Empirical population synthesis: New directions.

Empirical population synthesis (EPS) is a technique to decompose composite galaxy stellar populations into their component populations. From this decomposition, a stellar luminosity function is derived and used to analyze the star formation history of the observed galaxy population. In this study, EPS is used to investigate the nuclear stellar populations of the spiral galaxies NGC 1219, NGC 3177, NGC 3512, and NGC 4032. An EPS algorithm based on linear combinations of stellar spectra is implemented and tested. A maximum entropy variant is also tested. The effects of stellar spectra degeneracy on EPS solutions are investigated. Stellar degeneracy is quantified using linear correlation analysis. A Monte Carlo technique is implemented to estimate the uncertainties of the derived individual stellar components. Medium resolution (12A) data spanning 3500-9000A were acquired for the four galaxies above. A stellar spectral library with good luminosity and temperature discrimination was also constructed. Using these data and EPS, luminosity functions are derived for the observed spiral nuclei. Component uncertainties are calculated and the effects of uncertain continuum determination on these uncertainties is discussed. Data for the irregular galaxies NGC 1569 and NGC 4449 were also acquired but detailed luminosity functions are not derived. It is concluded that NGC 4032 has undergone constant, continual star formation over the last 10 Gyr. NGC 3512 luminosity function analysis indicates a declining mean star formation rate (SFR) with a burst of star formation 1-2 Gyr ago. While NGC 3177 and NGC 3512 appear spectroscopically similar, NGC 3177 is found to have a much higher current SFR than NGC 3512. The NGC 3177 result is shown to be less certain. No conclusive star formation history can be derived for NGC 1219 using the current data. The nature of the observed stellar populations in NGC 1569 and NGC 4449 are discussed. Evidence of young, red supergiants in the brightest star forming region of NGC 1569 suggests that the dominant population in this region must be very young ($\leq$10 Myr).PhDAstronomyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/105444/1/9124105.pdfDescription of 9124105.pdf : Restricted to UM users only