The UV spectral slope β and stellar population of most active star-forming galaxies at z ∼ 4

Abstract

We investigate a stellar population of star-forming galaxies at z ∼ 4 by focusing on the slope of their rest-frame ultraviolet (UV) continuum, β, where f_λ ∝ λ^β. We investigate a sample of bright Lyman break galaxies (LBGs) with i' ≤ 26.0 in the Subaru/XMM-Newton Deep Survey field by using a spectral energy distribution fitting analysis. We find that the apparently redder (β_obs > -1.73) LBGs tend to be dusty (A_V > 1.0), and have young stellar populations (β_int < -2.42) and intrinsically active star-forming galaxies (SFR ≳ a few × 10² M_{⊙} yr^{-1}). This means that a significant fraction of the UV-selected LBGs at z ∼ 4 contains on-going, active, and dust-obscured star-forming galaxies. We compare the infrared to UV luminosity ratio, which is estimated from our optical/near-infrared data assuming dust attenuation laws, with sub-millimeter observations from previous works. The result suggests that the Calzetti-like dust attenuation law is preferable for active and dusty star-forming LBGs at z = 4. We also find that an extrapolation of the β_int-M_{UV, int} relation toward the fainter magnitude range below our sample magnitude limit intersects the β_obs-M_{UV, obs} relation previously obtained in deeper narrow-area observations at M_UV = -18.9 and β = -1.94, which coincides with the break point of the β_obs-M_{UV, obs} relation observed so far. This coincidence suggests that we see an almost dust-free population at M_{UV, obs} ≳ -18.9.