  The traditional knowledge of the mechanisms that caused the formation and
evolution of early-type galaxies (ETG) in a hierarchical universe was
challenged by the unexpected finding by ATLAS3D that 86% of the ETGs show signs
of a fast-rotating disk. This implies a common origin of most spiral galaxies,
followed by a quenching phase, while only a minority of the most massive
systems are slow rotators and were likely to be the products of merger events.
Our aim is to improve our knowledge on the content and distribution of ionized
hydrogen and their usage to form stars in a representative sample of ETGs for
which the kinematics and detailed morphological classification were known from
ATLAS3D. Using narrow-band filters centered on the redshifted Halpha line along
with a broad-band (r-Gunn) filter to recover the stellar continuum, we observed
or collected existing imaging observations for 147 ETGs (including members of
the Virgo cluster) that are representative of the whole ATLAS3D survey.
Fifty-five ETGs (37%) were detected in the Halpha line above our detection
threshold, (Halpha E.W. <= -1 AA), and 21 harbor a strong source (Halpha E.W.
<=-5 AA). The strong Halpha emitters appear associated with low-mass (M 10^10
M_odot) S0 galaxies that contain conspicuous stellar and gaseous discs. These
harbor significant star formation at their interior, including their nuclei.
The weak Halpha emitters are almost one order of magnitude more massive,
contain gas-poor discs and harbor an AGN at their centers. Their emissivity is
dominated by [NII] and does not imply star formation. The 92 undetected ETGs
are gas-free systems that lack a disc and exhibit passive spectra even in their
nuclei. These pieces of evidence reinforce the conclusion that the evolution of
ETGs followed the secular channel for the less massive systems and the dry
merging channel for the most massive galaxies.
