Simulator of Galaxy Millimeter/Submillimeter Emission (SIGAME): The [CII]-SFR Relationship of Massive z=2 Main Sequence Galaxies

Abstract

We present S\'IGAME simulations of the [CII]157.7$\mu$ fine structure line emission from cosmological smoothed particle hydrodynamics (SPH) simulations of seven main sequence galaxies at z=2. Using sub-grid physics prescriptions the gas in our simulations is modeled as a multi-phased interstellar medium (ISM) comprised of molecular gas residing in giant molecular clouds, an atomic gas phase associated with photo-dissociation regions (PDRs) at the cloud surfaces, and a diffuse, ionized gas phase. Adopting logotropic cloud density profiles and accounting for heating by the local FUV radiation field and cosmic rays by scaling both with local star formation rate (SFR) volume density, we calculate the [CII] emission using a photon escape probability formalism. The [CII] emission peaks in the central $\lesssim$1 kpc of our galaxies as do the SFR radial profiles, with most [CII] ($\gtrsim$70%) originating in the molecular gas phase, whereas further out ($\gtrsim$2 kpc), the atomic/PDR gas dominates ($\gtrsim$90%) the [CII] emission, no longer tracing on-going star formation. Throughout, the ionized gas contribution is negligible ($\lesssim$3%). The [CII] luminosity vs. SFR ([CII]-SFR) relationship, integrated as well as spatially resolved (on scales of 1 kpc), delineated by our simulated galaxies is in good agreement with the corresponding relations observed locally and at high redshifts. In our simulations, the molecular gas dominates the [CII] budget at SFR$\gtrsim$20 M$_{\odot}$yr$^{-1}$ (SFR density $\gtrsim$0.5 M$_{\odot}$yr$^{-1}$kpc$^{-2}$), while atomic/PDR gas takes over at lower SFRs, suggesting a picture in which [CII] predominantly traces the molecular gas in high-density/pressure regions where star formation is on-going, and otherwise reveals the atomic/PDR gas phase.