Outflows and complex stellar kinematics in SDSS star-forming galaxies

¹ Institute for Astronomy, Department of Physics, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
e-mail: claudia.cicone@phys.ethz.ch
² Cavendish Laboratory, University of Cambridge 19 J. J. Thomson Avenue, Cambridge CB3 0HE, UK
³ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁴ Dipartimento di Fisica e Astronomia, Università degli studi di Firenze, via G. Sansone 1, 50019 Sesto Fiorentino (Firenze), Italy
⁵ INAF−Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 2 July 2014
Accepted: 17 January 2016

Abstract

We investigate the properties of star-formation-driven outflows by using a large spectroscopic sample of ~160 000 local “normal” star-forming galaxies drawn from the Sloan digital sky survey (SDSS), spanning a wide range of star formation rates (SFRs) and stellar masses (M_∗). The galaxy sample is divided into a fine grid of bins in the M_∗−SFR parameter space, for each of which we produced a composite spectrum by stacking the SDSS spectra of the galaxies contained in that bin together. We exploited the high signal-to-noise of the stacked spectra to study the emergence of faint features of optical emission lines that may trace galactic outflows and are otherwise too faint to detect in individual galaxy spectra. We have adopted a novel approach that relies on the comparison between the line-of-sight velocity distribution (LoSVD) of the ionised gas (as traced by the [OIII]λ5007 and Hα+[NII]λλ6548, 6583 emission lines) and the LoSVD of the stars, which are used as a reference for tracing virial motions. Significant deviations in the gas kinematics from the stellar kinematics in the high-velocity tail of the LoSVDs are interpreted as a signature of outflows. Our results suggest that the incidence of ionised outflows increases with SFR and specific SFR. The outflow velocity (v_out) is found to correlate tightly with the SFR for SFR> 1 M_⊙ yr^-1, whereas the dependence of v_out on SFR is nearly flat at lower SFRs. The outflow velocity appears to also increase with the stellar velocity dispersion (σ_∗), although this relation has a much larger scatter than the one with SFR, and we infer velocities as high as v_out ~ (6−8)σ_∗. Strikingly, we detect the signature of ionised outflows only in galaxies located above the main sequence (MS) of star-forming galaxies in the M_∗−SFR diagram, and the incidence of such outflows increases sharply with the offset from the MS. This result suggests that star-formation-driven outflows may be responsible for shaping the upper envelope of the MS by providing a self-regulating mechanism for star formation. Finally, our complementary analysis of the stellar kinematics reveals the presence of blue asymmetries of a few 10 km s^-1 in the stellar LoSVDs. The origin of such asymmetries is not clear, but a possibility is that they trace the presence of a large number of high velocity runaway stars and hypervelocity stars in radial trajectories in local galaxies.