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Session 9 - Dwarf Galaxies.
Display session, Monday, June 10
Great Hall,
Deep V and I CCD images in a central and an outer field of the Local Group dwarf elliptical galaxy NGC147 have been obtained with the WFPC2 on board of the Hubble Space Telescope \rm. The color-magnitude diagram (CMD) shows a number of interesting features, including a red horizontal branch (HB) in the outer field, a prominent red clump in both fields, and the presence of asymptotic giant branch (AGB) stars in both fields. A mean distance modulus of \mu=24.41 is derived based on both the HB and the RGB tip brightness. Analyses of the CMDs also reveal some interesting variations of the stellar contents with galactocentric distance: 1) The mean metallicity [Fe/H] deduced from the RGB color is found to be -0.91\pm0.03 in the inner field, and -1.01\pm0.03 in the outer field. The metallicity dispersion shows a smooth variation with galactocentric radius, decreasing from 0.7 at the center to 0.3 at a radius of about 4 arcmin. 2) The AGB stars are highly concentrated toward the center of the galaxy; the percentage of the AGB stars is about 0.6% at the center, and gradually drops to about 0.3% at a radius of 3 arcmin, and shows no changes beyond. 3) The ratio of HB stars to RGB stars in the outer field shows a tendency of decreasing towards smaller radius, in contrast to that of the AGB to RGB ratio. Implications of these properties about the star formation history in the galaxy are briefly discussed. A new maximum likelihood isochrone fitting method based on the Genetic Algorithms optimization techniques is developed and used for experimental fits to these data.
This work has been supported by NASA Contract NAS7-1260 to JPL.
