Halpha3: an Halpha imaging survey of HI selected galaxies from ALFALFA.
  III. Nurture shapes up the Hubble sequence in the Great Wall
  We present the analysis of Halpha3, an Halpha imaging survey of galaxies
selected from the HI ALFALFA Survey in the Coma Supercluster. By using the
Halpha line as a tracer of the "instantaneous" star formation, complemented
with optical colors from SDSS we explore the hypothesis that a morphological
sequence of galaxies of progressively earlier type, lower gas-content exists in
the neighborhood of the Coma cluster, with specific star formation activity
decreasing with increasing local galaxy density and velocity dispersion. In the
dwarf regime (8.5<\log(M*)< 9.5) we identify a 4-step sequence of galaxies with
progressively redder colors, i.e. of decreasing specific star formation, from
(1) HI-rich Late-Type Galaxies belonging to the blue-cloud exhibiting extended
plus nuclear star formation, (2) HI-poor LTGs with nuclear star formation only,
(3) HI-poor galaxies with no star formation either extended or nuclear, but
with nuclear Post-Star-Burst signature,(4) Early-type Galaxies in the
red-sequence, with no gas or star formation on all scales. Along this sequence
the quenching of the star formation proceeds radially outside-in. The
progression toward redder colors found along this "morphological" (gas content)
sequence is comparable to the one obtained increasing the local galaxy density,
from the cosmic filaments (1,2), to the rich clusters (2,3,4). In the dwarf
regime we find evidence for evolution of HI-rich LTGs into ETGs, passing
through HI-poor and PSB galaxies, driven by the environment. We identify
ram-pressure as the mechanism most likely responsible for this transformation.
We conclude that infall of low-mass galaxies has proceeded for the last 7.5 Gyr
building up the Coma cluster at a rate of approximately 100 galaxies per Gyr.
