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 .
These observations comprise some of the best probes of the central 300 pc of NGC253 , the nearest massive galaxy undergoing a powerful starburst , and will allow us to gain more insight into the nature of the centers of starburst galaxies .
We get an estimate of the stellar kinematic center location corresponding to an area of r \sim 1.2 ^ { \prime \prime } centered \sim 0.7 ^ { \prime \prime } southwest from the radio core , and historically presumed nucleus , TH2 .
Newly processed Chandra data reveal a central point-like hard X-ray source ( X-1 ) lying \sim 0.4 \arcsec southwest from the kinematic center .
Very accurate alignment between radio , infrared and X-ray sources in the nuclear region shows that TH2 , the IR photometric center and X-1 are not associated with each other .
As the kinematic center is consistent with the positions of TH2 and X-1 , and both could be a manifestation of nuclear activity , we consider the two as possible galactic nucleus candidates .
Although TH2 is the strongest compact 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 ( N _ { H } = 7.5 \times 10 ^ { 23 } cm ^ { -2 } ) only detected at energies > 2 keV ( L _ { 2 - 10 keV } \sim 10 ^ { 40 } ergs s ^ { -1 } ) .
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 , a situation also observed in the nuclear starburst of M82 .
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 ( e.g .
M82 , NGC5253 , NGC4945 ) 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 dissaperance of an AGN .