The stellar content of elliptical galaxies: optical and infrared colour profiles of M32 and NGC 205.
We have obtained high quality surface photometry of the two galaxies of M 31 in optical and infrared colours. We find that in M 32 all colours, as well as the CO band, do not depend on radius from the center. This is consistent with profiles in the Mg_2_ index available for this galaxy in the literature, but not in other, mostly weaker, lines. In this respect M 32 follows the trend for compact elliptical galaxies, which contrary to giant ellipticals in general do not display any colour gradients. Since all compact ellipticals are companions to brighter galaxies and are tidally truncated we presume that the tidal interaction must be responsible for the removal of the interstellar material at a certain epoch, so that all star formation stopped. In NGC 205 we find a very blue center and colours that get redder slowly when going outward. The colour gradient reverses sign at ~60". Because of the presence of very young stars near the center we attribute the gradient in the inner areas to a difference in age. Optical and optical-infrared gradients are consistent with being caused by metallicity or age.
Astronomy and Astrophysics
- Pub Date:
- April 1993