  Using a sample of cosmic voids identified in the Sloan Digital Sky Survey
Data Release 7, we study the star formation activity of void galaxies. The
properties of galaxies living in voids are compared with those of galaxies
living in the void shells and with a control sample, representing the general
galaxy population. Void galaxies appear to form stars more efficiently than
shell galaxies and the control sample. This result can not be interpreted as a
consequence of the bias towards low masses in underdense regions, as void
galaxy subsamples with the same mass distribution as the control sample also
show statistically different specific star formation rates. This highlights the
fact that galaxy evolution in voids is slower with respect to the evolution of
the general population. Nevertheless, when only the star forming galaxies are
considered, we find that the star formation rate is insensitive to the
environment, as the main sequence is remarkably constant in the three samples
under consideration. This fact implies that environmental effects manifest
themselves as fast quenching mechanisms, while leaving the non-quenched
galaxies almost unaffected, as their star formation activity is largely
regulated by the mass of their halo. We also analyse galaxy properties as a
function of void-centric distance and find that the enhancement in the star
formation activity with respect to the control sample is observable up to a
radial distance 1.5 Rvoid. This result can be used as a suitable definition of
void shells. Finally, we find that larger voids show an enhanced star formation
activity in the shells with respect to their smaller counterparts, that could
be related to the different dynamical evolution experienced by voids of
different sizes.
