Star formation at different stages of ram-pressure stripping as observed through far-ultraviolet imaging of 13 GASP galaxies

Abstract

Galaxies undergoing ram-pressure stripping develop gaseous tails that can extend several kiloparsecs outside the galaxy disc. Under favourable conditions, star formation can occur in the stripped tail, but there are cases where it does not happen, and this can be attributed to several factors, including the properties of the intracluster medium and different stages of stripping, although a clear consensus has not yet been reached. We used FUV and Hα imaging from the GASP survey to investigate how different stages of stripping affect star formation properties in the tail and disc of 13 galaxies undergoing stripping. FUV imaging provides a direct opportunity to study star formation, unlike Hα, which is an indirect tracer and can have other mechanisms responsible for the emission. The 13 galaxies have different stripping strengths, as identified from the MUSE integral field spectroscopy. The star-forming knots in the disc and tails show a good correspondence between the measured FUV and Hα flux. This is especially true for strong and extreme cases of stripping, which have developed extended ionised gaseous tails featuring clumpy structures. The mechanism behind the Hα emission on the tails of these regions, which correlates well with FUV emission, is photoionisation caused by young massive stars. Optical emission line ratio maps enable us to understand the emission mechanism, which can be attributed to star formation, LINER activity, or a combination of both phenomena and AGN. The star-forming regions in the emission line maps correspond well to the areas with significant FUV flux in these galaxies. FUV imaging reveals that six galaxies exhibit minimal star formation in their tails. In two cases, star formation is limited to the central regions, and their discs are truncated. In galaxies with truncated discs, star formation is confined to a smaller region on the disc, as indicated by the FUV flux, compared to Hα. Galaxies with strong stripping, marked by significant FUV and Hα emission along their tails, are undergoing recent star formation and are likely recent infalls. In contrast, galaxies with truncated discs confine star formation to the centre, likely because they have completed a cluster crossing that depleted most of their outer gaseous disc. Galaxies with little FUV flux along their tails show unresolved Hα emission, particularly in the extended Hα tail where no FUV emission is present. The unresolved Hα emission along the tail may be the result of processes other than star formation.