Star Formation in Young Stellar Clumps of Ram-Pressure Stripped Galaxies as seen by HST

Hα and UV clumps are found embedded in the gaseous tails of galaxies undergoing ram-pressure stripping (RPS) in galaxy clusters, so-called jellyfish galaxies. These clumps offer the opportunity to study star formation (SF) under extreme conditions, in the absence of an underlying disk and embedded within the hot intracluster medium. Yet, a comprehensive, high-spatial resolution study of these systems is missing. We obtained UVIS/HST ∼140 pc-resolution data of the first statistical sample of clumps in six jellyfish galaxies from the GASP survey, combining broad-band (UV to I) filters and a narrow-band Hα filter. After the data are reduced minimizing residual cosmic rays, I detect Hα and UV clumps, tracing SF on different timescales, and optical complexes. I consider clumps in galaxy disks, in the stripped tails and those formed in stripped gas but still close to the disk, called extraplanar. I detect 2406 Hα clumps (1708 in disks, 375 in extraplanar regions, and 323 in tails), 3745 UV clumps (2021 disk clumps, 825 extraplanar clumps and 899 tail clumps) and 424 optical complexes (all in the tails). Only ∼15% of clumps are spatially resolved. I study the luminosity and size distribution functions (LDFs and SDFs, respectively) and the luminosity-size relation. The average LDF slope is 1.79±0.09, while the average SDF slope is 3.1±0.5, suggesting that also in this peculiar environment clumps form through the turbulent scale-free collapse of the interstellar medium, as in main-sequence galaxies. All the clumps, whether they are in disks or tails, have an enhanced Hα luminosity at a given size, compared to the clumps in main-sequence galaxies, and closer to that of clumps in starburst galaxies. That indicates that RPS is able to enhance this quantity, most likely as a consequence of gas compression. After that, I characterize the morphology of the optical complexes in the tails, by connecting them with those of the Hα and UV clumps that they contain. I find that more than half of the complexes contain no Hα clumps, while most of them host at least one UV clump. The clump number and size increase with the complex size, while the median complex filling factor is larger for UV clumps (0.27) than for Hα clumps (0.10) and does not correlate with almost any morphological property. This suggests that the clump number and size grow with the complex keeping the filling factor constant. When studying the position of the clumps inside their complexes, Hα clumps, and UV clumps to a lesser extent, show a displacement from the complex center of 0.1−1 kpc. This is in accordance with the fireball configuration, already observed in the tails of stripped galaxies. Finally, I parameterize the clump mass completeness and the discrepancy between the intrinsic and observed mass by performing a large set of clump mock observations, generated by modelling the real clumps and spanning a wide range of masses. These effects are taken into account when fitting the tail clumps mass function to a single power law in a Bayesian framework. I obtain slopes equal to 2.07±0.10 and 2.28±0.08 for Hα and UV tail clumps, respectively, which further confirm that even the gas undergoing RPS and embedded in the intracluster medium collapses following a scale-free cascade driven by turbulence. To conclude, the high-resolution statistical sample of clumps that I obtain helped answering some of the open questions about the effects of RPS on the SF mechanism, showing that this process highly enhances the clumps Hα luminosity at a given size and affects their morphology, yet not affecting their formation channel, which is the same observed and theorized for clumps in main-sequence galaxies.