The giant branch of the Fornax dwarf spheroidal galaxy

Abstract

A color-magnitude diagram for a large sample of stars in the Fornax dwarf spheroidal galaxy has been obtained from BV electronographic observations. These reach to the level of the horizontal branch, below V = 21 mag. The area of investigation is located ~ 18 arcmin away in a southwest direction from the center of the Fornax galaxy. The wide field of the McMullan camera enabled some 1300 stars to be measured and made possible an analysis of structure in the giant branch. The standard deviation of the intrinsic width of the giant branch is ~0.14 mag in (B-V) color at a given luminosity, which yields a standard deviation of 0.3 in Fe/H, if a spread in metallicity alone is responsible for the intrinsic width. Otherwise a combination of both variation in age from 3 to 17 billion yr and a spread in metallicity equivalent to a standard deviation of 0.27 in Fe/H can account for the intrinsic width of the giant branch. A comparison with galactic globular clusters as well as theoretical isochrones suggests a mean value of [Fe/H] = -1.5 ± 0.1 for the Fornax giant branch stars.