STAR FORMATION HISTORY AND DUST PRODUCTION: NGC147 AND NGC185

Abstract

NGC147 and NGC185, paired satellites of the Andromeda galaxy, possess the same order of mass and analogous structures, but they show different star formation and different amounts of interstellar gas and dust. Therefore, we present the first reconstruction of the star formation history of NGC147 and NGC185. Asymptotic Giant Branch stars are highly evolved stars that are brightest in K-band. This maximum K-band magnitude is related to the birth mass of stars. As a result, we have found a 9.9 Gyr old single star formation epoch for NGC185 followed by relatively continuous star formation. NGC147, however, has passed through two star formation episodes; one is as old as ~6 Gyr and the other is as recent as ~850 Myr. Asymptotic Giant Branch stars are also important dust factories; by fitting Spectral Energy Distributions to observed near and mid infrared data for each star, we were able to measure the dust production rates of individual stars; on order of <TEX>$10^{-5}M_{\odot}yr^{-1}$</TEX>. Hence, we estimate the total mass entering the interstellar medium to be <TEX>$1.06{\times}10^{-4}M_{\odot}yr^{-1}$</TEX> and <TEX>$2.89{\times}10^{-4}M_{\odot}yr^{-1}$</TEX> for NGC147 and NGC185.