A characteristic scale for cold gas

作者：McCourt, Michael^*; Oh, S. Peng^*; O'Leary, Ryan; Madigan, Ann-Marie

来源：Monthly Notices of the Royal Astronomical Society, 2018, 473(4): 5407-5431.

DOI：10.1093/mnras/stx2687

• MHD
• plasmas
• ISM: clouds
• Galaxy: centre
• Galaxy: halo

We find that clouds of optically thin, pressure-confined gas are prone to fragmentation as they cool below similar to 10(6) K. This fragmentation follows the lengthscale similar to c(s)t(cool), ultimately reaching very small scales (similar to 0.1 pc/n), as they reach the temperature similar to 10(4) K at which hydrogen recombines. While this lengthscale depends on the ambient pressure confining the clouds, we find that the column density through an individual fragment N-cloudlet similar to 10(17) cm(-2) is essentially independent of environment; this column density represents a characteristic scale for atomic gas at 10(4) K. We therefore suggest that 'clouds' of cold, atomic gas may, in fact, have the structure of a mist or a fog, composed of tiny fragments dispersed throughout the ambient medium. We show that this scale emerges in hydrodynamic simulations, and that the corresponding increase in the surface area may imply rapid entrainment of cold gas. We also apply it to a number of observational puzzles, including the large covering fraction of diffuse gas in galaxy haloes, the broad-line widths seen in quasar and AGN spectra and the entrainment of cold gas in galactic winds. While our simulations make a number of assumptions and thus have associated uncertainties, we show that this characteristic scale is consistent with a number of observations, across a wide range of astrophysical environments. We discuss future steps for testing, improving and extending our model.

• 出版日期2018-2