Reliability of PAH emission as a tracer of H2 on small spatial scales

Determine whether mid-infrared polycyclic aromatic hydrocarbon (PAH) emission, such as Spitzer/IRAC 8 μm emission, accurately reflects the amount (column density) of molecular hydrogen (H2) at small, sub-parsec scales, in order to validate its use as a tracer of low-density molecular gas in the Large Magellanic Cloud.

Background

In presenting a multi-wavelength view of N113, the study overlays Spitzer/IRAC 8 μm PAH emission with H I and CO to trace different gas phases. The authors note that PAH emission is commonly used to indicate the approximate distribution of low-density molecular gas not traced by CO.

However, they explicitly acknowledge that it remains debated whether PAH emission accurately reflects the amount of H2, especially on smaller spatial scales. This uncertainty affects how robustly PAH emission can be used as a quantitative tracer of molecular gas in environments like the LMC.

References

Although it is still debated whether PAH emission accurately reflects the amount of H$_2$ molecules especially at smaller scale, it is certain that molecular gas exists in the direction of PAH emission \citep[see][and references therein]{Leroy2023}.

— A multi-scale molecular and atomic gas view on the HII region N113 in the Large Magellanic Cloud:Evidence for high-mass star formation triggered by supersonically-colliding HI flows  (2603.29899 - Yamada et al., 31 Mar 2026) in Section 1 (Introduction), footnote associated with Figure 1 description of 8 μm PAH emission