Molecular gas and star formation in GASP jellyfish galaxies

Abstract

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. 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. 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 Hαionized 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. 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.

Figures (10)

• Figure 1: CO(2-1) and CO(1-0) zero-moment maps for JO201, JO204, JO206, and JW100. The red contour shows the extent of the stellar disk, as defined in GASPXXI. The beam size is the orange ellipse in the lower left corner of each map. Colored contours delineate the H$\alpha$ emission derived from MUSE data.
• Figure 2: Distribution of the CO(2-1) derived molecular gas mass surface densities using the MW $\alpha_{CO}$ (continuous black histogram) and a varying $\alpha_{CO}$ that follows the stellar mass density distribution (dashed blue histogram), using the relation given in Sandstrom+2013. The vertical dashed and continuous red lines show the average density at the galaxy centers, estimated over the central $\simeq$1 kpc$^2$, assuming the variable $\alpha_{CO}$ and the MW $\alpha_{CO}$, respectively.
• Figure 3: Star formation rate density as a function of the $\rm H_{2}$ surface mass density from the CO(2-1) measured on 1 kpc scale for the GASP galaxies JO201, JO204, JO206, and JW100 (from top left to bottom right). The shaded region encompasses the zone with depletion times between 0.1 and 10 Gyr. The continuous black line is the fit to the disk regions (in black) that takes into account errors on both coordinates, the dashed blue line is the local relation on 1 kpc scale from Bigiel2011, and the dashed green line was derived for VERTICO galaxies on a 1.2 kpc scale from jimenez+2023. The red squares show the measurements corresponding to the tails.
• Figure 4: As in Fig. \ref{['fig:ks_global_21']} but for the CO(1-0) emission.
• Figure 5: Depletion time maps for the four jellyfish galaxies JO201 (top left), JO204 (top right), JW100 (middle right left), and JO206 (bottom). The gray background layer shows the extent of the H$\alpha$ emission from MUSE.