Circumnuclear stellar population, morphology, and environment of Seyfert 2 galaxies : an evolutionary scenario

Abstract

We investigate the relation between the characteristics of the circumnuclear stellar population and both the galaxy morphology and the presence of close companions for a sample of 35 Seyfert 2 nuclei. Fifteen galaxies present unambiguous signatures of recent episodes of star formation within ≈300 pc of the nucleus. When we relate this property to the Hubble type of the host galaxy, we find that the incidence of recent circumnuclear star formation increases along the Hubble sequence; it seems to be greater than that in non-Seyfert galaxies for the early Hubble types S0 and Sa but similar to that in non-Seyfert galaxies for later Hubble types. In both early-type and late-type Seyfert galaxies, the presence of recent circumnuclear star formation is related to the galaxy morphology in the inner few kiloparsecs, as observed in Hubble Space Telescope images through the filter F606W by Malkan et al., who have assigned a late "inner Hubble type" to most Seyfert 2 galaxies with recent circumnuclear star formation. This new classification is due to the presence of dust lanes and spiral structures in the inner region. The presence of recent star formation around Seyfert 2 nuclei is also related to interactions : among the 13 galaxies of the sample with close companions or in mergers, nine have recent star formation in the nuclear region. These correlations between the presence of companions, the inner morphology, and the incidence of recent star formation suggest an evolutionary scenario in which the interaction is responsible for sending gas inward, which both feeds the active galactic nucleus and triggers star formation. The starburst then fades with time and the composite Seyfert 2 + starburst nucleus evolves to a "pure" Seyfert 2 nucleus with an old stellar population. This scenario can reconcile the hypothesis that the active nucleus in Seyfert galaxies is triggered by interactions with the results of previous studies, which find only a small excess of interacting galaxies in Seyfert samples when compared with non-Seyfert samples. The large excess can only be found early after the interaction, in the phase in which a composite (Seyfert + starburst) nucleus is observed.