The Density-Magnetic Field Relation in the Atomic ISM

TL;DR

This study uses numerical simulations to explore how magnetic field strength correlates with density in the cold atomic interstellar medium, revealing the influence of turbulence, gravity, and initial conditions on this relationship.

Contribution

It provides new insights into the B-n correlation in the CNM through comprehensive MHD simulations, highlighting the roles of self-gravity and turbulence in shaping this relation.

Findings

Positive B-n correlation develops in forced simulations.

Correlation threshold density depends on initial magnetic field.

Self-gravity modifies the slope of the B-n relation at high densities.

Abstract

We present numerical experiments aimed to study the correlation between the magnetic field strength, $B$, and the density, $n$, in the cold atomic interstellar medium (CNM). We analyze 24 magneto-hydrodynamic models with different initial magnetic field intensities ($B_0=$0.4, 2.1, 4.2, and 8.3 $\mu$G) and/or mean densities (2, 3, and 4 cm$^{-3}$), in the presence of driven and decaying turbulence, with and without self-gravity, in a cubic computational domain with 100 pc by side. Our main findings are: i) For forced simulations, which reproduce the main observed physical conditions of the CNM in the Solar neighborhood, a positive correlation between $B$ and $n$ develops for all the $B_0$ values. ii) The density at which this correlation becomes significant ($\lesssim 30$ cm$^{-3}$) depends on $B_0$ but is not sensitive to the presence of self-gravity. iii) The effect of self-gravity, when noticeable, consists of producing a shallower correlation at high densities, suggesting that, in the studied regime, self-gravity induces motions along the field lines. iv) Self-gravitating decaying models where the CNM is subsonic and sub-Alfv\'enic with $\beta\lesssim 1$ develop a high density positive correlation whose slopes are consistent with a constant $\beta(n)$. v) Decaying models where the low density CNM is subsonic and sub-Alfv\'enic with $\beta>1$ show a negative correlation at intermediate densities, followed by a high density positive correlation.