The Impact of neutral hydrogen on the current evolution of early-type galaxies

About 20 percent of all nearby early-type galaxies (ETGs) 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 my PhD thesis, I have for the first time studied the effect of these large HI reservoirs (hosting potential material for star formation) on their host galaxies, which are commonly considered to be passive (i.e., not forming new stars). I have found that (i) some of the large HI discs detected with radio telescopes do host a significant amount of star formation; (ii) if the metallicity of the gas is different, the star formation efficiency can dramatically change; (iii) in the outer regions of ETGs with HI, this gas is converted into new stars with a similar efficiency as in the outer regions spiral galaxies; (iv) since the relatively low amount of HI is distributed over a large area, this formation of new stars cannot significantly change the appearance of the host galaxy any time soon; (v) the presence of the HI distributed out to very large distances from the main stellar body of galaxies is also felt in their central regions. Indeed, ETGs with large amounts of HI at large radii are also characterised by a large amount of dust in their centre. This shows that gas at large radii can feed the inner region of galaxies, where new stars can then form