Physical properties of circumnuclear ionising clusters

Circumnuclear star-forming regions (CNSFRs) found close to galactic nuclei are ionised by massive clusters. These entities give us an excellent opportunity to study star formation in environments with high metallicity, and to relate it with active galactic nuclei. Our principal aim is to derive the physical properties and dynamical masses of the CNSFRs in the two rings of the spiral NGC 7469, categorised as a luminous infrared galaxy (LIRG) and hosting a Seyfert 1 nucleus. We used archival data obtained with the MUSE spectrograph. The galaxy shows two prominent star-forming rings, one of them very close to their active galactic nucleus, within 1.5 arcsec from the galaxy centre. We constructed 2D flux maps of the different emission lines and two continuum bands. A map of the EW(Hα) emission shows the circumnuclear regions within the rings having EW(Hα) > 50 Å, consistent with the presence of recent star formation. All emission lines appear to have at least two kinematical components. We ascribe the most intense and narrow component to the emission lines originated by the ionising star-forming complexes (SFC) since they follow the radial velocity of the galaxy disc. For each HII region, we derived the number rate of Lyman continuum photons; the gas electron density; the ionisation parameter; the filling factor; and the mass of ionised hydrogen. We used sulphur as a tracer for chemical abundances with the temperature-sensitive [SIII]λ 6312 Å emission line having been measured in ∼50% of the whole sample, allowing the derivation of abundances by the direct method. The evolutionary state of the SFC was inferred with the help of population synthesis models yielding mean ages of 5.7 Ma, agreeing with the presence of the broad Wolf–Rayet (WR) carbon feature at λ 5800 Å detected in all the regions. Ionising (lower limits) and photometric SFC masses were estimated from the number of Lyman continuum photons and absolute r-magnitudes using stellar population synthesis techniques, and give median values of 2.3 × 106 and 6.9 × 106, respectively. The dynamical masses (upper limit) were derived from the measured absorption CaT velocity dispersion and the sizes of each cluster were measured on continuum light, assuming virialisation, and yield a median value of 6.7 × 108. Regions in the studied galaxy show sizes larger than implied by simple photo-ionisation models, which can be explained by the stellar winds produced by WR stars. The inner ring regions seem to be more compact than the outer ones. The young stellar population of the clusters has contributions of ionising populations with ages around 5 Ma, and its masses constitute less than 1% of the total dynamical mass of each SFC. Finally, the comparison between the characteristics of the inner and outer ring ionising clusters, together with their derived dynamical masses, point to circumnuclear regions close to the active galactic nucleus being more compact and having higher gas density.