  The shape of emission lines in the optical spectra of star-forming galaxies
reveals the kinematics of the diffuse gaseous component. We analyse the shape
of prominent emission lines in a sample of ~53,000 star-forming galaxies from
the Sloan Digital Sky Survey, focusing on departures from gaussianity.
Departures from a single gaussian profile allow us to probe the motion of gas
and to assess the role of outflows. The sample is divided into groups according
to their stellar velocity dispersion and star formation rate. The spectra
within each group are stacked to improve the signal-to-noise ratio of the
emission lines, to remove individual signatures, and to enhance the effect of
star formation rate on the shapes of the emission lines. The moments of the
emission lines, including kurtosis and skewness, are determined. We find that
most of the emission lines in strong star-forming systems unequivocally feature
negative kurtosis. This signature is present in H$\beta$, H$\alpha$, [N II] and
[S II] in massive galaxies with high star formation rates. We attribute it as
evidence of radial outflows of ionised gas driven by the star formation of the
galaxies. Also, most of the emission lines in low-mass systems with high star
formation rates feature negative skewness, and we interpret it as evidence of
dust obscuration in the galactic disk. These signatures are however absent in
the [O III] line, which is believed to trace a different gas component. The
observed trend is significantly stronger in face-on galaxies, indicating that
star formation drives the outflows along the galactic rotation axis, presumably
the path of least resistance. The data suggest that outflows driven by star
formation exert accumulated impacts on the interstellar medium, and the outflow
signature is more evident in older galaxies as they have experienced a longer
total duration of star formation.
