HST Observations of the Starburst Nucleus of M83=NGC 5236
Session 31 -- Star-Forming Galaxies
Display presentation, Tuesday, 9:30-6:30, Heller Lounge Room

## [31.04] HST Observations of the Starburst Nucleus of M83=NGC 5236

S.R. Heap (NASA/GSFC), J. Holbrook (USRA/GSFC), E. Malumuth (CSC), S. Shore (CSC), W. Waller (NRC/GSFC)

We have used the Planetary Camera on HST to obtain images of the nuclear region of M83 in four filters: F336W (U'' band), F555W (V'' band), F785LP (I'' band) and F658N (H$\alpha$ redshifted by +504 km s$^{-1}$). These pictures reveal an arc of numerous OB clusters south and west of the conventional nucleus. Each of the clusters is resolved with a typical FWHM = 0.2" (4 pc).

We have used the wide-band images to construct color-color diagrams and to derive absolute visual magnitudes of each of the OB star clusters. The absolute visual magnitudes range from $M_v$ =-10.4 to $M_v$=-13.4, while the conventional nucleus has an $M_v$ = -15.2. The unreddened colors indicate ages of the OB clusters from about 2 to 6 million years. The cluster masses inferred from their age and $M_v$ range from $10^4$ to $10^5$ $M_o$ for a Salpeter initial mass function.

When the continuum contribution to the F658N fluxes are subtracted out, it is clear that the nucleus is not a source of H$\alpha$ emission. However, intense H$\alpha$ emission covers the western OB clusters. The inferred ionizing flux is about $10^{50}$ - $10^{51}$ photons s$^{-1}$ for individual regions, and $3x10^{52}$ photons s$^{-1}$ for the nuclear region as a whole.