  In the local Universe, there is a handful of dwarf compact star-forming
galaxies with extremely low oxygen abundances. It has been proposed that they
are young, having formed a large fraction of their stellar mass during their
last few hundred Myr. However, little is known about the fraction of young
stellar populations in more massive galaxies. In a previous article, we
analyzed 280 000 SDSS spectra to identify a surprisingly large sample of more
massive Very Young Galaxies (VYGs), defined to have formed at least $50\%$ of
their stellar mass within the last 1 Gyr. Here, we investigate in detail the
properties of a subsample of 207 galaxies that are VYGs according to all three
of our spectral models. We compare their properties with those of control
sample galaxies (CSGs). We find that VYGs tend to have higher surface
brightness and to be more compact, dusty, asymmetric, and clumpy than CSGs.
Analysis of a subsample with HI detections reveals that VYGs are more gas-rich
than CSGs. VYGs tend to reside more in the inner parts of low-mass groups and
are twice as likely to be interacting with a neighbour galaxy than CSGs. On the
other hand, VYGs and CSGs have similar gas metallicities and large scale
environments (relative to filaments and voids). These results suggest that
gas-rich interactions and mergers are the main mechanisms responsible for the
recent triggering of star formation in low-redshift VYGs, except for the lowest
mass VYGs, where the starbursts may arise from a mixture of mergers and gas
infall.
