The tight correlation of PAH and CO emission from z~0-4

Abstract

Aim: The cold molecular gas mass is one of the crucial, yet challenging parameters in galaxy evolution studies. Here, we introduce a new calibration for estimating molecular gas masses using mid-infrared (MIR) photometry. This topic is timely, as JWST now allows us to detect the MIR emission of typical main-sequence galaxies across a wide range of masses and star formation rates with modest time investments. This Letter highlights the strong synergy between ALMA and JWST for studies of dust and gas at cosmic noon. Methods: We combine a sample of 14 main sequence galaxies at z=1-3 with robust CO detections and multi-band MIR photometry, along with a literature sample at z=0-4 with CO and PAH spectroscopy, to study the relationship between PAH, CO(1-0), and total IR luminosities. PAH luminosities are derived from modeling rest-frame UV to sub-mm data. The new z=1-3 sample extends previous high-z studies to about an order-of-magnitude lower PAH and CO luminosities, into the regime of local starbursts for the first time. Results: The PAH-to-CO luminosity ratio remains constant across a wide range of luminosities, for various galaxy types, and throughout the explored redshift range. In contrast, the PAH-to-IR and CO-to-IR luminosity ratios deviate from a constant value at high L(IR). The intrinsic scatter in the L(PAH)-L'(CO) relation is 0.21 dex, with a median of 1.40, and a power-law slope of $1.07 \pm 0.04$. Both the PAH-IR and CO-IR relations are sub-linear. Given the tight and uniform PAH-CO relation over ~3 orders of magnitude, we provide a recipe to estimate the cold molecular gas mass of galaxies from PAH luminosities, with a PAH-to-molecular gas conversion factor of $\alpha_{\rm PAH7.7} = (3.08 \pm 1.08)(4.3/\alpha_{\rm CO})\,M_{\odot}/L_{\odot}$. This method opens a new window to explore the gas content of galaxies beyond the local Universe using multi-wavelength JWST/MIRI imaging.