SUBARU DEEP SURVEY. V. A CENSUS OF LYMAN BREAK GALAXIES AT z approx= 4 AND 5 IN THE SUBARU DEEP FIELDS: PHOTOMETRIC PROPERTIES

摘要

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′ approx< 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) approx> 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 approx> 100 h_(70)~(-2) solar mass 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 approx= 4 LBGs with M < M~* (approx= —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 A, 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 approx= 60% at z approx= 5, and thus about half of star formation probably occurs in LAEs at z approx= 5. We obtain a constraint on the escape fraction of UV ionizing photons (i.e., UV continuum in 900 A) produced by LBGs, f_(esc) approx> 0.13. This implies that the escape fraction of LBGs may be larger than that of star-forming galaxies at z = 0.