Star formation associated with neutral hydrogen in the outskirts of
  early-type galaxies
  About 20 percent of all nearby early-type galaxies ($M_{\star} \gtrsim 6
\times 10^{9}$ M$_{\odot}$) outside the Virgo cluster are surrounded by a disc
or ring of low-column-density neutral hydrogen (HI) gas with typical radii of
tens of kpc, much larger than the stellar body. In order to understand the
impact of these gas reservoirs on the host galaxies, we analyse the
distribution of star formation out to large radii as a function of HI
properties using GALEX UV and SDSS optical images. Our sample consists of 18
HI-rich galaxies as well as 55 control galaxies where no HI has been detected.
In half of the HI-rich galaxies the radial UV profile changes slope at the
position of the HI radial profile peak. To study the stellar populations, we
calculate the FUV-NUV and UV-optical colours in two apertures, 1-3 and 3-10
R$_{eff}$ . We find that HI -rich galaxies are on average 0.5 and 0.8 mag bluer
than the HI-poor ones, respectively. This indicates that a significant fraction
of the UV emission traces recent star formation and is associated with the HI
gas. Using FUV emission as a proxy for star formation, we estimate the
integrated star formation rate in the outer regions (R > 1R$_{eff}$) to be on
average $6 \times 10^{-3}$ M$_{\odot}$ yr$^{-1}$ for the HI-rich galaxies. This
rate is too low to build a substantial stellar disc and, therefore, change the
morphology of the host. We find that the star formation efficiency and the gas
depletion time are similar to those at the outskirts of spirals.
