The VIMOS Ultra Deep Survey: Lyα emission and stellar populations of star-forming galaxies at 2 < z < 2.5

Abstract

The aim of this paper is to investigate spectral and photometric properties of 854 faint (I<SUB>AB</SUB> ≲ 25 mag) star-forming galaxies (SFGs) at 2 < z < 2.5 using the VIMOS Ultra-Deep Survey (VUDS) spectroscopic data and deep multi-wavelength photometric data in three extensively studied extragalactic fields (ECDFS, VVDS, COSMOS). These SFGs were targeted for spectroscopy as a result of their photometric redshifts. The VUDS spectra are used to measure the UV spectral slopes (β) as well as Lyα equivalent widths (EW). On average, the spectroscopically measured β (-1.36 ± 0.02), is comparable to the photometrically measured β (-1.32 ± 0.02), and has smaller measurement uncertainties. The positive correlation of β with the spectral energy distribution (SED)-based measurement of dust extinction E<SUB>s</SUB>(B-V) emphasizes the importance of β as an alternative dust indicator at high redshifts. To make a proper comparison, we divide these SFGs into three subgroups based on their rest-frame Lyα EW: SFGs with no Lyα emission (SFG<SUB>N</SUB>; EW ≤ 0 Å), SFGs with Lyα emission (SFG<SUB>L</SUB>; EW > 0 Å), and Lyα emitters (LAEs; EW ≥ 20 Å). The fraction of LAEs at these redshifts is ~10%, which is consistent with previous observations. We compared best-fitSED-estimated stellar parameters of the SFG<SUB>N</SUB>, SFG<SUB>L</SUB> and LAE samples. For the luminosities probed here (~ L<SUP>∗</SUP>), we find that galaxies with and without Lyα in emission have small but significant differences in their SED-based properties. We find that LAEs have less dust, and lower star-formation rates (SFR) compared to non-LAEs. We also find that LAEs are less massive compared to non-LAEs, though the difference is smaller and less significant compared to the SFR and E<SUB>s</SUB>(B-V). When we divide the LAEs according to their Spitzer/IRAC 3.6 μm fluxes, we find that the fraction of IRAC-detected (m<SUB>3.6</SUB> ≲ 25 mag) LAEs is much higher than the fraction of IRAC-detected narrow band (NB)-selected LAEs at z ≃ 2-3. This could imply that UV-selected LAEs host a more evolved stellar population, which represents a later stage of galaxy evolution, compared to NB-selected LAEs. <P />Based on data obtained with the European Southern Observatory Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791.