The supersonic nature of jellyfish galaxies

Abstract

All gas-rich galaxies in cluster environments are expected to experience ram-pressure stripping from the intra-cluster medium. However, only a fraction of these develop ongoing star-formation in their stripped tail, becoming the so-called ``jellyfish'' galaxies. In this work we provide observational evidence that magnetic fields can signal differences in the extraplanar star formation and explore what are the physical conditions that lead to the formation of a jellyfish galaxy. We first focus on JO147, a jellyfish galaxy that features weak star formation activity in its tail. Using MeerKAT radio continuum observations, we discover polarized emission only in a small fraction of its tail, with an average fraction of $~10\%$, and a low Mach number $\mathcal{M}=1.3-1.6$, which suggests a possible association between magnetic field draping, shock-compression of the gas, and extraplanar star formation activity. Then, we test this scenario in a sample of 17 jellyfish galaxies from the GASP project. We combine dynamical models for their orbits within the host clusters with realistic cluster temperature profiles to infer their Mach number, and we find a positive correlation between it and the star formation activity in their tail. We conclude that supersonic motion is a necessary condition for triggering star formation in the stripped tails of jellyfish galaxies. Our findings provide empirical evidence that the critical factor preventing the stripped gas evaporation is the shock compression induced by the supersonic motion through the cluster. This process likely enhances the magnetic field surrounding the galaxy and the properties of the stripped material.

Figures (8)

• Figure 1: Composite MUSE--MeerKAT image of the jellyfish galaxy JO147. We show the stellar disk (dashed black contour), the H$\alpha$ emission (orange to black colormap), the radio continuum emission at 1.4 GHz (blue-scale contours, from signal-to-noise of 3 up to 600, angular resolution $12\times12$ arcsec$^{2}$, noise level of 7 $\mu$Jy beam$^{-1}$), and the polarized emission with a signal-to-noise ratio higher than five (green contours). The green contours' intensity and the magnetic field vectors' length (black lines) are proportional to the polarization fraction. The physical scale is shown in the top-right corner, whereas the blue and red circles in the left corner show, respectively, the radio continuum and H$\alpha$ images resolution.
• Figure 2: Composite MUSE--MeerKAT image of the jellyfish galaxy JO147. All quantities are identical to those shown in Figure \ref{['image']}, except that the color-filled contours now represent the Faraday RM map.
• Figure 3: Radio flux density vs. distance from the stellar disk edge. In the left corner, we show the sampling grid overlayed on the radio continuum emission shown in Figure \ref{['image']}. The best-fitting profile is shown by the blue line. The blue-shaded region indicates the $1\sigma$ uncertainties on the fit.
• Figure 4: Extraplanar star formation efficiency vs. "projected" Mach number. Extraplanar specific star formation rate, log$_{10}(\mathrm{sSFR}_{\text{tail}})$ (top) and star formation rate fraction, log$_{10}(F_{\text{tail}})$ (bottom), vs. projected Mach number lower limits, $\mathcal{M}$. Each galaxy is color-coded for its corresponding total star formation rate Gullieuszik2020. JO147 and JO206 are marked with red and blue circles, respectively.
• Figure 5: Outcomes of Monte Carlo analysis for JO147. Top: Simulated orbits projected on the phase-space plane, with the red dot showing the projected phase-space coordinates for JO147 and the blue line indicating the corresponding ICM sound speed profile (left) and 2D Distribution of the $V_i$-$R_i$ pairs associated with the projected JO147 coordinates, the dashed lines indicate JO147 projected coordinates, and the shaded area covers the rejected $V_i$-$R_i$ solutions (right); Bottom: $\mathcal{M}_i$ (left) and $P_{ram,i}$ (right) distributions, the vertical lines indicate the median and the 16$^{\text{th}}$ and 84$^{\text{th}}$ percentiles.