UVIT survey of the host galaxies of active galactic nuclei. I. star formation scenarios

Abstract

Circumnuclear star formation (SF) is generally seen in galaxies hosting active galactic nuclei (AGN); however, the connection between the AGN activity and SF in them is less well understood. To explore this connection on scales of a few tens of parsecs to a few tens of kiloparsecs and larger, we carried out an investigation of SF in seven Seyfert-type AGN and one low-ionization nuclear emission-line region galaxy, using observations with the Ultraviolet Imaging Telescope on board AstroSat in the near-ultraviolet (2000–3000 Å) and far-ultraviolet (1300−1800 Å) bands. A total of 1742 star-forming regions were identified, having size scales of 0.010–63.642 kpc2. Considering all the galaxies, we found a positive correlation between their total surface density of SF (ΣSFR) and extinction. For five galaxies, namely NGC 1365, NGC 4051, NGC 4321, NGC 5033, and NGC 6814, we found a gradual decrease of both extinction and ΣSFR from the centre to the outer regions. Four sources are found to lie in the main sequence of star-forming galaxies, and the other four lie away from it. We found the ratio of the star formation rate (SFR) in the nuclear region to the total SFR to be positively correlated with the Eddington ratio. This points to the influence of AGN in enhancing the SF characteristics of the hosts. However, the impact is dominant only in the central nuclear region and has no significant effect on the larger scales probed in this work.