The origin of blue cores in Hubble deep field E/S0 galaxies

In this Letter we address the problem of the origin of blue cores and inver se color gradients in early-type galaxies reported in the Hubble Deep Field -North and Hubble Deep Field-South by Menanteau, Abraham, & Ellis. We use a multizone single-collapse model. This model accounts for the observed blue cores by adopting a broad spread in formation redshifts for elliptical gal axies, allowing some of these galaxies to begin forming no more than simila r to1 Gyr before the redshift of observation. The single-zone collapse mode l then produces cores that are bluer than the outer regions because of the increase of the local potential well toward the center, which makes star fo rmation more extended in the central region of the galaxy. We compare the p redicted V-606 -I-814 (r) color gradients with those observed using the red shift of formation (z(F)) of the elliptical as the only free parameter. We find that the model can account with relatively good agreement for the blue cores and inverse color gradients found in many spheroidals and at the sam e time for the red and smooth color profiles reported. Based on the model, our analysis suggests two populations of field elliptical galaxies, one for med recently, within less than or similar to1 Gyr, and another much older, formed greater than or similar to4 Gyr since the redshift of observation.