The H I content of early-type galaxies from the ALFALFA survey II. The case of low density environments

Aims. 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, A&A, 474, 851, Paper I). Our aim is both to investigate how the cool interstellar medium (ISM) of these systems depends on the galaxy mass and the environment and to relate the properties of the neutral hydrogen to the warm phases of the ISM. Methods. We have selected a sample of 62 nearby ETGs (V < 3000 km s(-1)) in an area of the sky where the ALFALFA data are already available (8(h) < RA < 16(h), 4 degrees < Dec < 16 degrees), avoiding the region of the Virgo cluster. Among these, 39 have absolute B magnitudes fainter than M(B) = -17. Results. Fifteen out of 62 galaxies have been firmly detected with ALFALFA (similar to 25%). Five additional galaxies show a weaker HI emission (S/N similar to 4) and they will need deeper observations to be confirmed. Eight objects had 21-cm measurements reported in the literature. One by one comparison with the available material confirms, as expected, that ALFALFA data are, with rare exceptions, of equal or better quality than the best spectra previously obtained for these objects. 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. Conclusions. 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.