Uncovering the nucleus candidate for NGC 253

TL;DR

This study uses near-infrared spectroscopy to identify the true galactic nucleus of NGC 253, revealing that the brightest infrared source TH7 is the most probable nucleus candidate due to its dynamical and kinematic features.

Contribution

The paper provides new evidence that the infrared source TH7 is the actual nucleus of NGC 253, combining spectroscopic, kinematic, and morphological data.

Findings

TH7 is the most massive compact infrared object near the galaxy's center.

A kinematic residual suggests an outflow centered on TH7.

TH7 aligns with the rotation center of the molecular gas disk.

Abstract

NGC253 is the nearest spiral galaxy with a nuclear starburst which becomes the best candidate to study the relationship between starburst and AGN activity. However, this central region is veiled by large amounts of dust, and it has been so far unclear which is the true dynamical nucleus. The near infrared spectroscopy could be advantageous in order to shed light on the true nucleus identity. Using Flamingos-2 at Gemini South we have taken deep K-band spectra along the major axis and through the brightest infrared source. We present evidence showing that the brightest near infrared and mid infrared source in the central region, already known as radio source TH7 and so far considered just a stellar supercluster, in fact, presents various symptoms of a genuine galactic nucleus. Therefore, it should be considered a valid nucleus candidate. It is the most massive compact infrared object in the central region, located at 2.0" of the symmetry center of the galactic bar. Moreover, our data indicate that this object is surrounded by a large circumnuclear stellar disk and it is also located at the rotation center of the large molecular gas disk of NGC 253. Furthermore, a kinematic residual appears in the H2 rotation curve with a sinusoidal shape consistent with an outflow centered in the candidate nucleus position. The maximum outflow velocity is located about 14 pc from TH7, which is consistent with the radius of a shell detected around the nucleus candidate observed at 18.3 {\mu}m (Qa) and 12.8 {\mu}m ([NeII]) with T-ReCS. Also, the Br_gamma emission line profile is blue-shifted and this emission line has also the highest equivalent width at this position. All these evidences point out TH7 as the best candidate to be the galactic nucleus of NGC 253.