The history of star-forming regions in the tails of six GASP jellyfish galaxies observed with the Hubble Space Telescope

Aims: In this work, we aim to characterize the stellar populations of star-forming regions detached from the stellar disks of galaxies undergoing ram-pressure stripping.
Methods: Using images collected with the WFC3 camera on board of the Hubble Space Telescope, we detected stellar clumps in continuum-subtracted Hα and the ultraviolet (F275W filter); such clumps are often embedded in larger regions (star-forming complexes) detected in the optical (F606W filter). Our sample includes 347 Hα clumps, 851 F275W clumps, and 296 star-forming complexes. We modeled the photometry of these objects in five bands using BAGPIPES to obtain their stellar population parameters.
Results: The median mass-weighted stellar ages are 27 Myr for Hα clumps and 39 Myr for F275W clumps and star-forming complexes, but the oldest stars in the complexes can be older than ∼300 Myr which indicates that star formation is sustained for long periods of time. Stellar masses vary from 10^3.5 to 10^7.1 M_⊙, with star-forming complexes being more massive objects in the sample. Clumps and complexes found further away from the host galaxy are on average younger, less massive, and less obscured by dust. We interpret these trends as due to the effect of ram pressure in different phases of the interstellar medium. Hα clumps form a well-defined sequence in the stellar mass-SFR plane with a slope of 0.73. Some F275W clumps and star-forming complexes follow the same sequence while others stray away from it and passively age. The difference in mean stellar age between a complex and its youngest embedded clump scales with the distance between the clump and the center of the optical emission of the complex, with the most displaced clumps being hosted by the most elongated complexes. This is consistent with a fireball-like morphology, where star formation proceeds in a small portion of the complex while older stars are left behind producing a linear stellar population gradient. The stellar masses of star-forming complexes are consistent with the ones of globular clusters, but their stellar mass surface densities are lower by 2 dex, and their properties are more consistent with the population of dwarf galaxies in clusters.

The catalog of physical properties is available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (ftp://130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/682/A162