Probing high-redshift galaxy formation at the highest luminosities: New insights from deimos spectroscopy

Abstract

We present Keck DEIMOS spectroscopic observations of the most UV-luminous star-forming galaxies at redshifts 3.2 < z < 4.6. Our sample, selected in the Boötes field of the NOAO Deep Wide-Field Survey, contains galaxies with luminosities of L* ≲ L UV ≲ 7 L* and is one of the largest samples to date of the most UV-luminous galaxies at these redshifts. Our spectroscopic data confirm 41 candidates as star-forming galaxies at 3.2 < z < 4.6 and validate the relatively clean selection of the photometric candidates with a contamination rate of 11%-28%. We find that the fraction of Lyα emitting galaxies increases with decreasing UV luminosity. None of the 12 galaxies with M UV < -22 (i.e., L UV > 3 L*) exhibit strong Lyα emission. We find strong evidence of large-scale outflows, transporting the neutral/ionized gas in the interstellar medium away from the galaxy. Galaxies exhibiting both interstellar absorption and Lyα emission lines show a significant offset between the two features, with a relative velocity of 200-1150 km s-1. We find tentative evidence that this measure of the outflow velocity increases with UV luminosity and/or stellar mass. The luminosity- and mass-dependent outflow strengths suggest that the efficiency of feedback and enrichment of the surrounding medium depend on these galaxy parameters. We also stack the individual spectra to construct composite spectra of the absorption-line-only and Lyα-emitting subsets of the UV luminous galaxies at z ≃ 3.7. The composite spectra are very similar to those of lower-redshift and lower-luminosity Lyman break galaxy (LBG) samples, but with some subtle differences. Analyses of the composite spectra suggest that the UV luminous LBGs at z ≃ 3.7 may have a higher covering fraction of absorbing gas, and may be older (or have had more prolonged star formation histories) than their lower-redshift and lower-luminosity counterparts. In addition, we have discovered that five galaxies in the sample belong to a massive overdensity at z = 3.78. Finally, two galaxies each show two distinct sets of interstellar absorption features. The latter may be a sign of a final stage of major merger, or clumpy disk formation. These systems are not expected in our sample: their presence implies that the frequency of such sources among our luminous z ≃ 3.7 LBGs may be an order of magnitude higher than in lower-redshift and lower-luminosity samples. © 2013. The American Astronomical Society. All rights reserved.