Thermal Phases of the Neutral Atomic Interstellar Medium – from Solar Metallicity to Primordial Gas. (arXiv:1902.06764v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Bialy_S/0/1/0/all/0/1">Shmuel Bialy</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sternberg_A/0/1/0/all/0/1">Amiel Sternberg</a>
We study the thermal structure of the neutral atomic (H {small I})
interstellar medium across a wide range of metallicities, from supersolar down
to vanishing metallicity, and for varying UV intensities and cosmic-ray
ionization rates. We calculate self-consistently the gas temperature and
species abundances (with a special focus on the residual H$_2$), assuming
thermal and chemical steady-state. For solar metallicity, $Z’ equiv 1$, we
recover the known result that there exist a pressure range over which the gas
is multiphased, with the warm ($sim 10^4$ K, WNM) and cold ($sim 100$ K, CNM)
phases coexisting at the same pressure. At a metallicity $Z’ approx 0.1$, the
CNM is colder (compared to $Z’=1$) due to the low efficiency of photoelectric
heating. For $Z’ lesssim 0.1$, cosmic-ray ionization becomes the dominant
heating mechanism and the WNM-to-CNM transition shifts to ever increasing
pressure/density as the metallicity is reduced. For metallicities $Z’ lesssim
0.01$, H$_2$ cooling becomes important, lowering the temperature of the WNM
(down to $approx 600$ K), and smoothing out the multiphase phenomenon. At
vanishing metallicities, H$_2$ heating becomes effective and the multiphase
phenomenon disappears entirely. We derive analytic expressions for the critical
densities for the warm-to-cold phase transition in the different regimes, and
the critical metallicities for H$_2$ cooling and heating. We discuss potential
implications on the star-formation rates of galaxies and self-regulation
theories.
We study the thermal structure of the neutral atomic (H {small I})
interstellar medium across a wide range of metallicities, from supersolar down
to vanishing metallicity, and for varying UV intensities and cosmic-ray
ionization rates. We calculate self-consistently the gas temperature and
species abundances (with a special focus on the residual H$_2$), assuming
thermal and chemical steady-state. For solar metallicity, $Z’ equiv 1$, we
recover the known result that there exist a pressure range over which the gas
is multiphased, with the warm ($sim 10^4$ K, WNM) and cold ($sim 100$ K, CNM)
phases coexisting at the same pressure. At a metallicity $Z’ approx 0.1$, the
CNM is colder (compared to $Z’=1$) due to the low efficiency of photoelectric
heating. For $Z’ lesssim 0.1$, cosmic-ray ionization becomes the dominant
heating mechanism and the WNM-to-CNM transition shifts to ever increasing
pressure/density as the metallicity is reduced. For metallicities $Z’ lesssim
0.01$, H$_2$ cooling becomes important, lowering the temperature of the WNM
(down to $approx 600$ K), and smoothing out the multiphase phenomenon. At
vanishing metallicities, H$_2$ heating becomes effective and the multiphase
phenomenon disappears entirely. We derive analytic expressions for the critical
densities for the warm-to-cold phase transition in the different regimes, and
the critical metallicities for H$_2$ cooling and heating. We discuss potential
implications on the star-formation rates of galaxies and self-regulation
theories.
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