Jellyfish galaxies with the IllustrisTNG simulations: I. Gas-stripping phenomena in the full cosmological context (1810.00005v2)

Abstract: We use IllustrisTNG, a suite of gravity and MHD simulations, to study the demographics and properties of jellyfish galaxies in the full cosmological context. By jellyfish galaxies, we mean satellites orbiting in massive groups and clusters that exhibit highly asymmetric distributions of gas and gas tails. We use the TNG100 run and select galaxies at redshifts $z\le0.6$ with stellar mass exceeding $10^{9.5}{\rm M_\odot}$ and with host halo masses of $10^{13}-10^{14.6}\,{\rm M_\odot}$. Among more than about 6000 (2600) galaxies with stars (and some gas), we identify 800 jellyfish galaxies by visually inspecting their gas and stellar mass maps in random projections. About $31\%$ of cluster satellites are found with signatures of ram-pressure stripping and gaseous tails stemming from the main luminous bodies. This is a lower limit, since the random orientation entails a loss of about $30\%$ of galaxies that in an optimal projection would otherwise be identified as jellyfish. The connection with ram-pressure stripping is further confirmed by a series of findings: jellyfish galaxies are more frequent at intermediate and large cluster-centric distances ($r/R_{\rm 200c}\gtrsim 0.25$); they move through the ICM with larger bulk velocities and Mach numbers than the general cluster population, typically orbiting supersonically and experiencing larger ram pressures. Furthermore, the gaseous tails usually extend in opposite directions to the galaxy trajectory, with no relation between tail orientation and the host's center. The frequency of jellyfish galaxies shows a very weak dependence on redshift $(0\le z\le0.6)$ but larger fractions of disturbed gaseous morphologies occur in more massive hosts and at smaller satellite masses. Finally, jellyfish galaxies are late infallers ($< 2.5-3$ Gyrs ago, at $z=0$) and the emergence of gaseous tails correlates well with the presence of bow shocks in the ICM.

• The paper identifies jellyfish galaxies in the IllustrisTNG100 simulation through visual inspection of gas asymmetry to study their formation and evolution in cosmological contexts.
• Jellyfish morphology is strongly associated with ram-pressure stripping in massive host halos, constituting about 31% of gas-bearing satellites, with prevalence increasing in denser environments.
• Jellyfish galaxies are typically found at intermediate to large cluster distances moving at supersonic velocities, supporting ram-pressure over tidal stripping as the dominant mechanism and suggesting they are recent cluster infallers.

Jellyfish Galaxies in Cosmological Simulations: An Analysis with IllustrisTNG

The paper presented focuses on the phenomena of jellyfish galaxies within the framework of the IllustrisTNG cosmological simulations, particularly at low redshifts. Jellyfish galaxies, characterized by asymmetric gas distributions and tails, are intriguing objects found predominantly in dense environments such as massive galaxy clusters and groups. This paper conducts a comprehensive examination of these galaxies, leveraging the capabilities of the IllustrisTNG suite to explore the conditions under which these galaxies form and evolve.

Methodological Approach

The authors employed a visual inspection method to identify jellyfish galaxies from the TNG100 simulation dataset. The classification relied on the asymmetry in the gas distribution rather than specific wavelength observations, aiming to emulate the observational approaches applied to identify similar galaxies in real astronomical data. The sample was restricted to satellite galaxies with a stellar mass threshold of $10^{9.5} M_\odot$ within haloes of mass ranging from $10^{13} M_\odot$ to $10^{14.6} M_\odot$.

Analysis of Jellyfish Formation

One of the key findings is the association of jellyfish morphology with ram-pressure stripping, an environmental process expected in high-density regions of massive clusters. The paper reports that jellyfish galaxies constitute a significant fraction of satellites that retain their gas—about 31% of satellites with gas are jellyfish. The prevalence of jellyfish galaxies rises with host mass, suggesting that more massive halo environments intensify the ram-pressure stripping effect.

Orbital and Dynamical Characteristics

The spatial and kinematic analyses revealed that jellyfish galaxies are typically found at intermediate and large cluster-centric distances, often at supersonic velocities relative to the intra-cluster medium (ICM). This contrasts with the notion that tidal stripping, which would mainly affect galaxies closer to cluster centers, is the primary mechanism for their morphology. The paper found that the tails of jellyfish galaxies are predominantly anti-aligned with the direction of motion, corroborating the theory that ram-pressure stripping from the ICM is the driving factor.

Implications and Future Research

These findings underscore the significant role that environmental factors play in galaxy evolution, particularly in dense cluster environments. The paper suggests that jellyfish galaxies are recent infallers into clusters, as evidenced by their relatively short time since crossing the cluster virial radius.

Future work is expected to refine the classification of jellyfish galaxies through automated methods and to extend the analysis to larger simulation volumes and higher resolution datasets within the IllustrisTNG project. Furthermore, understanding the distinct phases of gas—whether HI, molecular, or ionized—affected in jellyfish galaxies could provide valuable insights into the ram-pressure stripping mechanism and its overall impact on galaxy evolution.

Conclusion

This paper provides a robust framework for identifying jellyfish galaxies in cosmological simulations and offers significant insights into the environmental processes that lead to their formation. The paper enhances our understanding of galaxy evolution in clusters, highlighting the conditions that favor the development of jellyfish-like morphologies. By advancing the methods to quantify and classify such galaxies, future research will likely deepen our comprehension of the dynamic interplay between galaxies and their environments in the cosmic web.