The Stellar Kinematic Center and the True Galactic Nucleus of NGC253

Abstract

We present the first sub-arcsecond resolution two-dimensional stellar kinematics and X-ray observations of the prototypical starburst galaxy NGC253 which define the position and nature of the galactic nucleus. We get an estimate of the stellar kinematic center location corresponding to an area of r~1.2" centered 0.7" southwest from the radio core (TH2). Newly processed Chandra data reveal a central hard X-ray source (X-1) lying 0.4" southwest from the kinematic center. Very accurate alignment between radio, infrared and X-ray sources shows that TH2, the IR photometric center and X-1 are not associated with each other. As the kinematic center is consistent with TH2 and X-1, we consider the two as possible galactic nucleus candidates. Although TH2 is the strongest radio source in the nuclear region, it does not have any infrared, optical or X-ray counterparts. If the kinematic center is associated with this source, by analogy we suggest that the nucleus of NGC253 resembles our Galactic Center SgrA*. On the other hand, X-1 is a heavily absorbed object only detected at energies >2 keV. If X-1 is instead associated with the kinematic center, the nucleus of NGC253 is compatible with an obscured low luminosity active galactic nucleus (AGN) or a spatially resolved super star cluster (SSC) brightening up in X-rays most probably due to young supernovae or supernova remnants. If no SSC is associated with the kinematic center, we conclude that NGC253 is a galaxy in which a strong starburst and a weak AGN (either TH2 or X-1) coexist. Results from few other high resolution studies of nearby starburst galaxies indicate that the AGN in these systems, if present, is always in the low luminosity regime. This may indicate that the onset of nuclear activity in galaxies is closely related with the occurrence of star formation, and that we are witnessing the emergence or disappearance of an AGN.