Star formation associated with neutral hydrogen in the outskirts of early-type galaxies

About 20 per cent of all nearby early-type galaxies (M⋆≳ 6 × 109 M⊙) outside the Virgocluster are surrounded by a disc or ring of low-column-density neutralhydrogen (H I) gas with typical radii of tens of kpc, much larger thanthe stellar body. In order to understand the impact of these gasreservoirs on the host galaxies, we analyse the distribution of starformation out to large radii as a function of H I properties using GALEXUV and SDSS optical images. Our sample consists of 18 H I-rich galaxiesas well as 55 control galaxies where no H I has been detected. In halfof the H I-rich galaxies, the radial UV profile changes slope at theposition of the H I radial profile peak. To study the stellarpopulations, we calculate the FUV-NUV and UV-optical colours in twoapertures, 1-3 and 3-10 Reff. We find that H I-rich galaxiesare on average 0.5 and 0.8 mag bluer than the H I-poor ones,respectively. This indicates that a significant fraction of the UVemission traces recent star formation and is associated with the H Igas. Using FUV emission as a proxy for star formation, we estimate theintegrated star formation rate in the outer regions (R >1Reff) to be on average ˜6 × 10-3M⊙ yr-1 for the H I-rich galaxies. This rateis too low to build a substantial stellar disc and, therefore, changethe morphology of the host. We find that the star formation efficiencyand the gas depletion time are similar to those at the outskirts ofspirals.