V, R, I and Hα 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 Hα+[N ii] emission lines, for a sample of circumnuclear star-forming regions (CNSFR), located in four galaxies with different kinds of activity in their nuclei: NGC 7469 (Seyfert 1), NGC 1068 (Seyfert 2), NGC 7177 (LINER) and NGC 3310 (starburst). Hα luminosities for the CNSFR range from 0.02 to 7×1040 erg s−1 (uncorrected for internal extinction), comparable to those observed in other galaxies, with NGC 7177 showing the lowest luminosity in average. No systematic differences in the broad-band colours are found for the CNSFR in the different galaxies, except for those in NGC 3310 which are considerably bluer. This is found to be partially because of a younger stellar population. The colours have been analysed in the light of theoretical evolutionary synthesis models. In some cases they can be reproduced by single populations with 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 Hα, which require the presence of a younger population. In the cases of NGC 1068, 7177 and 7469, acceptable fits are found for a two-burst population model at solar metallicity: the younger burst, with an age between 2 and 8 Myr, provides the bulk of the ionization and the older one (8–20 Myr) is responsible for the continuum light at wavelengths longer than Hβ. 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 the cluster. This would be consistent with the younger burst being originated by the supernova activity from the previous one. Models of this kind also reproduce the regions in NGC 3310, but for younger ionizing population ages (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 predicted 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 population, 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 circumnuclear star-forming regions of NGC 3310 (starburst), NGC 1068 (Seyfert 2) and NGC 7469 and NGC 7177 (Seyfert 1 and LINER respectively) to be progressively older. Whether this implies a relation between the evolutionary state of the regions and the nuclear type of the parent galaxy remains to be explored.