Title:Localized Deviations from the CO-PAH Relation in PHANGS-JWST Galaxies: Faint PAH Emission or Elevated CO Emissivity?

Abstract:Polycyclic aromatic hydrocarbon (PAH) emission is widely used to trace the distribution of molecular gas in the interstellar medium (ISM), exhibiting a tight correlation with CO(2-1) emission across nearby galaxies. Using PHANGS-JWST and PHANGS-ALMA data, we identify localized regions where this correlation fails, with CO flux exceeding that predicted from 7.7$\mu$m PAH emission by more than an order of magnitude. These outlier regions are found in 20 out of 70 galaxies and are mostly located in galaxy centers and bars, without signs of massive star formation. We explore two scenarios to explain the elevated CO-to-PAH ratios, which can either be due to suppressed PAH emission or enhanced CO emissivity. We examine PAH emission in other bands (3.3$\mu$m and 11.3$\mu$m) and the dust continuum dominated bands (10$\mu$m and 21$\mu$m), finding consistently high CO-to-PAH (or CO-to-dust continuum) emission ratios, suggesting that 7.7$\mu$m PAH emission is not particularly suppressed. In some outlier regions, PAH size distributions and spectral energy distribution of the illuminating radiation differ slightly compared to nearby control regions with normal CO-to-PAH ratios, though without a consistent trend. We find that the outlier regions generally show higher CO velocity dispersions ($\Delta v_{\mathrm{CO}}$). This increase in $\Delta v_{\mathrm{CO}}$ may lower CO optical depth and raise its emissivity for a given gas mass. Our results favor a scenario where shear along the bar lanes and shocks at the tips of the bar elevate CO emissivity, leading to the breakdown of the CO-PAH correlation.