The Case for Thermalization as a Contributor to the [CII] Deficit

TL;DR

This study investigates the causes of the [CII] deficit in galaxies, finding that thermalization in HII regions significantly contributes to the observed decrease in [CII] emission relative to infrared emission.

Contribution

It provides new evidence that thermalization effects in HII regions are a key factor in the [CII] deficit, using detailed analysis of local galaxies with multi-wavelength data.

Findings

Thermalization in HII regions impacts [CII] emission.

[CII] deficit correlates with conditions in star-forming regions.

Analysis of 18 local galaxies supports thermalization as a major cause.

Abstract

The [CII] deficit, which describes the observed decrease in the ratio of [CII] 158 micron emission to continuum infrared emission in galaxies with high star formation surface densities, places a significant challenge to the interpretation of [CII] detections from across the observable universe. In an attempt to further decode the cause of the [CII] deficit, the [CII] and dust continuum emission from 18 Local Volume galaxies has been split based on conditions within the interstellar medium where it originated. This is completed using the Key Insights in Nearby Galaxies: a Far-Infrared Survey with Herschel (KINGFISH) and Beyond the Peak (BtP) surveys and the wide-range of wavelength information, from UV to far-infrared emission lines, available for a selection of star-forming regions within these samples. By comparing these subdivided [CII] emissions to isolated infrared emission and other properties, we find that the thermalization (collisional de-excitation) of the [CII] line in HII regions plays a significant role in the deficit observed in our sample.