Cold, warm, and hot gas phases are kinematically aligned but we don’t know why
Absorption -line studies of the CGM often find that absorption features from ions that trace a wide range of temperatures (10^4-10^6K) have the same line-of-sight velocity (Figure 1). It is not yet understood why this kinematic alignment is present and what implications it has for the structure of the CGM.
Figure 1: Example of kinematic alignment from the COS-Halos survey. Figure is taken from Tumlinson, Peeples, Werk 2017. HI, SiII, SiIII, SiIV, and OVI all trace a wide range of gas densities and temperatures, and yet their absorption features all have the same line-of-sight velocity. This hints at some physical relationship between the different gas phases.
We show that cosmic rays can significantly alter the kinematics of cold CGM gas
In this work, study the effect of cosmic rays on the kinematics of the CGM using two simulated case-study galaxies: one with a thermal-pressure-supported CGM and one with a cosmic-ray-pressure-supported CGM. In the cosmic-ray-pressure-supported CGM, we find that cold CGM gas has low densities, which leads to large, buoyant cold gas clouds that have radial velocities similar to that of the hot gas phase (Figure 2).
We generate realistic synthetic absorption spectra from our simulations and show that in a cosmic-ray-pressure supported CGM, not only is there more kinematic alignment between multiphase ions, but that kinematic alignment is also indicative of physical, spatial alignment of that gas (Figure 3)
Figure 2: Snapshots of the CGM of the two case-study galaxies. The different panels show the density, temperature, and line-of-sight velocity in the inner CGM supported by thermal pressure (top) and cosmic-ray pressure (bottom) at redshift 0.25. The cosmic-ray-pressure-supported CGM builds up a reservoir of cool, low-density gas, which has qualitatively different velocities from the small, dense cold clouds in the thermal-pressure-supported CGM.
Figure 3: An example of a synthetic spectrum generated from a simulated cosmic-ray-pressure-supported CGM. The top panel shows the line-of-sight velocities of Si III and O VI. The remaining panels show the physical properties along the line of sight of the particles that contributed to generating the spectra. This specific plot is an example of kinematic alignment due to a large, rotating cool gas cloud in the inner CGM enveloped by a warmer, co-rotating medium.