An AGN in the Antennae galaxies ?

TL;DR

This study uses ALMA observations over 2.5 months to investigate potential active galactic nucleus activity in the Antennae galaxies, identifying a likely AGN in NGC 4039 through variability analysis.

Contribution

First detection of time variability at 100 GHz in the Antennae galaxies, suggesting the presence of a possible AGN in NGC 4039.

Findings

Source S4 shows variability with a 13-day timescale, indicating a compact emission region.

S4's properties are consistent with a Compton-thick AGN.

Source S3 could be either a young stellar cluster or an AGN.

Abstract

Time variability is a strong probe of energetic phenomena which occur at small spatial scales, like Active Galactic Nuclei (AGN). We use ALMA observations at 100 GHz executed over a period of 2.5 months to look for time variability in the Antennae galaxies, a prototypical early stage merger galaxy pair, for which there are no previous signatures of an AGN in the optical, infrared or X-ray. Most 100 GHz detections in the Antennae are spatially extended and associated with star forming regions, but two sources in the southern galaxy NGC 4039 are compact. One of these compact sources, S3, is offset by 1 arcsecond in the northeast direction from the stellar peak of NGC 4039, and marginally resolved at 10 parsec resolution. The other source, S4, is co-spatial with the stellar peak of NGC 4039 and unresolved even at a resolution of 4 parsec. We examine the time variability of these two sources using their power spectrum. We find that S4 varies with a characteristic timescale of 13+/-3 days, indicating that the phenomena responsible for the 100 GHz emission is smaller than 0.01 parsecs. By comparing the observed flux of the two sources with various candidate origins, we show that while S3 can be explained either by a young massive stellar cluster or an AGN, S4 is likely to be an AGN that is possibly Compton-thick.