Understanding the [CII] Deficit in NGC 3627 and NGC 1365

The [CII] 157.7 micron emission line is the brightest observable emission line in most galaxies, which has made it a standard tracer for galactic properties and interstellar medium (ISM) conditions. Its far-infrared wavelength allows it to pass through dust with minimal attenuation, and places it within the ALMA bandwidth for galaxies at z>4. However, there is an currently unaccounted for decrease in the relative [CII] emission in galaxies undergoing bursts of star formation that makes this tracer less than ideal. This effect, commonly referred to as the [CII] deficit, limits our ability to use [CII] emission to study evolving ISM conditions. One possible cause of this deficit is decreases in the photoelectric heating efficiency, driven by a changing population of the smallest dust grains, the polycyclic aromatic hydrocarbons (PAHs). After analyzing the [CII] emission from regions in two nearby galaxies, NGC1365 and NGC3627, we measured the PAH emission using JWST observations. With this data, we can then compare tracers of cooling ([CII] emission) to tracers of heating (PAH emission) in each region across NGC 1365 and NGC 3627. No clear correlations were found between PAH properties and normalized [CII] emission, indicating that changes to PAH charge or size are not the dominant cause of the [CII] deficit in these galaxies.