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

TL;DR

This study investigates star formation in the outer regions of early-type galaxies associated with neutral hydrogen gas, revealing recent star formation activity that does not significantly alter galaxy morphology.

Contribution

It provides new insights into the relationship between neutral hydrogen gas and star formation in early-type galaxies' outskirts, using multi-wavelength observations.

Findings

HI-rich galaxies are bluer than HI-poor ones, indicating recent star formation.

Star formation rates in outer regions are low, insufficient to change galaxy morphology.

Star formation efficiency is similar to that in spiral galaxy outskirts.

Abstract

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.