Hubble Space Telescope ( HST ) ultraviolet STIS imaging and spectroscopy of the low luminosity AGN ( LLAGN ) NGC 4303 have identified the previously detected UV-bright nucleus of this galaxy , as a compact , massive and luminous stellar cluster .
The cluster with a size ( FWHM ) of 3.1 pc , and an ultraviolet luminosity log L _ { 1500 \AA } ( erg s ^ { -1 } Å ^ { -1 } ) = 38.33 is identified as a nuclear super star cluster ( SSC ) like those detected in the circumnuclear regions of spirals and starburst galaxies .
The UV spectrum showing the characteristic broad P Cygni lines produced by the winds of massive young stars , is best fitted by the spectral energy distribution of a massive cluster of 10 ^ { 5 } M _ { \odot } ( for a Salpeter IMF law with lower-mass cutoff of 1 M _ { \odot } ) generated in an instantaneous burst 4 Myr ago .
The ionizing energy produced by this cluster exceeds the flux needed to explain the nuclear H \alpha luminosity .
No evidence for an additional non-thermal ionizing source associated with an accreting black hole is detected in the ultraviolet .
These new HST/STIS results show unambiguously the presence of a compact , super star cluster in the nucleus of a low luminosity AGN , that is also its dominant ionizing source .
We hypothesize that at least some LLAGNs in spirals could be understood as the result of the combined ionizing radiation emitted by an evolving SSC ( i.e .
determined by the mass and age ) and a black hole ( BH ) accreting with low radiative efficiency ( i.e .
radiating at low sub-Eddington luminosities ) , coexisting in the inner few parsecs region .
Complementary multifrequency studies give the first hints of the very complex structure of the central 10 pc of NGC 4303 where a young SSC apparently coexists with a low efficiency accreting black hole and with an intermediate/old compact star cluster , and where in addition an evolved starburst could also be present .
If structures as such detected in NGC 4303 are common in the nuclei of spirals , the modeling of the different stellar components , and their contribution to the dynamical mass , has to be established accurately before deriving any firm conclusion about the mass of central black holes of few to several million solar masses .