The influence of magnetic field on the cold neutral medium mass fraction and its alignment with density structures

Abstract

To contribute to the understanding of the magnetic field's influence on the segregation of cold neutral medium (CNM) in the solar neighbourhood we analyse magnetohydrodynamic simulations that include themain physical characteristics of the local neutral atomic interstellar medium. The simulations have a continuous solenoidal Fourier forcing in a periodic box of 100 pc per side and an initial uniform magnetic field (B 0 ) with intensities ranging between ~0.4 and ~8 μG. Our main results are as follows. (i) The CNM mass fraction diminishes with the increase in magnetic field intensity. (ii) There is a preferred alignment between CNM structures and B in all our B 0 range but the preference weakens as B 0 increases. It is worth noticing that this preference is also present in two-dimensional projections making an extreme angle (0 or π/2) with respect to B 0 and it is only lost for the strongest magnetic field when the angle of projection is perpendicular to B 0 . (iii) The aforementioned results are prevalent despite the inclusion of self-gravity in our continuously forced simulations with a mean density similar to the average value of the solar neighbourhood. (iv) Given a fixed B 0 and slightly higher mean densities, up to double, the effects of self-gravity are still not qualitatively significant.