GMASS ultradeep spectroscopy of galaxies at z ∼ 2 VI. Star formation, extinction, and gas outflows from UV spectra

Aims. We use rest-frame UV spectroscopy to investigate the properties related to large-scale gas outflow, and both the dust extinction and star-formation rates (SFRs) of a sample of z similar to 2 star-forming galaxies from the Galaxy Mass Assembly ultradeep Spectroscopic Survey (GMASS). Methods. Dust extinction is estimated from the rest-frame UV continuum slope and used to obtain dust-corrected SFRs for the galaxies in the sample. A composite spectrum is created by averaging all the single spectra of our sample, and the equivalent widths and centroids of the absorption lines associated with the interstellar medium are measured. We then calculate the velocity offsets of these lines relative to the composite systemic velocity, which is obtained from photospheric stellar absorption lines and nebular emission lines. Finally, to investigate correlations between galaxy UV spectral characteristics and galaxy general properties, the sample is divided into two bins that are equally populated, according to the galaxy properties of stellar mass, color excess, and SFR. A composite spectrum for each group of galaxies is then created, and both the velocity offsets and the equivalent widths of the interstellar absorption lines are measured. Results. For the entire sample, we derive a mean value of the continuum slope = -1.11 +/- 0.44 (rms). For each galaxy, we calculate the dust extinction from the UV spectrum and then use this to correct the flux measured at 1500 angstrom (rest-frame), before converting the corrected UV flux into a SFR. We find that our galaxies have an average SFR of < SFR > = 52 +/- 48 M-circle dot yr(-1) (rms) and that there is a positive correlation between SFR and stellar mass, in agreement with other works, the logarithmic slope of the relation being 1.10 +/- 0.10. We discover that the low-ionization absorption lines associated with the interstellar medium measured in the composite spectrum, are blueshifted with respect to the rest frame of the system, which indicates that there is outflowing gas with typical velocities of about similar to 100 km s(-1). Finally, investigating the correlations between the galaxy UV spectral characteristics and general galaxy properties, we find a possible correlation between the equivalent width of the interstellar absorption lines and SFR, stellar mass, and color excess similar to that previously reported to hold at different redshifts.