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), H2 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 5–35 μm and photometry from 3.4–250 μm correlate with SFR and molecular gas. In general, we find MIR emission features (i.e., PAHs and H2 rotational lines) trace both CO and SFR better than CO and SFR trace one another. H2 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 μm, 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 in our sample. The 17 μm/7.7 μm 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 and 70 μm better tracing SFR, while longer wavelengths better trace CO.

The Size–Linewidth Relation and Signatures of Feedback from Quiescent to Active Star Forming Regions in the LMC

“…The variation in 8 μm emission around these parsec-scale CO peaks reflects the differences in dust heating by local variations in the interstellar radiation field. This is not directly comparable with studies that integrate over much larger (∼ kpc) scales, in which 8 μm is found to trace star formation rate, as well as CO emission, with indications that 8 μm is a stronger CO tracer than a star formation rate tracer (Leroy et al 2023;Whitcomb et al 2023). Because we adopt a CO clump-finding method, we are studying variations in 8 μm brightness among CO-bright regions, rather than studying variations in 8 μm brightness across the ISM unbiased to the presence of molecular gas.…”

“…The Spitzer 8 μm band is centered on a prominent PAH feature. Recent work (e.g., Whitcomb et al 2023;Leroy et al 2023) has shown a strong correlation between CO and mid-infrared bands on sub-kpc and galactic scales. Bendo et al (2008) and Cortzen et al (2019) have found the 8 and 6.2 μm PAH bands to trace cold dust rather than warm dust.…”

A Two-Component Probability Distribution Function Describes the Mid-IR Emission from the Disks of Star-forming Galaxies

“…In the more diffuse regions away from H II regions, the diffuse mid-IR emission should be a good tracer of column density, as the radiation field varies more weakly than the gas column density. This appears to be borne out by a very good local correlation between CO emission, tracing molecular gas column density, and mid-IR emission on ∼100-200 pc scales (see Whitcomb et al 2023a;Leroy et al 2023;Sandstrom et al 2023). In this case, we might expect the lognormal PDFs that we observe to trace the underlying gas column density distribution and to reflect the convolution of turbulence and self-gravity in the diffuse ISM (Burkhart et al 2017).…”

Active Galactic Nuclei and Star Formation in nearby Galaxies (AGNSTRONG). I. Sample and Strategy

“…One of the significant challenges in the study of AGN feedback is the difficulty of estimating SFRs (e.g., Kennicutt 1998;Kennicutt et al 2009;Leroy et al 2012;Whitcomb et al 2023). SFR estimation is a complex endeavor with considerable uncertainties, often reaching approximately 0.7 dex (Salim et al 2016).…”