Physical properties of circumnuclear ionising clusters. I. NGC 7742

ABSTRACT This work aims to derive the physical properties of the circumnuclear star-forming region in the ring of the face-on spiral NGC 7742 using integral field spectroscopy observations. We have selected 88 individual ionizing clusters that power H ii regions populating the ring of the galaxy that may have originated in a minor-merger event. For the H ii regions, the rate of Lyman continuum photon emission is between 0.025 and 1.5 × 1051, which points to these regions being ionized by star clusters. Their electron density, ionization parameter, filling factor, and ionized hydrogen mass show values consistent with those found in other studies of similar regions and their metal abundances, as traced by sulphur have been found to be between 0.25 and 2.4 times solar, with most regions showing values slightly below solar. The equivalent temperature of the ionizing clusters is relatively low, below 40 000 K, which is consistent with the high elemental abundances derived. The young stellar population of the clusters has contributions of ionizing and non-ionizing populations with ages around 5 and 300 Ma, respectively. The masses of ionizing clusters once corrected for the contribution of underlying non-ionizing populations were found to have a mean value of 3.5 × 104 M⊙, comparable to the mass of ionized gas and about 20 per cent of the corrected photometric mass.