The HI content of early-type galaxies from the ALFALFA survey. II. The
  case of low density environments
  We present the analysis of the HI content of a sample of early-type galaxies
(ETGs) in low-density environments (LDEs) using the data set provided by the
Arecibo Legacy Fast ALFA (ALFALFA) survey. We compare their properties to the
sample in the Virgo cluster that we studied in a previous paper (di Serego
Alighieri et al. 2007, Paper I). We have selected a sample of 62 nearby ETGs
(V< 3000 km/s) in an area of the sky where the ALFALFA data are already
available (8h<RA<16h, 4 deg<DEC<16deg), avoiding the region of the Virgo
cluster. Among these, 39 have absolute B magnitudes fainter than M_B = -17.
Fifteen out of 62 galaxies have been firmly detected with ALFALFA (\sim 25%).
Five additional galaxies show a weaker HI emission (S/N \sim 4) and they will
need deeper observations to be confirmed. All together, our analysis doubles
the number of known gas-rich ETGs in this area. The HI detection rate is 44% in
luminous ETGs (M_B < -17) and 13% in dwarf ETGs (M_B > -17). In both cases it
is 10 times higher than that of the Virgo cluster. The presence of gas can be
related to a recent star formation activity: 60% of all ETGs with HI have
optical emission line ratios typical of star-forming galaxies and blue colours
suggesting the presence of young stellar populations, especially in the dwarf
subsample. We show that the HI detection rate of ETGs depends both on the
environment and mass. The fraction of early-type systems with neutral hydrogen
is higher in more massive objects when compared to early-type dwarfs. The ETGs
in LDEs seem to have more heterogeneous properties than their Virgo cluster
counterparts, since they are able to retain a cold interstellar gas component
and to support star formation activity even at recent epochs.
