V, R, I and H alpha photometry of circumnuclear star-forming regions in four galaxies with different levels of nuclear activity

We present photometry, in the V, R and I continuum bands and in the H alpha + [N II] emission lines, for a sample of circumnuclear star-forming region s (CNSFR), located in four galaxies with different kinds of activity in the ir nuclei: NGC 7469 (Seyfert 1), NGC 1068 (Seyfert 2), NGC 7177 (LINER) and NGC 3310 (starburst). H alpha luminosities for the CNSFR range from 0.02 to 7 x 10(40) erg s(-1) (uncorrected for internal extinction), comparable to those observed in othe r galaxies, with NGC 7177 showing the lowest luminosity in average. No syst ematic differences in the broad-band colours are found for the CNSFR in the different galaxies, except for those in NGC 3310 which are considerably bl uer. This is found to be partially because of a younger stellar population. The colours have been analysed in the light of theoretical evolutionary syn thesis models. In some cases they can be reproduced by single populations w ith ages ranging between 7 and 300 Myr and modest values of extinction (0.5 -1.5 mag). However, in many cases, this population is unable to provide the observed equivalent widths of H alpha, which require the presence of a you nger population. In the cases of NGC 1068, 7177 and 7469, acceptable fits are found for a tw o-burst population model at solar metallicity: the younger burst, with an a ge between 2 and 8 Myr, provides the bulk of the ionization and the older o ne (8-20 Myr) is responsible for the continuum light at wavelengths longer than H beta. The age difference between both populations is around 5-7 Myr and the younger burst involves from 3 to 61 per cent of the total mass of t he cluster. This would be consistent with the younger burst being originate d by the supernova activity from the previous one. Models of this kind also reproduce the regions in NGC 3310, but for younger ionizing population age s (between 1 and 3 Myr) and a metallicity 0.25 times solar. In most cases an excess in the observed (R - I) colour over the model predi cted one is found, which is not consistent with a normal reddening law. If this excess is attributed to the red supergiants present in the older popul ation, this seems to imply that this population is not properly taken into account by the models. In this two-population scenario there seems to be a trend for the circumnuc lear star-forming regions of NGC 3310 (starburst), NGC 1068 (Seyfert 2) and NGC 7469 and NGC 7177 (Seyfert 1 and LINER respectively) to be progressive ly older. Whether this implies a relation between the evolutionary state of the regions and the nuclear type of the parent galaxy remains to be explor ed.