[Paper Review] LoTSS jellyfish galaxies: II. Ram pressure stripping in groups versus clusters

Ian Roberts, R. J. van Weeren+4 more|arXiv (Cornell University)|Jan 1, 2021

Galaxies: Formation, Evolution, Phenomena138 references70 citations

TL;DR

This study uses LOFAR's 144 MHz radio continuum survey to identify jellyfish galaxies—galaxies with ram pressure stripped tails—in ~500 low-redshift galaxy groups (10^12.5 < M_group < 10^14 h⁻¹ M⊙), comparing their properties to those in clusters. It finds that ram pressure stripping occurs in groups, but is less frequent and less efficient than in clusters, with group jellyfish galaxies showing no enhanced star formation and slower stripping dynamics.

ABSTRACT

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.

Motivation & Objective

• To investigate the prevalence and efficiency of ram pressure stripping (RPS) in galaxy groups, the most common local Universe environment.
• To compare RPS in groups (lower mass) with that in clusters (higher mass) using a uniform radio continuum detection method.
• To determine whether jellyfish galaxies in groups exhibit enhanced star formation or faster stripping, as seen in clusters.
• To assess the role of group environments in pre-processing galaxies before cluster infall, particularly via RPS.

Proposed method

• Utilized the LOFAR Two-metre Sky Survey (LoTSS) at 144 MHz to detect extended, asymmetric radio continuum tails indicative of cosmic-ray and relativistic electron stripping.
• Selected jellyfish galaxies in groups based on radio tail morphology and alignment opposite to galaxy motion, confirming ram pressure stripping.
• Cross-matched with the R21b sample of jellyfish galaxies in clusters (z < 0.05) to enable direct comparison across three decades in halo mass.
• Analyzed phase-space positions and tail orientations to infer stripping timescales and pericentric passage history.
• Measured star formation rates (SFRs) via Hα fluxes from SDSS imaging to compare with 'normal' star-forming group galaxies.
• Used group mass estimates from SDSS group catalogs (M_group from 10^12.5 to 10^14 h⁻¹ M⊙) to assess mass dependence of RPS frequency.

Experimental results

Research questions

• RQ1 What is the frequency of jellyfish galaxies in galaxy groups compared to clusters, across a range of group masses?
• RQ2 How does the efficiency of ram pressure stripping differ between groups and clusters, as indicated by tail morphology and kinematics?
• RQ3 Do jellyfish galaxies in groups show enhanced star formation rates, as observed in cluster jellyfish galaxies?
• RQ4 Is there evidence that group jellyfish galaxies have already passed pericentre, suggesting slower stripping compared to cluster galaxies?
• RQ5 To what extent do group environments contribute to pre-processing of galaxies prior to cluster accretion?

Key findings

• The frequency of jellyfish galaxies is highest in clusters and decreases significantly toward the lowest mass groups (10^12.5 < M_group < 10^14 h⁻¹ M⊙).
• Jellyfish galaxies in groups are more likely to have already passed pericentre, indicating slower stripping timescales compared to cluster galaxies, which are typically still in the process of stripping.
• Unlike in clusters, jellyfish galaxies in groups do not show systematically enhanced star formation rates, remaining consistent with 'normal' star-forming group galaxies.
• The orientation of radio tails in groups is consistent with ongoing, but less intense, ram pressure stripping, suggesting weaker ICM densities or lower relative velocities than in clusters.
• There is clear evidence for ram pressure stripping in galaxy groups down to masses as low as 10^12.5 h⁻¹ M⊙, though the process is less efficient than in clusters.
• Differences in stripping efficiency between groups and clusters likely contribute to the observed positive correlation between quenched fraction and host halo mass in the local Universe.