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

Very Large Telescope adaptive optics images of NGC 253 with resolutions down to 200 mas resolve the central 300 pc of this galaxy in similar to 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 II alpha equivalent width of about 80 angstrom, sizes of similar to 3 pc, magnitudes in L band of about 12 mag and relatively high extinction, A(V) similar to 7 mag. Their spectral energy distributions (SEDs) look very similar, characterized by a maximum at 20 mu m and a gentle bump in the 1-2 mu m 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.