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Register a new userJellyfish: Ram-pressure stripping as a diagnostic tool in studies of cluster collisions
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Prompted by the discovery of A1758N_JFG1, a spectacular case of ram-pressure stripping (RPS) in the galaxy cluster A1758N, we investigate the properties of other galaxies suspected to undergo RPS in this equal-mass, post-collision merger. Exploiting constraints derived from Hubble Space Telescope images and Keck longslit spectroscopy, our finding of apparent debris trails and dramatically enhanced star formation rates in an additional seven RPS candidates support the hypothesis that RPS, and hence rapid galaxy evolution in high-density environments, is intricately linked to cluster collisions. Unexpectedly, we find the vast majority of RPS candidates in A1758N to be moving toward us, and in a shared direction as projected on the plane of the sky. We hypothesize that this directional bias is the result of two successive events: (1) the quenching, during and after the first core passage, of star formation in galaxies with an approximately isotropic velocity distribution within the central region of the merger, and (2) RPS events triggered in late-type galaxies falling into the merging system along a filament, possibly enhanced by a shock front expanding into the outskirts of the south-eastern subcluster. Since this explanation implies that the merger axis of A1758N must be significantly inclined with respect to the plane of the sky, our findings open the possibility of RPS events becoming important diagnostic tools to constrain the geometry of cluster collisions that, due to the orientation of the merger axis, lack the classic observational signatures of face-on mergers.
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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.
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.
We report the detection of H$alpha$ trails behind three new intermediate-mass irregular galaxies in the NW outskirts of the nearby cluster of galaxies Abell 1656 (Coma). Hints that these galaxies possess an extended component were found in earlier, deeper H$alpha$ observations carried out with the Subaru telescope. However the lack of a simultaneous $r$-band exposure, together with the presence of strong stellar ghosts in the Subaru images, prevented us from quantifying the detections. We therefore devoted one full night of H$alpha$ observation to each of the three galaxies using the San Pedro Martir 2.1m telescope. One-sided tails of H$alpha$ emission of 10-20 kpc projected size were detected, suggesting an ongoing ram pressure stripping event. We added these 3 new sources of extended ionized gas (EIG) added to the 12 found by Yagi et al. (2010), NGC 4848 (Fossati et al. 2012), and NGC 4921 whose ram pressure stripping is certified by HI asymmetry. This brings the number sources with H$alpha$ trails to 17 gaseous tails out of 27 (63 %) late-type galaxies (LTG) galaxies members of the Coma cluster with direct evidence of ram pressure stripping. The 27 LTG galaxies, among these the 17 with extended H$alpha$ tails, have kinematic properties that are different from the rest of the early-type galaxy (ETG) population of the c ore of the Coma cluster, as they deviate in the phase-space diagram $Delta$V/$sigma$ versus $r/R_{200}$.
X-ray studies of jellyfish galaxies play a crucial role in understanding the interactions between the interstellar medium (ISM) and the intracluster medium (ICM). In this paper, we focused on the jellyfish galaxy JO201. By combining archival Chandra observations, MUSE H$alpha$ cubes, and maps of the emission fraction of the diffuse ionised gas, we investigated both its high energy spectral properties and the spatial correlation between its X-ray and optical emissions. The X-ray emission of JO201 is provided by both the Compton thick AGN (L$_{text{X}}^{0.5-10 text{keV}}$=2.7$cdot$10$^{41}$ erg s$^{-1}$, not corrected for intrinsic absorption) and an extended component (L$_{text{X}}^{0.5-10 , text{keV}}approx$1.9-4.5$cdot$10$^{41}$ erg s$^{-1}$) produced by a warm plasma (kT$approx$1 keV), whose luminosity is higher than expected from the observed star formation (L$_{text{X}}sim$3.8$cdot10^{40}$ erg s$^{-1}$). The spectral analysis showed that the X-ray emission is consistent with the thermal cooling of hot plasma. These properties are similar to the ones found in other jellyfish galaxies showing extended X-ray emission. A point-to-point analysis revealed that this X-ray emission closely follows the ISM distribution, whereas CLOUDY simulations proved that the ionisation triggered by this warm plasma would be able to reproduce the [OI]/H$alpha$ excess observed in JO201. We conclude that the galactic X-ray emitting plasma is originated on the surface of the ISM as a result of the ICM-ISM interplay. This process would entail the cooling and accretion of the ICM onto the galaxy, which could additionally fuel the star formation, and the emergence of [OI]/H$alpha$ excess in the optical spectrum.
In the current epoch, one of the main mechanisms driving the growth of galaxy clusters is the continuous accretion of group-scale halos. In this process, the ram pressure applied by the hot intracluster medium on the gas content of the infalling group is responsible for stripping the gas from its dark-matter halo, which gradually leads to the virialization of the infalling gas in the potential well of the main cluster. Using deep wide-field observations of the poor cluster Hydra A/A780 with XMM-Newton and Suzaku, we report the discovery of an infalling galaxy group 1.1 Mpc south of the cluster core. The presence of a substructure is confirmed by a dynamical study of the galaxies in this region. A wake of stripped gas is trailing behind the group over a projected scale of 760 kpc. The temperature of the gas along the wake is constant at kT ~ 1.3 keV, which is about a factor of two less than the temperature of the surrounding plasma. We observe a cold front pointing westwards compared to the peak of the group, which indicates that the group is currently not moving in the direction of the main cluster, but is moving along an almost circular orbit. The overall morphology of the group bears remarkable similarities with high-resolution numerical simulations of such structures, which greatly strengthens our understanding of the ram-pressure stripping process.
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