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Register a new userLoTSS jellyfish galaxies: II. Ram pressure stripping in groups versus clusters
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Numerous examples of ram pressure stripping in galaxy clusters are present in literature; however, substantially less work has been focused on ram pressure stripping in lower mass groups. In this work we use the LOFAR Two-metre Sky Survey (LoTSS) to search for jellyfish galaxies in ~500 SDSS groups (z<0.05), making this the most comprehensive search for ram pressure stripping in groups to date. We identify 60 jellyfish galaxies in groups with extended, asymmetric radio continuum tails, which are found across the entire range of group mass from $10^{12.5} < M_mathrm{group} < 10^{14},h^{-1},mathrm{M_odot}$. We compare the group jellyfish galaxies identified in this work with the LoTSS jellyfish galaxies in clusters presented in Roberts et al. (2021), allowing us to compare the effects of ram pressure stripping across three decades in group/cluster mass. We find that jellyfish galaxies are most commonly found in clusters, with the frequency decreasing towards the lowest mass groups. Both the orientation of observed radio continuum tails, and the positions of group jellyfish galaxies in phase space, suggest that galaxies are stripped more slowly in groups relative to clusters. Finally, we find that the star formation rates of jellyfish galaxies in groups are consistent with `normal star-forming group galaxies, which is in contrast to cluster jellyfish galaxies that have clearly enhanced star formation rates. On the whole, there is clear evidence for ongoing ram pressure stripping in galaxy groups (down to very low group masses), though the frequency of jellyfish galaxies and the strength of ram pressure stripping appears smaller in groups than clusters. Differences in the efficiency of ram pressure stripping in groups versus clusters likely contributes to the positive trend between quenched fraction and host halo mass observed in the local Universe.
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Ram-pressure stripping by the gaseous intra-cluster medium has been proposed as the dominant physical mechanism driving the rapid evolution of galaxies in dense environments. Detailed studies of this process have, however, largely been limited to relatively modest examples affecting only the outermost gas layers of galaxies in nearby and/or low-mass galaxy clusters. We here present results from our search for extreme cases of gas-galaxy interactions in much more massive, X-ray selected clusters at $z>0.3$. Using Hubble Space Telescope snapshots in the F606W and F814W passbands, we have discovered dramatic evidence of ram-pressure stripping in which copious amounts of gas are first shock compressed and then removed from galaxies falling into the cluster. Vigorous starbursts triggered by this process across the galaxy-gas interface and in the debris trail cause these galaxies to temporarily become some of the brightest cluster members in the F606W passband, capable of outshining even the Brightest Cluster Galaxy. Based on the spatial distribution and orientation of systems viewed nearly edge-on in our survey, we speculate that infall at large impact parameter gives rise to particularly long-lasting stripping events. Our sample of six spectacular examples identified in clusters from the Massive Cluster Survey, all featuring $M_{rm F606W}<-$21 mag, doubles the number of such systems presently known at $z>0.2$ and facilitates detailed quantitative studies of the most violent galaxy evolution in clusters.
It is well known that galaxies falling into clusters can experience gas stripping due to ram-pressure by the intra-cluster medium (ICM). The most spectacular examples are galaxies with extended tails of optically-bright stripped material known as jellyfish. We use the first large homogeneous compilation of jellyfish galaxies in clusters from the WINGS and OmegaWINGS surveys, and follow-up MUSE observations from the GASP MUSE programme to investigate the orbital histories of jellyfish galaxies in clusters and reconstruct their stripping history through position vs. velocity phase- space diagrams. We construct analytic models to define the regions in phase-space where ram-pressure stripping is at play. We then study the distribution of cluster galaxies in phase-space and find that jellyfish galaxies have on average higher peculiar velocities (and higher cluster velocity dispersion) than the overall population of cluster galaxies at all clustercentric radii, which is indicative of recent infall into the cluster and radial orbits. In particular, the jellyfish galaxies with the longest gas tails reside very near the cluster cores (in projection) and are moving at very high speeds, which coincides with the conditions of the most intense ram-pressure. We conclude that many of the jellyfish galaxies seen in clusters likely formed via fast (~1- 2 Gyr), incremental, outside-in ram-pressure stripping during first infall into the cluster in highly radial orbits.
We investigate the effects of magnetic fields and turbulence on ram pressure stripping in elliptical galaxies using ideal magnetohydrodynamics simulations. We consider weakly-magnetised interstellar medium (ISM) characterised by subsonic turbulence, and two orientations of the magnetic fields in the intracluster medium (ICM) - parallel and perpendicular to the direction of the galaxy motion through the ICM. While the stronger turbulence enhances the ram pressure stripping mass loss, the magnetic fields tend to suppress the stripping rates, and the suppression is stronger for parallel fields. However, the effect of magnetic fields on the mass stripping rate is mild. Nevertheless, the morphology of the stripping tails depends significantly on the direction of the ICM magnetic field. The effect of the magnetic field geometry on the tail morphology is much stronger than that of the level of the ISM turbulence. The tail has a highly collimated shape for parallel fields, while it has a sheet-like morphology in the plane of the ICM magnetic field for perpendicular fields. The magnetic field in the tail is amplified irrespectively of the orientation of the ICM field. More strongly magnetised regions in the ram pressure stripping tails are expected to have systematically higher metallicity due to the strong concentration of the stripped ISM than the less magnetised regions. Strong dependence of the morphology of the stripped ISM on the magnetic field could potentially be used to constrain the relative orientation of the ram pressure direction and the dominant component of the ICM magnetic field.
While galaxies move through the intracluster medium of their host cluster, they experience a ram pressure which removes at least a significant part of their interstellar medium. This ram pressure stripping appears to be especially important for spiral galaxies: this scenario is a good candidate to explain the differences observed between cluster spirals in the nearby universe and their field counterparts. Thus, ram pressure stripping of disk galaxies in clusters has been studied intensively during the last decade. I review advances made in this area, concentrating on theoretical work, but continuously comparing to observations.
We use 3-dimensional SPH/N-BODY simulations to study ram pressure stripping of gas from spiral galaxies orbiting in clusters. We find that the analytic expectation of Gunn & Gott (1972) relating the gravitational restoring force provided by the disk to the ram pressure force, provides a good approximation to the radius that gas will be stripped from a galaxy. However, at small radii it is also important to consider the potential provided by the bulge component. A spiral galaxy passing through the core of a rich cluster such as Coma, will have its gaseous disk truncated to $sim 4$ kpc, thus losing $sim 80%$ of its diffuse gas mass. The timescale for this to occur is a fraction of a crossing time $sim 10^7$ years. Galaxies orbiting within poorer clusters, or inclined to the direction of motion through the intra-cluster medium will lose significantly less gas. We conclude that ram-pressure alone is insufficient to account for the rapid and widespread truncation of star-formation observed in cluster galaxies, or the morphological transformation of Sabs to S0s that is necessary to explain the Butcher-Oemler effect.
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