About 20 percent of all nearby early-type galaxies ( M _ { \star } \gtrsim~ { } 6 ~ { } \times 10 ^ { 9 } ~ { } \mathrm { M } _ { \odot } ) outside the Virgo cluster are surrounded by a disc or ring of low-column-density neutral hydrogen ( H i ) 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 H i properties using GALEX UV and SDSS optical images .
Our sample consists of 18 H i -rich galaxies as well as 55 control galaxies where no H i has been detected .
In half of the H i -rich galaxies the radial UV profile changes slope at the position of the H i 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 _ { \mathrm { eff } } .
We find that H i -rich galaxies are on average 0.5 and 0.8 mag bluer than the H i -poor ones , respectively .
This indicates that a significant fraction of the UV emission traces recent star formation and is associated with the H i gas .
Using FUV emission as a proxy for star formation , we estimate the integrated star formation rate in the outer regions ( R > 1 R _ { \mathrm { eff } } ) to be on average \sim 6 \times 10 ^ { -3 } M _ { \odot } ~ { } \mathrm { yr } ^ { -1 } for the H i -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 .