2001ApJ...562L..23M

In this Letter we address the problem of the origin of blue cores and inverse 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 galaxies, allowing some of these galaxies to begin forming no more than ∼1 Gyr before the redshift of observation. The single-zone collapse model 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 formation more extended in the central region of the galaxy. We compare the predicted V₆₀₆-I₈₁₄(r) color gradients with those observed using the redshift 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 same time for the red and smooth color profiles reported. Based on the model, our analysis suggests two populations of field elliptical galaxies, one formed recently, within ≲1 Gyr, and another much older, formed ≳4 Gyr since the redshift of observation.