Double-peak emission line galaxies in the SDSS catalogue. A minor merger
  sequence
  Mergers can be detected as double-peak narrow emission line galaxies but they
are difficult to disentangle from disc rotations and gas outflows. We aim to
properly detect such galaxies and distinguish the underlying mechanisms.
Relying on RCSED, we developed an automated selection procedure and found 5663
double-peak emission line galaxies at z<0.34 corresponding to 0.8% of the
parent database. To characterise these galaxies, we built a single-peak no-bias
control sample (NBCS) with the same redshift and stellar mass distributions as
the double-peak sample (DPS). These two samples are indeed very similar in
terms of absolute magnitude, [OIII] luminosity, colour-colour diagrams, age and
specific star formation rate, metallicity, and environment. We find an
important excess of S0 galaxies in the DPS, not observed in the NBCS, and which
cannot be accounted for by the environment, as most of these galaxies are
isolated or in poor groups. Similarly, we find a relative deficit of pure discs
in the DPS late-type galaxies, that are preferentially of Sa type. In parallel,
we observe a systematic central excess of star formation and extinction for DP
galaxies. Finally, there are noticeable differences in the kinematics: the gas
velocity dispersion is correlated with the galaxy inclination in the NBCS,
whereas this relation does not hold for the DPS. Furthermore, the DP galaxies
show larger stellar velocity dispersions and they deviate from the Tully-Fisher
relation for both late-type and S0 galaxies. These discrepancies can be
reconciled if one considers the two peaks as two different components.
Considering the morphological biases in favour, bulge-dominated galaxies and
star-formation central enhancement, we suggest a scenario of multiple
sequential minor mergers driving the increase of the bulge size, leading to
larger fractions of S0 galaxies and a deficit of pure disc galaxies.
