The regulation of star formation by cosmic ray heating in the low–metallicity interstellar medium
Vittoria Brugaletta
Low-metallicity environments experience a more inefficient cooling due to the lack of coolants. Therefore, appropriately modelling of the main heating rates is essential to describe the evolution of the multi-phase interstellar medium in metal-poor conditions. Since the photoelectric heating is dependent on the dust-to-gas-ratio, which in turn decreases for low metallicity, its efficiency is lowered. On the other side, the heating due to low-energy cosmic rays is independent on metallicity. Therefore, at a metallicity threshold of 0.02 Z⊙ the photoelectric heating is comparable, if not lower, than the cosmic-ray heating (Brugaletta et al. in prep.). In our work we describe the evolution of the interstellar medium with a metallicity of 0.02 Z⊙ using two new models for the characterization of the photoelectric heating and the cosmic-ray heating. In previous works, we assumed both of them to depend on a constant interstellar radiation field strength in Habing units, G0, and a constant cosmic-ray ionization rate, ζ, respectively. In this work (Brugaletta et al. in prep.) we include the AdaptiveG0 module from Rathjen et al. in prep. , which computes the parameter G0 self-consistently with the present stellar population, and a newly-implemented method to compute ζ from the energy density of cosmic rays.
The different impact that these two methods have on the metal-poor gas can be seen in Fig. 1, where we show the temperature-density and pressure-density phase plots. If we assume constant G0 = 1.7 and ζ = 3 x 10-17 s-1 (run Z0.02), the heating rates are so high that the gas is unable to cool down, but it exists in a warm phase. If we assume a variable G0 and a constant ζ (run Z0.02-vG0) the interstellar medium is able to cool down to a certain temperature, however it is not sufficient to form stars. Assuming both G0 and ζ to be variable (runs Z0.02-vG0-vζ and Z0.02-vG0-vζ-BS) lets the gas cool down and form stars. The difference between the last two simulations is a different scaling of the dust-to-gas ratio with metallicity.