On the regulation of the cold neutral medium mass fraction by magnetic fields

In the Galactic plane, the ratio of the cold HI mass to the total HI mass appears to remain approximately constant for galactocentric radii, Rg, between ∼10 and ∼20 kpc, despite differences in physical conditions. To explore the possible role of the mean magnetic field in the radial regulation of the cold HI (CNM) mass fraction, we performed numerical simulations of turbulent cubic boxes of 250 and 125 pc by side, including cooling functions and initial conditions tailored to model the HI gas at Rg equal to 8.5, 11, 15, and 18 kpc. We find that for these distances, a sufficiently large forcing scale is enough to maintain the CNM mass fraction approximately constant between 11 and 18 kpc albeit at values that are too high compared to those observed. In the presence of a magnetic field, the same radial behaviour is obtained but with values that are more consistent with those observed. When a radial reduction of the mean magnetic field is taken into account, the radial trend in the cold gas mass fraction is stronger and, in addition, both the ratio between the thermal and magnetic pressure, as well as the Alfvénic Mach number in the CNM, also remain approximately constant with Rg, suggesting the possibility that some properties of the CNM related to the magnetic field may also be approximately constant with the galactocentric distance, while the properties of the warm HI gas have a radial dependence.