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 analysed to provide estimates of its hot young star population. Our approach differs from previous investigations in that we are able to distinguish between the regions in which different infrared fine-structure lines form, using 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 originates in the galactic core. We use photoionization modelling coupled with the latest theoretical O-star flux distributions to derive effective stellar 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 supported by more sophisticated calculations using evolutionary synthesis models We assess the contribution that Wolf-Rayet stars may make to highly ionized nebular lines (e.g, [O IV]). From our Br alpha flux, the 2-arcsec nucleus contains the equivalent of approximately 1000 O7 V star equivalents and the starburst there is 2-3 Myr old: the 20-arcsec core contains about 2500 O7 V star equivalents, with a representative 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 good 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 aperture may result in confusion. Neglecting the spatial distribution of nebular emission in NGC 5253 implies 'global' stellar temperatures (or ages) of 36 kK (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.