Intermediate-age stars as the origin of low stellar velocity dispersion nuclear rings: the case of Mrk 1157

Abstract

We have used the Gemini Near-Infrared Integral-Field Spectrograph 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 on 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 dispersion (σ*). Low-σ* 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-σ* 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 – is found for Mrk 1157.