Pressure Regulated Formation of Molecular Clouds and Stars: The case of the Milky Way

Abstract

We present a steady-state analytical model for pressure-regulated formation of molecular clouds (MC) and stars (SF) in gaseous galactic disks and apply it to the Milky Way (MW). MC formation depends on midplane interstellar pressure $P_{\text{ISM}}$ and metallicity $Z$, and for galactocentric distances $R\gtrsim5$ kpc, $P_{\text{ISM}}(R)$ scales approximately linearly with molecular gas surface density $\Sigma_{\rm mol}(R)$. The molecularization of the cold neutral medium (CNM) is due to the opacity of small dust grains that protect the center of the cloud from dissociating radiation when the column density is $\Sigma_d\geq 5\ (Z_\odot/Z)M_\odot\text{ pc}^{-2}$. The H$_2$ formation rate per hydrogen atom is $F\sim10^{-15}(P_{\text{ISM}}/P_\odot)T_{100}^{-1/2}\text{s}^{-1}$, and the corresponding formation rate per unit area is $\dot{\Sigma}^{+}_{\rm mol}\sim 5\times10^{-2}\left(P_{\text{ISM}}/{P_\odot}\right)T_{100}^{-1/2}M_\odot~\text{kpc}^{-2}~\text{yr}^{-1}$, where $P_\odot$ is the pressure at the solar circle and $T_{100}=T/100\text{ K}$ is the temperature of the cloud. In equilibrium, this equals the molecular gas destruction rate $\dot{\Sigma}^{-}_{\rm mol}$ due to SF. Self-gravity sets in when the column density of a cloud reaches $\Sigma_{\rm sg}=\Sigma_{\rm sg,\odot}(P_{\text{ISM}}/P_\odot)^{1/2}$, with $\Sigma_{\rm sg,\odot}\sim30\ M_\odot\ \text{pc}^{-2}$. Given the distribution of $P_{\text{ISM}}(R)$ and $Z(R)$ in the MW, the SF process at $5\lesssim R\lesssim11$ kpc follows a two-step track: first, MCs form from CNM gas and then they form stars when self-gravity sets in. The resulting SFR surface density is $\Sigma_\text{SFR}(R)\approx (1.6-4)\times10^{-3}\left(P_{\text{ISM}}/P_\odot\right)\ \text{M}_\odot~\text {kpc}^{-2}\text{yr}^{-1}$ with an average final SF efficiency of $\epsilon_{\rm sf}\sim (3-8)\times 10^{-2}$.