Spectroscopy and stellar populations of star-forming galaxies at z ∼ 3 in the Hubble Deep Field - South

Abstract

We present results of VLT/FORS2 spectroscopy of galaxies at z ∼ 3 in the Hubble Deep Field - South (HDF-S). A sample of galaxies was drawn from the photometric redshift catalogue based on the HST/WFPC2 optical images and the deep near-infrared images obtained with VLT/ISAAC as a part of the Faint Infrared Extragalactic Survey (FIRES) project. We selected galaxies with photometric redshift between 2.5 and 4. Most of the selected galaxies are bright in rest-frame UV wave-lengths and satisfy color selection criteria of Lyman break galaxies (LBGs) at z ∼ 3. The number of target galaxies with I AB ≤ 25.0 was 15. We identified new 5 firm and 2 probable redshifts in addition to confirmation of 6 previously known galaxies at z ∼ 3. We found that 6 among these 13 galaxies lie in a quite narrow redshift range at z = 2.80 ± 0.01. Their spatial distribution is fairly concentrated and is at the edge of the HDF-S field, suggesting the possible existence of larger galaxy clustering. We examined stellar populations of the galaxies with spectroscopic redshifts through comparisons of their optical and near-infrared photometry data with template spectra generated by a population synthesis code. The ages from the onset of star formation for these star-forming galaxies with I ≤ 25.0 are typically 50-200 Myr, and their stellar masses are between (0.5-5) × 1010 M⊙, consistent with previous studies. We also compared these SED fitting results with those for "distant red galaxies" (DRGs) at z > 2 discovered by FIRES. DRGs have larger stellar masses, higer dust attenuation than our UV-luminous LBG sample, and their star formation rates are often comparable to LBGs. These trends suggest that majority of the DRGs are indeed the most massive systems at that redshift and are still in the active star-forming phase. Unless the number density of DRGs is much smaller than LBGs, estimates based on a UV selected sample could miss substantial part of stellar mass density at z ∼ 3. © ESO 2005.