Age-divided mean stellar populations from full spectrum fitting as the simplified star formation and chemical evolution history of a galaxy: methodology and reliability

We introduce a practical methodology for investigating the star formation and chemical evolution history of a galaxy: age-divided mean stellar populations (ADPs) from full spectrum fitting. In this method, the mass-weighted mean stellar populations and mass fractions (f (mass)) of young and old stellar components in a galaxy are separately estimated, which are divided with an age cut (selected to be 10(9.5) yr approximate to 3.2 Gyr in this paper). To examine the statistical reliability of ADPs, we generate 10 000 artificial galaxy spectra, each of which consists of five random simple stellar population components. Using the Penalized PiXel-Fitting ( PPXF) package, we conduct full spectrum fitting to the artificial spectra with noise as a function of wavelength, imitating the real noise of Sydney-Australian Astronomical Observatory Multi-object Integral field spectrograph (SAMI) galaxies. As a result, the Delta (= output - input) of age and metallicity appears to significantly depend on not only signal-to-noise ratio (S/N), but also luminosity fractions (f lum) of young and old components. At given S/N and f lum, Delta of young components tends to be larger than Delta of old components; e.g. sigma(Delta[M/H]) similar to 0.40 versus 0.23 at S/N = 30 and f lum = 50 per cent. The age-metallicity degeneracy appears to be insignificant, but Delta log(age/yr) shows an obvious correlation with Delta f (mass) for young stellar components (R similar to 0.6). The impact of dust attenuation and emission lines appears to be mostly insignificant. We discuss how this methodology can be applied to spectroscopic studies of the formation histories of galaxies, with a few examples of SAMI galaxies.