Nuclear region and discrete source population

Chandra has resolved the starburst nuclear region of the face-on grand-design spiral M83. Eighty-one point sources are detected (above 3.5-σ) in the ACIS S3 image, and 15 of them are within the inner 16 region of the galaxy. A point source with Lx ≈ 3 × 10 38 erg s in the 0.3–8.0 keV band is found to coincide with the infrared nuclear photometric peak, one of the two dynamical nuclei of the galaxy. No point-like sources are resolved (at a 2.5-σ level) at the centre of symmetry of the outer optical isophote ellipses, suspected to be another dynamical nucleus. About 50% of the total emission in the nuclear region is unresolved; of this, about 70% can be attributed to hot thermal plasma, and the rest is probably due to unresolved point sources (eg, faint X-ray binaries). The azimuthally-averaged radial distribution of the unresolved emission has a King-like profile, with no central cusp. Strong emission lines are seen in the spectrum of the optically thin plasma component. The high abundances of C, Ne, Mg, Si and S with respect to Fe suggest that the interstellar medium in the nucleus is enriched and heated by type-II supernova explosions and winds from massive stars. The cumulative luminosity distribution of the discrete X-ray sources is neither a single nor a broken power law. Separating the sources in the nuclear region (within a distance of 60 from the X-ray centre) from the rest reveals that the two groups have different luminosity distributions. The log N(>S) – log S curve of the sources in the inner region (nucleus and stellar bar) is a single power law, which we interpret as due to continuous, ongoing star formation. Outside the central region, there is a smaller fraction of sources brighter than the Eddington limit for an accreting neutron star.