Intermediate-age Stars as Origin of Low Stellar velocity Dispersion
  Nuclear Rings: the case of Mrk 1157
  We have used the Gemini Near-infrared Integral Field Spectrograph (NIFS) to
map the age distribution of the stellar population in the inner 400 pc of the
Seyfert 2 galaxy Mrk 1157 (NGC 591), at a spatial resolution of 35 pc. We have
performed wavelet and principal component analysis in the data in order to
remove instrumental signatures. An old stellar population component (age > 5
Gyr) is dominant within the inner ~ 130 pc which we attribute to the galaxy
bulge. Beyond this region, up to the borders of the observation field, young to
intermediate age components (0.1--0.7 Gyr) dominate. As for Mrk 1066,
previously studied by us, we find a spatial correlation between this
intermediate age component and a partial ring of low stellar velocity
dispersions (sigma*). Low-sigma* nuclear rings have been observed in other
active galaxies and our results for Mrk 1157 and Mrk 1066 reveal that they are
formed by intermediate age stars. Such age is consistent with a scenario in
which the origin of the low-sigma* rings is a past event which triggered an
inflow of gas and formed stars which still keep the colder kinematics of the
gas from which they have formed. No evidence for the presence of an unresolved
featureless continuum and hot dust component -- as found in Mrk1066 -- are
found for Mrk 1157.
