# Evidence of AGN activity in the gamma-ray emission from two starburst galaxies

## TL;DR

This study presents evidence that two starburst galaxies exhibit gamma-ray emissions likely dominated by obscured AGN activity, challenging the typical starburst origin of gamma-ray emissions in such galaxies.

## Contribution

The paper identifies gamma-ray variability and outlier behavior in two starburst galaxies, suggesting the presence of obscured AGNs contributing significantly to their gamma-ray emission.

## Key findings

- NGC 3424 and UGC 11041 show significant gamma-ray flux variability.
- These galaxies are outliers in the gamma-ray to IR and radio luminosity relations.
- The results imply AGN activity dominates gamma-ray emission in these galaxies.

## Abstract

Starburst galaxies are huge reservoirs of cosmic rays (CRs) and these CRs convert a significant fraction of their energy into gamma-rays by colliding with the interstellar medium (ISM). The produced GeV gamma-ray emission is temporally stable and the flux is found to correlate well with indicators of star formation rates, such as the total infrared (IR) luminosity $L_{\rm IR}$ and monochromatic radio continuum luminosity at 1.4 GHz $L_{\rm 1.4\ GHz}$, i.e., following $L_{\gamma}-L_{\rm IR}$ and $L_{\gamma}-L_{\rm 1.4\ GHz}$ relations. Recently, gamma-ray excesses are reported to be spatially coincident with two starburst galaxies NGC 3424 and UGC 11041 in the fourth \textsl{Fermi} Large Area Telescope (LAT) source catalog (4FGL). Different from other starburst galaxies detected by \textsl{Fermi}-LAT, we find that the gamma-ray emission associated with NGC 3424 and UGC 11041 show significant flux variability. With relatively weak infrared and radio emission, NGC 3424 and UGC 11041 appear as outliers of the $L_{\gamma}-L_{\rm IR}$ and $L_{\gamma}-L_{\rm 1.4\ GHz}$ relations of starburst galaxies. These results suggest that NGC 3424 and UGC 11041 may harbor obscured active galactic nuclei (AGNs) and the AGN activities provide the dominant contribution to the gamma-ray emission as compared to that provided by the starburst activities.