Clump formation in ram-pressure stripped galaxies: Evidence from mass function

Context. The mass function (MF) of young stellar clumps (age ≲ 200 Myr) is an indicator of the mechanism driving the collapse of the interstellar medium (ISM) into the giant molecular clouds. Typically, the MF of clumps in main-sequence galaxies is described by a power law (d N /d M * ∝ M * − α ) with slope α = 2, hinting that the collapse is driven by turbulence. Aims. To understand whether the local environment affects star formation, we modelled the clump MF of six cluster galaxies, from the GASP survey, undergoing strong ram-pressure stripping (RPS). This phenomenon, exerted by the hot and high-pressure intracluster medium (ICM), has produced long tails of stripped ISM, where clumps form far away from the galactic disk and are surrounded by the ICM itself. Methods. Clumps were selected from HST -UVIS/WFC3 images, covering from near-UV to red-optical bands and including H α emission-line maps. The catalogue comprises 398 H α clumps (188 in tails, 210 in the disk outskirts known as the extraplanar region) and 1270 UV clumps (593 in tails, 677 extraplanar). Using mock images, we quantified the mass completeness and bias of our sample. Taking these two effects into account, we adopted a Bayesian approach to fit the clump mass catalogue to a suitable MF. Results. The resulting MFs are steeper than the expected value of 2. In the tails the H α clumps have slope α = 2.31 ± 0.12, while the UV slope is larger (2.60 ± 0.09), in agreement with ageing effects. Similar results are found in the extraplanar region, with H α slope α = 2.45 −0.16 +0.20 and UV slope α = 2.63 −0.18 +0.20 , even though in this case they are consistent within the uncertainties. Conclusions. We suggest that the steepening results from the higher-than-usual turbulent environment, arising from the interaction between the ISM and the ICM. As shown by recent works, this process can favour the fragmentation of the largest ISM clouds, inhibiting the formation of very massive clumps.