The TYPHOON stellar population synthesis survey: I. The young stellar population of the Great Barred Spiral NGC 1365

Abstract

We analyze TYPHOON long slit absorption line spectra of the starburst barred spiral galaxy NGC 1365 obtained with the Progressive Integral Step Method covering an area of 15 square kpc. Applying a population synthesis technique, we determine the spatial distribution of ages and metallicity of the young and old stellar population together with star formation rates, reddening, extinction and the ratio R$_V$ of extinction to reddening. We detect a clear indication of inside-out growth of the stellar disk beyond 3 kpc characterized by an outward increasing luminosity fraction of the young stellar population, a decreasing average age and a history of mass growth, which was finished 2 Gyrs later in the outermost disk. The metallicity of the young stellar population is clearly super solar but decreases towards larger galactocentric radii with a gradient of -0.02 dex/kpc. On the other hand, the metal content of the old population does not show a gradient and stays constant at a level roughly 0.4 dex lower than that of the young population. In the center of NGC 1365 we find a confined region where the metallicity of the young population drops dramatically and becomes lower than that of the old population. We attribute this to infall of metal poor gas and, additionally, to interrupted chemical evolution where star formation is stopped by AGN and supernova feedback and then after several Gyrs resumes with gas ejected by stellar winds from earlier generations of stars. We provide a simple model calculation as support for the latter