, the cosmic star formation rate (SFR) density (with correction for dust extinction) remains constant within the error bars, or possibly has a slight decline, from z = 3 to 5. We estimate the stellar mass density from the cosmic SFR thus obtained and find that this stellar mass density is consistent with those derived directly from the stellar mass function at z = 0-1 but exceeds those at z ~ 3 by a factor of 3. We find that the ratio of the UV-luminosity density of Lyα emitters (LAEs) to that of LBGs is ≃60% at z ≃ 5, and thus about half of star formation probably occurs in LAEs at z ≃ 5. We obtain a constraint on the escape fraction of UV ionizing photons (i.e., UV continuum in 900 Å) produced by LBGs, f esc ≳ 0.13. This implies that the escape fraction of LBGs may be larger than that of star-forming galaxies at z = 0.