Ram pressure stripping in clusters: Gravity can bind the ISM but not the CGM (2404.02035v2)

Abstract: We explore the survival of a galaxy's circumgalactic medium (CGM) as it experiences ram pressure stripping (RPS) moving through the intracluster medium (ICM). For a satellite galaxy, the CGM is often assumed to be entirely stripped/evaporated, an assumption that may not always be justified. We carry out 3D-hydrodynamic simulations of the interstellar and circumgalactic media (ISM+CGM) of a galaxy like JO201 moving through the ICM. The CGM can survive long at cluster outskirts ($\gtrsim2 \rm \ Gyr$) but at smaller cluster-centric distances, 90\% of the CGM mass is lost within $\sim 500$ Myr. The gravitational restoring force on the CGM is mostly negligible and the CGM-ICM interaction is analogous to \textit{cloud-wind interaction'}. The CGM stripping timescale does not depend on the ram pressure but on the CGM to ICM density contrast $\chi$. Two distinct regimes emerge for CGM stripping: the $\chi \>1$ regime, which is the well-known \textit{cloud crushing'} problem, and the $\chi <1$ regime, which we refer to as the (relatively unexplored) \textit{`bubble drag'} problem. The first pericentric passage near the cluster core can rapidly -- over a crossing time $t_{\rm drag} \sim R/v_{\rm rel}$ -- strip the CGM in the \textit{bubble drag} regime. The ISM stripping criterion unlike the CGM criterion, still depends on the ram pressure $\rho_{\rm ICM} v_{\rm rel}^2$. The stripped tails of satellites contain contributions from both the disk and the CGM. The X-ray plume in M89 in the Virgo cluster and a lack of it in the nearby M90 might be attributed to their orbital histories. M90 has likely undergone stripping in the bubble drag regime due to a pericentric passage close to the cluster center.

• The paper demonstrates that 3D hydrodynamic simulations reveal gravity effectively binds the ISM against ram pressure while leaving the CGM vulnerable.
• The paper identifies two distinct CGM stripping regimes: cloud crushing when denser than the ICM and bubble drag when less dense.
• The paper shows that prolonged CGM stripping compromises gas reservoirs, ultimately influencing star formation and accelerating galaxy quenching.

Overview: Gravity Can Bind the ISM but Not the CGM

This paper provides a detailed investigation into the dynamics of ram pressure stripping (RPS) affecting satellite galaxies as they move through the intracluster medium (ICM). By utilizing three-dimensional hydrodynamic simulations, the paper elucidates how the interplay between a galaxy's interstellar medium (ISM), circumgalactic medium (CGM), and the encompassing ICM dictates the mass exchange and ionization processes over a galaxy's orbital lifetime. The work extensively explores the underlying mechanisms that differentiate the stripping of the CGM from the ISM due to their respective interactions with gravitational forces, i.e., the CGM is largely unaffected by gravitational binding, contrasting the more strongly bound ISM.

Simulation Framework and Key Results

The simulations incorporate models of galaxies similar in characteristics to jellyfish galaxies such as JO201, employing a mass of $10^{12} M_{\odot}$ for dark matter halos and accounting for both ISM and CGM components, thereby providing a comprehensive view of their interaction with the ICM. Key parameters include relative velocities ranging from 400-3200 km/s and ambient ICM densities between $10^{-5}$ to $10^{-2}$ cm$^{-3}$, effectively covering a broad spectrum of atmospheric conditions representative of different cluster-centric distances.

Significantly, the paper delineates two distinct regimes relevant to CGM stripping based on the density contrast $\chi$ between the CGM and the ICM:

• Cloud Crushing Regime ($\chi > 1$): Here, the CGM is denser than the ICM. The gravitational forces exert minimal resistance, resulting in CGM undergoing hydrodynamic instabilities similar to the well-studied `cloud crushing’ problem where the drag time is $\chi R_{\rm CGM}/v_{\rm rel}$.
• Bubble Drag Regime ($\chi < 1$): In this novel regime, the CGM, being less dense than the ICM, interacts via a drag process that is predominantly independent of density contrast but directly proportional to the CGM size relative to the wind velocity, i.e., $R_{\rm CGM}/v_{\rm rel}$.

The Implications of CGM and ISM Stripping

The findings underscore a key implication for galaxy evolution: while the ISM is subject to rapid stripping driven by the Gunn & Gott criterion for ram pressure, the CGM is subjected to a prolonged stripping process, governed by its classification in the above regimes. This extended stripping process modifies the gas reservoir around the galaxy, ultimately affecting the sustained star formation rate, which could explain the observed high fraction of quenched galaxies in cluster environments.

Moreover, the simulations highlight that CGM dynamics significantly mediate the ram pressure experienced by the ISM. As observed, CGM provides a buffer that initially delays the direct impact of ICM wind on ISM, thereby shielding the star-forming regions for durations that could extend beyond several hundreds of Myr. This shielding delays immediate quenching but eventually contributes to long-term galaxy strangulation due to a lack of reservoir fueling future star formation.

Theoretical and Observational Insights

Observationally, the outcomes suggest new diagnostics for distinguishing stripping regimes based on X-ray and neutral gas studies of jellyfish galaxies. For instance, extended X-ray tails and ionized gas features can be reinterpreted acknowledging the differential survival timelines of CGMs in low-versus-high density contrast regimes. Similarly, the work implies that metallicity gradients within stripped tails, commonly observed in galaxies, are inherently linked with interaction histories between ISM-CGM-ICM triads.

Future Developments and Questions

Future work could expand on these insights by integrating factors like radiative cooling, gravitational feedback, and magnetic fields into the simulations. Detailed exploration into how AGN and supernovae feedback modifies the ISM-CGM interface could lead to refined models, potentially narrowing the broad assumptions about CGM’s passive role. Additionally, incorporating turbulent ICM simulations could delineate more realistic dynamics, given the inherent energies involved in cluster-scale interactions.

In summarizing, this paper significantly enriches our theoretical understanding of RPS by dissecting the nuanced roles of CGM versus ISM interactions. While the role of gravity on these interactions is re-evaluated, the paper presents comprehensive modeling that lays the groundwork for future explorations into galaxy evolution amid the dense and active environments of galaxy clusters.