Shock enhanced [CII] emission from the infalling galaxy Arp 25

TL;DR

This study provides the first direct evidence that shocks from ram pressure in galaxy groups can significantly enhance [CII] emission, complicating its use as a star formation indicator.

Contribution

It demonstrates that ram pressure shocks can increase [CII] emission by 60% locally and 25% globally, revealing a non-stellar contribution to [CII] in galaxy groups.

Findings

[CII] emission is enhanced by 60% along the impact front.

Global [CII] emission increases by approximately 25%.

Shocks significantly contribute to [CII] emission, affecting star formation estimates.

Abstract

We present SOFIA observations with HAWC+ and FIFI-LS of the peculiar galaxy Arp 25, also known as NGC 2276 or UGC 3740, whose morphology is deformed by its impact with the intra-group medium of the NGC 2300 galaxy group. These observations show the first direct proof of the enhancement of [CII] emission due to shocks caused by ram pressure in a group of galaxies. By comparing the [CII] emission to UV attenuation, dust emission, PAH, and CO emission in different regions of the galaxy, we find a clear excess of [CII] emission along the impact front with the intra-group medium. We estimate that the shock due to the impact with the intra-group medium increases the [CII] emission along the shock front by 60% and the global [CII] emission by approximately 25% with respect to the predicted [CII] emission assuming only excitation caused by stellar radiation. This result shows the danger of interpreting [CII] emission as directly related to star formation since shocks and other mechanisms can significantly contribute to the total [CII] emission from galaxies in groups and clusters.