GMASS ultradeep spectroscopy of galaxies at z  ~  2

VI. Star formation, extinction, and gas outflows from UV spectra

¹ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: margherita.talia2@unibo.it
² INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Max Planck Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching bei München, Germany
⁴ CEA-Saclay, DSM/Irfu/Service d’Astrophysique, 91191 Gif-sur-Yvette Cedex, France
⁵ Department of Astronomy, University of Massachusetts, 710 North Pleasant Street, Amherst, MA 01003, USA
⁶ NOAO-Tucson, 950 North Cherry Avenue, Tucson, AZ 85719, USA
⁷ Università di Padova, Dipartimento di Astronomia, vicolo dell’Osservatorio 2, 35122 Padova, Italy
⁸ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁹ INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁰ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany

Received: 11 July 2011
Accepted: 10 November 2011

Abstract

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 ~ 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  ⟨β⟩  =  −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 Å (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_⊙   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  ~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.