Origin of color gradients in elliptical galaxies

The origin of color gradients in elliptical galaxies is examined by compari ng model gradients with those observed in the Hubble Deep Field. The models are constructed so as to reproduce color gradients in local elliptical gal axies either by a metallicity gradient or by an age gradient. By looking-ba ck a sequence of the color gradient as a function of redshift, the age-meta llicity degeneracy is solved. The observed color gradients in elliptical ga laxies at z = 0.1 to 1.0 agree excellently with those predicted by the meta llicity gradient, while they deviate significantly from those predicted by the age gradient even at z similar to 0.3, and the deviation becomes larger with increasing redshift. This result does not depend on cosmological para meters and parameters for an evolutionary model of an elliptical galaxy wit hin a reasonable range. Thus our results clearly indicate that the origin o f color gradients is not age but stellar metallicity.