Molecular cloud formation by compression of magnetized turbulent gas subjected to radiative cooling

Abstract

Complex turbulent motions of magnetized gas are ubiquitous in the interstellar medium. The source of this turbulence, however, is still poorly understood. Previous work suggests that compression caused by supernova shockwaves, gravity, or cloud collisions, may drive the turbulence to some extent. In this work, we present threedimensional (3D) magnetohydrodynamic (MHD) simulations of contraction in turbulent, magnetized clouds from the warm neutral medium (WNM) of the ISM to the formation of cold dense molecular clouds, including radiative heating and cooling. We test different contraction rates and find that observed molecular cloud properties, such as the temperature, density, Mach number, and magnetic field strength, and their respective scaling relations, are best reproduced when the contraction rate equals the turbulent turnover rate. In contrast, if the contraction rate is significantly larger (smaller) than the turnover rate, the compression drives too much (too little) turbulence, producing unrealistic cloud properties. The relation σ 2 s = ln(1 + b 2 M 2 ) between logarithmic density fluctuations (σ s ) and turbulent Mach number (M) is found to be consistent with previous theoretical models that were based on artificially-driven isothermal turbulence. Here we find that the effective turbulence driving parameter of contraction-driven MHD turbulence subject to heating and cooling grows from solenoidal (b ∼ 1/3) to compressive (b ∼ 1) during the contraction. Overall, the physical properties of the simulated clouds that contract at a rate equal to the turbulent turnover rate, indicate that large-scale contraction induced by processes such as supernova shockwaves, gravity, spiral-arm compression, or cloud collisions, may explain the origin and evolution of turbulence in the ISM.

Low star formation efficiency due to turbulent adiabatic compression in the Taffy bridge

“…In this case, we presume that the velocity dispersion of the turbulent substructures or clouds also increases (Fig. 2 of Mandal et al 2020). Such clouds were observed in the Galactic Center region by Oka et al (1998Oka et al ( , 2001.…”

“…In these collisions, one gaseous body is the turbulent clumpy multiphase ISM, while the other can be of different mean density and temperature (for example ISM, intragroup or intracluster gas): NGC 4438 is affected by ongoing ram pressure caused by its rapid motion through the Virgo intracluster medium (Vollmer et al 2005(Vollmer et al , 2009, and the intragroup gas of the Stephan's Quintet is compressed by a highvelocity intruder galaxy (Appleton et al 2017). We suggest that the common theme of all these gas-gas interactions is adiabatic large-scale compression of the ISM leading to an increase in the turbulent velocity dispersion of the gas (Robertson & Goldreich 2012;Mandal et al 2020).…”

“…Following Robertson & Goldreich (2012) and Mandal et al (2020), we expect turbulent adiabatic heating, for example, an increase in the turbulent velocity dispersion due to the p dV work, to occur if the timescale of large-scale gas compression…”

“…In this case, we presume that the velocity dispersion of the turbulent substructures or clouds also increases (Fig. 2 of Mandal et al 2020). In our toy model, we decided to suppress star formation during a cloud-cloud collision if the energy injection by large-scale gas compression exceeds that from stellar feedback expected from an ISM that forms stars according to a Kennicutt-Schmidt law.…”

The Molecular Cloud Lifecycle

“…The effect of magnetic fields on this phase transition has been studied by e.g. Inutsuka (2008, 2009), Körtgen and Banerjee (2015), Valdivia et al (2016), van Loo et al (2007, Mandal et al (2020).…”