To Survive or to Shatter: The Impact of Cosmic Rays on the Fate of Stripped Cold Clouds

TL;DR

This study demonstrates that cosmic-ray pressure can substantially alter the fate of ram-pressure stripped cold gas in the circumgalactic medium. Using high-resolution MFM simulations with varying CR energy densities in a Milky Way–like halo, the authors show that CRs puff up cold clouds, suppress fragmentation, and enhance mixing-layer cooling, leading to larger cloud masses and higher cold-gas content in the CGM. The strongest CR effects occur when CR energy density is in equipartition with the thermal gas, increasing the CGM cold gas mass by up to a factor of four and boosting the inflow of cold gas to the central galaxy by up to 2–8×, which in turn elevates the star formation rate at late times. These results imply a critical role for CRs in regulating the cold phase of the CGM and the fuelling of galaxies, especially in satellite-rich environments, and underscore the need for physically accurate CR transport models in simulations.

Abstract

Does cosmic ray (CR) pressure matter for the circumgalactic medium (CGM)? Despite growing interest, this remains a debated question, complicated by limited observational constraints and differing implementations of CR physics in simulations. While prior studies suggest that CRs influence the thermal and dynamical state of the CGM, their role in shaping cold gas structures remains underexplored. This paper investigates how CRs affect ram-pressure stripped cold gas clouds originating from satellite galaxies in a Milky Way-like halo. Using high-resolution simulations with varying CR energy densities, we find that CRs can significantly modify the size and survival of stripped clouds. Specifically, CR pressure puffs up the cold clouds, increasing their surface area and enabling more efficient mixing-layer cooling, allowing them to grow in mass. This enhanced growth results in higher cold gas inflow rates into the central galaxy, leading to an increase in the star formation rate compared to the no-CR case at a later time. Moreover, CRs can boost the total cold gas mass in the CGM by up to a factor of four. These effects are most pronounced in simulations where the CR energy density is in equipartition with the thermal gas. Our results demonstrate that CRs can play a critical role in regulating the cold phase of the CGM contributed by satellites and therefore their ability to feed galaxies.

Figures (9)

• Figure 1: The temperature distribution of different simulations (see Table \ref{['t:runs']}) at snapshot t = 1 Gyr. The colorbar varies both color and saturation based on $f_{\rm sat}$ and temperature, respectively. The parameter $f_{\rm sat}$ represents the local mass fraction that originated from the satellite, such that $f_{\rm sat}=1$ is entirely composed of satellite gas and $f_{\rm sat}=0$ is entirely host gas. The stripped cold gas is streaming behind the satellites and falling towards the central disk. There is also induced cool gas in the mixing layer of stripped cool gas and hot host gas. The top and bottom panels denote m10 and m09 runs, respectively. In each panel, there is an increasing amount of CRs from left to right, with the left-most being no-CR and the right-most being the highest amount of CRs. The ram-pressure-stripped clouds grow more and get bigger with an increase in the CR content (Left to Right).
• Figure 2: Size and mass distribution of cold gas clouds identified using the Friends-of-Friends (FoF) algorithm across three time bins up to 1.5 Gyr (top to bottom rows). The left and right panels correspond to the m09 and m10 simulations, respectively. Each panel includes one-dimensional histograms along the top and right axes, showing the distributions of cloud mass and size, respectively. Cold clouds are defined using a temperature cut of $T < 3 \times 10^4$ K and identified using a linking length of 2 kpc with a minimum group size of 10 particles. CRs enhance the formation of larger, more coherent cold gas clouds by suppressing fragmentation and enabling their long-term survival and growth through enhanced mixing-layer cooling.
• Figure 3: Time evolution of cold gas mass ($T \leq 3\times10^4$ K) beyond 40 kpc from the host galaxy center, split into the four categories defined in Section \ref{['S:Def']}: cold gas inside satellites (1), stripped cold gas (2), host gas cooled inside satellites (3), and host gas cooled outside satellites (4). Top panel: Total cold gas in the CGM and satellites (left; sum of middle and bottom panels) and total cold gas in the CGM only, excluding satellite-bound gas (right; sum of right panels in middle and bottom rows). Middle panel: Cold gas inside satellites (left) and stripped from satellites (right; defined as cold satellite gas beyond six satellite scale radii). Bottom panel: Host cold gas cooled inside satellites (left) and outside satellites (right). An increase in CR pressure enhances total stripped cold gas mass, mixing layer cooling along with the total cold gas mass, with the largest difference between no-CR and high (h) and mid (m2) CR runs.
• Figure 4: Covering fraction of cold gas ($T < 3 \times 10^4$ K) as a function of time for different CR strengths for the m10 (right panel) and m09 (left panel) runs. The bottom panel shows the covering fraction for a Milky Way (MW) view, with sightlines originating from the galactic center, while the top panel represents an external galaxy view using parallel sightlines. In both geometries, the covering fraction increases with CR content, with the high-CR runs showing the most significant enhancement, up to $\sim$2--8 times higher compared to no-CR.
• Figure 5: The time evolution of the net cold (T$\leq3\times10^4$K) gas mass flux (instantaneous by light-shaded and time-integrated by dark-shaded) at $40$ kpc (Right) and $80$kpc (Left) shell of $1$ kpc width for the m09 case (where negative indicates inflow). The top and bottom panels indicate the mass flux from satellite gas and host gas, respectively. Increasing CR content enhances the inflow of cold gas into the central galaxy, with the high-CR run exhibiting an inflow rate nearly $\sim2-4$ times that of the no-CR case.