The nucleus of NGC 253 and its massive stellar clusters at parsec scales

TL;DR

This study uses high-resolution infrared imaging to analyze the central 300 parsecs of NGC 253, revealing numerous young massive stellar clusters, their properties, and challenging previous assumptions about the galaxy's core activity.

Contribution

It provides detailed IR observations of NGC 253's nucleus, identifying young stellar clusters and offering a new template for extragalactic star-forming regions, while questioning the active nucleus hypothesis.

Findings

Resolved 37 IR-bright knots in the galaxy's center.

Identified IR counterparts for eight radio sources.

Challenged the active nucleus interpretation of the radio core.

Abstract

Very Large Telescope adaptive optics images of NGC 253 with resolutions down to 200 mas resolve the central 300 pc of this galaxy in ~ 37 infrared (IR) bright knots, a factor of 3 larger than previously reported, and extended diffuse emission. The angular resolution, comparable to that of available Very Large Array 2 cm maps, permits us a very accurate IR-radio registration. Eight radio sources are found to have an IR counterpart. The knots have H\alpha equivalent width of about 80 angstroms, sizes of ~ 3 pc, magnitudes in L-band of about 12 mag and relatively high extinction, Av ~ 7 mag. Their spectral energy distributions (SEDs) look very similar, characterized by a maximum at 20 micron and a gentle bump in the 1 - 2 micron range. These features can be well reproduced by considering an important contribution of very young stellar objects to the IR, efficiently heating their dust envelope. The evidence indicates that these are young massive clusters bursting from their dust cocoons. A median SED of the knots is provided, which may represent one of the most genuine templates of an extragalactic circumnuclear star-forming region. The lack of any optical or IR counterpart for the previously identified radio core calls into question its supposed active nucleus nature. This source may instead represent a scaled up version of Sgr A* at the Galactic Centre.