The Environmental Dependence of the X_CO Conversion Factor

Abstract

is the most widely used observational tracer of molecular gas. The observable luminosity is translated to mass via a conversion factor, , which is a source of uncertainty and bias. Despite variations in , the empirically determined solar neighborhood value is often applied across different galactic environments. To improve understanding of , we employ 3D magnetohydrodynamics simulations of the interstellar medium (ISM) in galactic disks with a large range of gas surface densities, allowing for varying metallicity, far-ultraviolet (FUV) radiation, and cosmic-ray ionization rate (CRIR). With the TIGRESS simulation framework we model the three-phase ISM with self-consistent star formation and feedback, and post-process outputs with chemistry and radiation transfer to generate synthetic CO (1–0) and (2–1) maps. Our models reproduce the observed CO excitation temperatures, line widths, and line ratios in nearby disk galaxies. decreases with increasing metallicity, with a power-law slope of −0.8 for the (1–0) line and −0.5 for the (2–1) line. also decreases at higher CRIR and is insensitive to the FUV radiation. As density increases, first decreases owing to increasing excitation temperature and then increases when the emission is fully saturated. We provide fits between and observable quantities such as the line ratio, peak antenna temperature, and line brightness, which probe local gas conditions. These fits, which allow for varying beam size, may be used in observations to calibrate out systematic biases. We also provide estimates of the CO-dark fraction at different gas surface densities, observational sensitivities, and beam sizes.

Atomic and molecular gas from the epoch of reionisation down to redshift 2

“…We warn that, in principle, H 2 production could be enhanced by grain processes at any z, as long as the sites concerned have local metallicities of Z 0.1 Z ⊙ . This conclusion is consistent with detailed ISM analyses by for example Gong et al (2020) (who consider Z within 0.5-2 Z ⊙ ) and Hu et al (2021) (who explore Z within 0.1-3 Z ⊙ ).…”

“…Moreover, exploiting [C II] emission in high-z enriched environments (as lately done by Heintz et al 2021) would possibly open the door to observing Ω neutral at z > 6, where our results predict different scenarios depending on the chosen UV background. On the other hand, estimates of the reservoir of H 2 masses in star forming environments from [C II] or CO observational data are challenging and the conversion factors usually adopted bear uncertainties and biases (Madden et al 2020;Gong et al 2020;Breysse et al 2021). The [C II] 158 micron line is a workhorse for (sub-)mm observations and is resolved on a kiloparsec scale by ALMA (Rybak et al 2019).…”

“…For this reason, H 2 mass in targeted objects is usually inferred through empirical fits, based for example on emission from CO (Scoville & Solomon 1975;Dickman et al 1986;Maloney & Black 1988;Hughes et al 2017) or [C II] (Madden et al 1997;Pak et al 1998;Hollenbach & Tielens 1999) calibrated in the local Universe. Heavy elements and C-rich molecules have been detected in metal-enriched collapsing sites, suggesting that CO cores exist surrounded by a [C II] envelope where H 2 is self-shielded from photodissociating radiation (Röllig et al 2006;Wolfire et al 2010;Genzel et al 2012;Bolatto et al 2013;Zanella et al 2018;Madden et al 2020;Gong et al 2020). Notwithstanding the uncertainties related to calibration techniques, completeness and conversion factors, the currently estimated H 2 molecular-mass density in the local Universe is ρ H 2 ≃ 6.8×10 6 M ⊙ /Mpc 3 and accounts for about 20% of the total abundance of cold gas (Fletcher et al 2021).…”

Dependence of $X_{\rm CO}$ on metallicity, intensity, and spatial scale in a self-regulated interstellar medium

“…Recently, it has become feasible to simulate a kpc-scale ISM patch and follow star formation and stellar feedback self-consistently, establishing a more realistic, self-regulated system with pc-scale resolution such that the ISM structure and stellar feedback are resolved without resorting to sub-grid models (Gatto et al 2017;Seifried et al 2017;Gong et al 2018;Seifried et al 2020;Smith et al 2020). However, most of these studies focus on solar-metallicity, solar-neighborhood conditions, and the only exception (Gong et al 2020) studied X CO with a limited range of 0.5 ≤ Z ≤ 2, which still does not cover the "low-metallicity" regime.…”

Low-J CO Line Ratios From Single Dish CO Mapping Surveys and PHANGS-ALMA

“…In contrast to commonly used "dense gas tracers" like HCN (1−0) and HCO + (1−0), the CO lines are bright and can be studied across a range of environments (see Usero et al 2015, regarding relative line strengths). Numerical simulations can now resolve CO chemistry and predict CO line emission over whole molecular clouds, large parts of a spiral galaxy, or even entire dwarf galaxies (e.g., Glover & Clark 2012;Peñaloza et al 2017aPeñaloza et al , 2018Gong et al 2020;Hu et al 2021), but such calculations remain extremely challenging for tracers of higher density gas (e.g., Onus et al 2018). A combined observational, numerical, and analytic approach that leverages ratios among the low-J CO lines and their isotopologues represents a promising path forward to diagnose physical conditions in the molecular gas of galaxies.…”