Long period variable stars in NGC 147 and NGC 185. I. Their star
  formation histories
  NGC 147 and NGC 185 are two of the most massive satellites of the Andromeda
galaxy (M 31). Close together in the sky, of similar mass and morphological
type dE, they possess different amounts of interstellar gas and tidal
distortion. The question therefore is, how do their histories compare? Here we
present the first reconstruction of the star formation histories of NGC 147 and
NGC 185 using long-period variable stars. These represent the final phase of
evolution of low- and intermediate-mass stars at the asymptotic giant branch,
when their luminosity is related to their birth mass. Combining near-infrared
photometry with stellar evolution models, we construct the mass function and
hence the star formation history. For NGC 185 we found that the main epoch of
star formation occurred 8.3 Gyr ago, followed by a much lower, but relatively
constant star formation rate. In the case of NGC 147, the star formation rate
peaked only 7 Gyr ago, staying intense until ~ 3 Gyr ago, but no star formation
has occurred for at least 300 Myr. Despite their similar masses, NGC 147 has
evolved more slowly than NGC 185 initially, but more dramatically in more
recent times. This is corroborated by the strong tidal distortions of NGC 147
and the presence of gas in the centre of NGC 185.
