Optical and near-IR spectrophotometry of the galaxy NGC 3310

Abstract

Spectrophotometric information in the range λλ 3500–9200 A has been obtained for 16 of the star forming regions in the circumnuclear ring of the galaxy NGC 1068. In 8 of these regions, the Hα luminosity is higher than 4 1039 erg s−1 cm−2, so they can be placed among the most luminous HII regions observed. In 10 of the 16 studied regions, the CaT infrared absortion lines are definitely present. In 7 of these regions the measured equivalent width is lower than 7 A, while in the other three, higher values are obtained (7.3, 8.6, 8.7). In these three cases, the lines might have their origin in a population of red supergiant stars. If we compare the CaT and the Ha equivalent width with theoretical models (Leitherer & Heckmann 1995, ApJS, 96, 9; Leitherer et al. 1999, ApJSS, 123, 3L), we find that the regions with low values of the CaT equivalent width are on the line standing for a 6.5 Myr single burst model of Salpeter initial mass function. For those with the highest values of the equivalent width, the data could be reproduced by a mixed population of young stars (2 – 8 Myr) ionizing the gas, and older ones (15 – 20 Myr) responsible for the CaT observed.