Gas Stripping in Simulated Galaxies with a Multiphase ISM
Cluster galaxies moving through the intracluster medium (ICM) are expected to lose some of their interstellar medium (ISM) through ISM-ICM interactions. We perform high resolution (40 pc) three-dimensional hydrodynamical simulations of a galaxy undergoing ram pressure stripping including radiative cooling in order to investigate stripping of a multiphase medium. The clumpy, multiphase ISM is self-consistently produced by the inclusion of radiative cooling, and spans six orders of magnitude in gas density. We find no large variations in the amount of gas lost whether or not cooling is involved, although the gas in the multiphase galaxy is stripped more quickly and to a smaller radius. We also see significant differences in the morphology of the stripped disks. This occurs because the multiphase medium naturally includes high density clouds set inside regions of lower density. We find that the lower density gas is stripped quickly from any radius of the galaxy, and the higher density gas can then be ablated. If high density clouds survive, through interaction with the ICM they lose enough angular momentum to drift towards the center of the galaxy where they are no longer stripped. Finally, we find that low ram pressure values compress gas into high density clouds that could lead to enhanced star formation, while high ram pressure leads to a smaller amount of high-density gas.
💡 Research Summary
This study investigates ram‑pressure stripping of a galaxy moving through the intracluster medium (ICM) using high‑resolution (40 pc) three‑dimensional hydrodynamical simulations that explicitly include radiative cooling. By allowing cooling to operate, the interstellar medium (ISM) naturally develops a multiphase structure spanning six orders of magnitude in density, from diffuse warm gas (∼10⁻⁴ cm⁻³) to dense molecular‑cloud‑like clumps (∼10² cm⁻³). Two sets of simulations are compared: one with cooling (producing a clumpy, multiphase disk) and one without cooling (a single‑phase, smooth disk). Both runs share identical galaxy mass models, orbital parameters, and ICM wind properties, enabling a clean assessment of how the internal phase structure influences stripping.
The main findings are as follows. First, the total amount of gas removed from the galaxy after several hundred Myr is remarkably similar in the cooling and non‑cooling cases, confirming that the integrated ram‑pressure force dominates the mass budget. However, the temporal evolution and spatial distribution of the stripped gas differ substantially. In the multiphase runs, low‑density gas is stripped efficiently from all radii, leaving behind a compact, high‑density core. The dense clumps survive longer because they are embedded in a low‑density background that is removed first; once exposed, they experience direct ICM‑cloud interaction. This interaction removes angular momentum from the clumps, causing them to drift inward toward the galactic centre where the gravitational potential shields them from further stripping. Consequently, the stripped disk radius is smaller and the morphology of the tail is more filamentary, with isolated high‑density knots embedded in a diffuse wake.
A second key result concerns the dependence on ram‑pressure strength. At low ram pressure, the ICM wind compresses the outer disk, converting some of the diffuse gas into higher‑density clouds. This compression can raise the fraction of gas above typical star‑formation thresholds, suggesting a possible burst of star formation triggered by the cluster environment. In contrast, at high ram pressure the wind is strong enough to ablate even the dense clumps, reducing the overall high‑density gas fraction and suppressing any compression‑induced star formation. Thus, the same physical process can either enhance or quench star formation depending on the wind’s intensity.
Methodologically, the 40 pc resolution is sufficient to resolve giant molecular cloud scales, allowing the simulations to capture cloud‑ICM drag, ablation, and angular‑momentum loss realistically. Nevertheless, the models omit magnetic fields, thermal conduction, and explicit stellar feedback (e.g., supernovae, radiation pressure). These processes could alter cloud survival times, modify the mixing layer between ICM and ISM, and affect the net star‑formation response. The authors acknowledge these limitations and propose future work incorporating magnetohydrodynamics and feedback to refine the picture.
In the broader astrophysical context, the results provide a physical explanation for several observational trends. Cluster spirals often display truncated H I disks but retain CO‑bright central concentrations; the simulations reproduce this by showing that low‑density atomic gas is stripped quickly while dense molecular clumps migrate inward. Moreover, observations of enhanced H α or UV emission in the leading edges of galaxies undergoing mild ram pressure are consistent with the compression‑induced high‑density cloud formation reported here. Conversely, galaxies experiencing strong ram pressure show little ongoing star formation, matching the simulated destruction of dense clumps at high wind strengths.
In summary, this work demonstrates that the internal multiphase structure of a galaxy’s ISM critically shapes the morphology, timescale, and star‑formation consequences of ram‑pressure stripping. While the total gas loss may be set by the external ram pressure, the fate of dense clouds—whether they survive, migrate inward, or are ablated—determines the residual star‑forming reservoir and the observable signatures of stripped galaxies in clusters. Future simulations that add magnetic fields and stellar feedback will be essential to fully capture the complex interplay between the ICM wind and a realistic, turbulent ISM.