Constraining Cosmic-ray transport with observations of the Circumgalactic medium
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Figure 1: Cosmic-ray transport is severely under-constrained. This toy plot highlights the fact that the existing observations only constrain cosmic-ray transport in the ~local ISM. It is not clear how to extrapolate these transport rates to other environments, for example, the ISM of other galaxies or the CGM. There is a wide range of existing cosmic-ray transport models that can match all of the relevant observations in the local ISM that then diverge by orders of magnitude in their predictions for the cosmic-ray transport rate in the CGM.
Cosmic rays fundamentally alter CGM structure
Galaxies evolve embedded in a vast gaseous halo that dwarfs the mass and spatial extent of its stars. In order to understand galaxy evolution, we must first understand the complex interplay between galaxies and their circumgalactic medium (CGM).
Much of my recent work has pioneered our understanding of how cosmic rays fundamentally alter the structure of the CGM, especially around low-redshift L* galaxies. Cosmic rays drive cool, mass-loaded outflows that enhance the CGM column densities of many metal ions (Butsky and Quinn 2018). Once in the CGM, cosmic-ray pressure support alters the morphology of cool gas, leading to large, low-density clouds that are out of thermal pressure equilibrium with the hot gas (Butsky et al. 2020). This effect leads to detectable differences in the kinematic signatures of multiphase CGM gas (Butsky et al. 2022).
Cosmic-ray Transport is severely under-constrained in the CGM
The problem is that all of our predictions for how cosmic rays affect the CGM (and in general, how cosmic rays affect galaxy evolution on all scales) is extremely sensitive to the invoked model of cosmic-ray transport, which is severely under-constrained in the CGM (Figure 1).
Estimating Cosmic-ray Transport using CGM observations
In this work, we build a toy model for estimating the cosmic-ray pressure profile as a function of distance from the galactic center (Figure 2). Using this profile, we show that we can estimate the lower-limit of the effective cosmic-ray transport rate in the CGM. using three observables: (1) the total hydrogen column density (at some projected radius from the galactic center) (2) the recent star formation rate, and (3) the circular velocity, measured at some projected radius from the galactic center, r. After vetting the toy model using FIRE-2 simulations, we use it to estimate the predicted lower-limit of the cosmic-ray transport rate in the COS-Halos galaxies (Werk et al. 2013; Figure 3). These first constraints show that the cosmic-ray transport rate is expected to increase in the CGM.
Figure 1: A (simplified) schematic of cosmic rays in the CGM. Galactic supernovae inject roughly 10% of their energy as cosmic-ray energy. Those cosmic rays move away from their injection site along tangled magnetic fields, creating a cosmic-ray pressure gradient that helps maintain hydrostatic equilibrium. This cosmic-ray pressure may be a significant, or even the dominant pressure source in the CGM around L* galaxies, however, the quantitative details (e.g., the exact cosmic-ray pressure or its radial extent) are sensitive to models of cosmic-ray transport, which remain unconstrained.
Figure 3: The first constraints to the cosmic-ray transport rate in the CGM. Using the analytic model in Butsky et al. 2023, the scattered points show the lower limit of the effective cosmic-ray transport rate for the COS-Halos galaxy sample (Werk at al. 2013). The clear increase in the effective cosmic-ray transport rate at large galactocentric distances rules out models of cosmic-ray transport in which the transport rate does not increase in the CGM.