Predicting the resolved CO emission of $z=1-3$ star-forming galaxies

TL;DR

This study uses simulations to predict CO emission in high-redshift star-forming galaxies, revealing significant spatial variation in CO-to-H2 ratios and excitation conditions, impacting how molecular gas is traced observationally.

Contribution

It introduces a synthetic observation framework for CO emission in $z=1-3$ galaxies, highlighting the spatial variability of CO-to-H2 conversion factors and excitation ratios.

Findings

CO(1-0)-to-H2 ratio varies significantly within galaxy disks

Higher-J CO transitions trace dense central gas, while CO(1-0) traces diffuse outskirts

Tracing molecular gas beyond 3-5 kpc at high redshift is challenging with current facilities

Abstract

(Abridged) Resolved observations of the CO emission from $z=1-3$ star-forming galaxies are becoming increasingly common, with new high-resolution surveys on the horizon. We aim to inform the interpretation of this resolved CO emission by creating synthetic observations and testing to what extent routinely observed CO transitions can be used to trace H$_2$ across galaxy disks. To this end, we extract $z=1-3$ massive star-forming galaxies (on and above the main sequence) from the SIMBA cosmological simulation and predict their spatially resolved CO(1$-$0)-to-CO(5$-$4) emission using the $\texttt{SLICK}$ pipeline, which combines sub-resolution modeling of the cloud population with the DESPOTIC spectral line calculation code. We find that the CO(1$-$0)-to-H$_2$ ratio ($\alpha_{\rm CO}$) varies significantly within these galaxy disks$-$from values of $\sim1-5$ $\mathrm{M_\odot}$ (K km s$^{-1}$ pc$^2$)$^{-1}$ in the central 1-3 kpc of the most massive galaxies to $>100$ $\mathrm{M_\odot}$ (K km s$^{-1}$ pc$^2$)$^{-1}$ at $\sim$ 15 kpc. Thus, the use of a single $\alpha_{\rm CO}$ to derive the H$_2$ surface density leads to severe underestimates of the H$_2$ contribution in its outskirts. As expected, higher-$J$ CO transitions trace molecular gas in the centers at higher densities, whereas CO(1$-$0) better traces the more diffuse, extended molecular gas. We see significant variations in the CO excitation, with CO(3$-$2)/CO(1$-$0) line luminosity ratios of the most massive galaxies at $z\sim2$ declining from $\sim$ 0.9 in the galaxy centers to $\sim$ 0.1 in the outskirts. On average, line ratios increase substantially toward higher redshifts and lower galaxy stellar masses. We predict that tracing molecular gas with CO beyond 3-5 kpc of cosmic noon galaxies will be challenging with current facilities due to the drastic increase in $\alpha_{\rm CO}$.