Molecular gas and star formation in GASP jellyfish galaxies

¹ INAF – Padova Astronomical Observatory, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
² INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁴ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 155, 2200 Copenhagen, Denmark
⁵ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Piero Gobetti 93/2, 40129 Bologna, Italy
⁶ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁷ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
⁸ INAF – Osservatorio Astronomico di Brera, Via Brera, 28, 20121 Milano, Italy
⁹ Instituto de Radioastronomia y Astrofisica, UNAM, Campus Morelia, AP 3-72, Morelia CP 58089, Mexico
¹⁰ Department of Physics, Faculty of Science, University of Zagreb, Bijenicka 32, 10 000 Zagreb, Croatia

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 19 November 2025
Accepted: 13 February 2026

Abstract

Context. Several studies have reported a nearly linear correlation between the molecular gas and star formation rate surface density, the so-called Kennicutt-Schmidt (KS) law.

Aims. We aim to retrieve the KS relation for a sample of four star-forming galaxies located in nearby clusters, disturbed by the effects of the ram pressure stripping, as testing this law in galaxies subject to different environmental conditions can provide key information on the physics of star formation.

Methods. To perform our analysis, we used ALMA band 6 and band 3 data coupled with MUSE data at spatial resolution of ∼1 kpc. Moreover, we analyzed data of star-forming complexes detected through their Hα ionized gas emission. We also derived the star formation efficiencies of the star-forming regions nested in these big complexes using the star formation rates derived from spatially resolved HST images and various recipes for the corresponding cold gas phase.

Results. We find that ram-pressure-stripped galaxies show normal-to-low star formation efficiencies, depending on the position within the galaxy and on the local gas density: the inner dense regions in the disk show higher efficiencies with respect to the outer regions, including the gaseous tails. The global relation between the star formation rate density and the molecular gas surface density is superlinear, likely suggesting the shortening of the depletion times at high gas mass densities caused by the ram pressure. Within the star-forming complexes, the star formation efficiency is very similar to the one observed at 1 kpc scale in undisturbed star-forming disks. Interestingly, this result holds also for the star-forming complexes located in the stripped gas tails. The analysis of HST resolved clumps suggests that the molecular gas is not uniformly distributed within the star-forming complexes, but its density distribution follows a steeper profile.