Star Formation and Molecular Gas Diagnostics with Mid- and Far-Infrared Emission

Abstract

With the start of JWST observations, mid-infrared (MIR) emission features from polycyclic aromatic hydrocarbons (PAHs), H$_2$ rotational lines, fine-structure lines from ions, and dust continuum will be widely available tracers of gas and star formation rate (SFR) in galaxies at various redshifts. Many of these tracers originate from dust and gas illuminated by UV photons from massive stars, so they generally trace both SFR and gas to varying degrees. We investigate how MIR spectral features from 5 to 35$\mu$m and photometry from 3.4 to 250$\mu$m correlate with SFR traced by ionized neon (15.6$\mu$m [Ne III] and 12.8$\mu$m [Ne II]) and molecular gas traced by carbon monoxide (CO). In general, we find MIR emission features (i.e. PAHs and H$_2$ rotational lines) trace both CO and SFR better than CO and SFR trace one another. H$_2$ lines and PAH features correlate best with CO. Fine-structure lines from ions correlate best with SFR. The [S III] lines at 18.7 and 33.5$\mu$m, in particular, have a very tight correlation with SFR, and we use them to calibrate new single-parameter MIR tracers of SFR that have negligible metallicity dependence. The 17$\mu$m/7.7$\mu$m PAH feature ratio increases as a function of CO emission which may be evidence of PAH growth or neutralization in molecular gas. The degree to which dust continuum emission traces SFR or CO varies as a function of wavelength, with continuum between 20 to 70$\mu$m better tracing SFR, while longer wavelengths better trace CO.