Halpha3: an Halpha imaging survey of HI selected galaxies from ALFALFA.
  IV. Structure of galaxies in the Local and Coma Superclusters
  We present the analysis of the galaxy structural parameters from Halpha3, an
Halpha narrow-band imaging follow-up survey of ~800 galaxies selected from the
HI ALFALFA Survey in the Local and Coma Superclusters. Taking advantage of
Halpha3 which provides the complete census of the recent star-forming, HI-rich
galaxies in the local universe, we aim to investigate the structural parameters
of both the young (<10 Myr) and the old (>1 Gyr) stellar populations. By
comparing the sizes of these stellar components we investigated the spatial
scale on which galaxies are growing at the present cosmological epoch and the
role of the environment in quenching the star-formation activity. We computed
the concentration, asymmetry, and clumpiness structural parameters. To quantify
the sizes we computed half-light radii and a new parameter dubbed EW/r. The
concentration index computed in the r band depends on the stellar mass and on
the Hubble type, these variables being related since most massive galaxies are
bulge dominated thus most concentrated. Going toward later spirals and
irregulars both the concentration index and the stellar mass decrease. Blue
Compact dwarfs represent an exception since they have similar stellar mass but
they are more concentrated than dwarf irregulars. The asymmetry and the
clumpiness increase along the spiral sequence then they decrease going into the
dwarf regime, where the light distribution is smooth and more symmetric. When
measured on Halpha images, the CAS parameters do not exhibit obvious
correlations with Hubble type. We found that the concentration index is the
main parameter that describes the current growth of isolated galaxies but, for
a fixed concentration, the stellar mass plays a second order role. At the
present epoch, massive galaxies are growing inside-out, conversely the dwarfs
are growing on the scale of their already assembled mass.
