Outflows and complex stellar kinematics in SDSS star forming galaxies
  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 SDSS, spanning a wide range of star formation rates and stellar
masses. The galaxy sample is divided into a fine grid of bins in the M_*-SFR
parameter space, for each of which we produce a composite spectrum by stacking
together the SDSS spectra of the galaxies contained in that bin. We exploit 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 would
otherwise be too faint to detect in individual galaxy spectra. We adopt 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]5007 and
Halpha+[NII]6548,6583 emission lines) and the LoSVD of the stars, which are
used as a reference tracing virial motions. Significant deviations of 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 sSFR. The outflow velocity
(v_out) correlates tightly with the SFR for SFR>1 M_Sun/yr, whereas at lower
SFRs the dependence of v_out on SFR is nearly flat. The outflow velocity,
although with a much larger scatter, increases also with the stellar velocity
dispersion, and we infer velocities as high as v_out~(6-8)*sigma_stars.
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. Our complementary analysis of the stellar kinematics reveals the
presence of blue asymmetries of a few 10 km/s in the stellar LoSVDs. [abridged]
