STACKED REST-FRAME ULTRAVIOLET SPECTRA OF Lyα-EMITTING AND CONTINUUM-SELECTED GALAXIES AT 2 z 3.5

摘要

We present properties of individual and composite rest-UV spectra of continuum- and narrowband-selected star-forming galaxies (SFGs) at a redshift of 2 z 3.5 discovered by the MUSYC collaboration in the Extended Chandra Deep Field-South. Among our sample of 81 UV-bright SFGs, 59 have R 25.5, of which 32 have rest-frame equivalent widths of W Lyα 20 ?, the canonical limit to be classified as an Lyα-emitting galaxy. We divide our data set into subsamples based on properties that we are able to measure for each individual galaxy: Lyα equivalent width, rest-frame UV colors, and redshift. Among our subsample of galaxies with R 25.5, those with rest frame W Lyα 20 ? have bluer UV continua, weaker low-ionization interstellar absorption lines, weaker C IV absorption, and stronger Si II* nebular emission than those with W Lyα 20 ?. We measure a velocity offset of Δv ~ 600?km?s–1 between Lyα emission and low-ionization absorption, which does not vary substantially among any of our subsamples. We find that the interstellar component, as opposed to the stellar component, dominates the high-ionization absorption line profiles. We find that the low- and high-ionization Si ionization states have similar kinematic properties, yet the low-ionization absorption is correlated with Lyα emission and the high-ionization absorption is not. These trends are consistent with outflowing neutral gas being in the form of neutral clouds embedded in ionized gas as previously suggested by Steidel et al. Moreover, our galaxies with bluer UV?colors have stronger Lyα emission, weaker low-ionization absorption, and more prominent nebular emission line profiles. From a redshift of 2.7 z 3.5 to 2.0 z 2.7, our subsample of galaxies with W Lyα 20 ? shows no significant evolution in their physical properties or the nature of their outflows. Among our data set, UV-bright galaxies with W Lyα 20 ? exhibit weaker Lyα emission at lower redshifts, although we caution that this could be caused by spectroscopic confirmation of low Lyα equivalent width galaxies being harder at z ~ 3 than z ~ 2.