Stellar populations as a function of radius in giant elliptical galaxies

Accurate surface photometry has been obtained in J and K for 12 giant elliptical galaxies. Ellipses have been fitted, to obtain luminosity, ellipticity, and major axis position angle profiles. The results have been combined with visual profiles from CCD observations. It is found that elliptical galaxies become bluer in V - K on the average by 0.3 (mag/sq arcsec x r/e/) going outward from the nucleus. Radial color gradients in V - K correlate strongly with those in U - V and B - V. By modeling, using theoretical isochrones, it is found that the observed color gradients can be explained by a gradient in metallicity, if, in addition, the proportion of hot stars rises very rapidly with decreasing metallicity