Properties of hot stars in the Wolf-Rayet galaxy NGC 5253 from ISO-SWS spectroscopy

ISO-SWS spectroscopy of the Wolf-Rayet galaxy NGC 5253 is presented, and an alysed to provide estimates of its hot young star population. Our approach differs from previous investigations in that we are able to distinguish bet ween the regions in which different infrared fine-structure lines form, usi ng complementary ground-based observations. The high-excitation nebular [S IV] emission is formed in a very compact region, which we attribute to the central super-star nucleus, and lower excitation [Ne Ir] nebular emission o riginates in the galactic core. We use photoionization modelling coupled wi th the latest theoretical O-star flux distributions to derive effective ste llar temperatures and ionization parameters of T-eff greater than or equal to 38 kK, log Q similar to 8.25 for the compact nucleus, with T-eff similar to 35 kK, log Q less than or equal to 8 for the larger core. Results are s upported by more sophisticated calculations using evolutionary synthesis mo dels We assess the contribution that Wolf-Rayet stars may make to highly io nized nebular lines (e.g, [O IV]). From our Br alpha flux, the 2-arcsec nucleus contains the equivalent of app roximately 1000 O7 V star equivalents and the starburst there is 2-3 Myr ol d: the 20-arcsec core contains about 2500 O7 V star equivalents, with a rep resentative age of similar to 5 Myr, The Lyman ionizing flux of the nucleus is equivalent to that of the 30 Doradus region. These quantities are in go od agreement with the observed mid-infrared dust luminosity of 7.8 x 10(8) L.. Since this structure of hot clusters embedded in cooler emission may be common in dwarf starbursts, observing a galaxy solely with a large apertur e may result in confusion. Neglecting the spatial distribution of nebular e mission in NGC 5253 implies 'global' stellar temperatures (or ages) of 36 k K (4.8 Myr) and 39 kK (2.9 or 4.4 Myr) from the observed [Ne III/II] and [S IV/III] line ratios, assuming log Q = 8.