The Stellar Kinematic Center and the True Galactic Nucleus of NGC253
  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.
