T. Storchi-bergmann et al., Circumnuclear stellar population, morphology, and environment of Seyfert 2galaxies: An evolutionary scenario, ASTROPHYS J, 559(1), 2001, pp. 147-156
We investigate the relation between the characteristics of the circumnuclea
r stellar population and both the galaxy morphology and the presence of clo
se companions for a sample of 35 Seyfert 2 nuclei. Fifteen galaxies present
unambiguous signatures of recent episodes of star formation within approxi
mate to 300 pc of the nucleus. When we relate this property to the Hubble t
ype 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 SO and Sa but
similar to that in non-Seyfert galaxies for later Hubble types. In both ear
ly-type and late-type Seyfert galaxies, the presence of recent circumnuclea
r star formation is related to the galaxy morphology in the inner few kilop
arsecs, as observed in Hubble Space Telescope images through the filter F60
6W by Malkan et al., who have assigned a late "inner Hubble type" to most S
eyfert 2 galaxies with recent circumnuclear star formation. This new classi
fication 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 nucl
ear region. These correlations between the presence of companions, the inne
r morphology, and the incidence of recent star formation suggest an evoluti
onary scenario in which the interaction is responsible for sending gas inwa
rd, which both feeds the active galactic nucleus and triggers star formatio
n. The starburst then fades with time and the composite Seyfert 2 + starbur
st nucleus evolves to a "pure" Seyfert 2 nucleus with an old stellar popula
tion. 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 (Se
yfert+starburst) nucleus is observed.