The structure and star formation history of early-type galaxies in the Ultra Deep Field/grapes survey

We present a two-pronged approach to the formation of early-type galaxies, using a sample of 18 galaxies at 0.5 ≲ z ≲ 1 from the HST ACS Ultra Deep Field and GRAPES surveys: (1) We combine slitless low-resolution spectroscopy from the GRAPES data set with simple models of galaxy formation to explore their star formation histories. (2) We also perform an analysis of their surface brightness distribution with the unprecedented details provided by the ACS superb angular resolution and photometric depth. Our spectroscopic analysis reveals that their stellar populations are rather homogeneous in age and metallicity and formed at redshifts Z_F ∼ 2-5. Evolving them passively, they become practically undistinguishable from elliptical galaxies at z = 0. Also, their isophotal shapes appear very similar to those observed for nearby elliptical galaxies, in that the percentages of disky and boxy galaxies at z ∼ 1 are close to the values measured at z = 0. Moreover, we find that the isophotal structure of z ∼ 1 early-type galaxies obeys the correlations already observed among nearby elliptical galaxies; i.e., disky elliptical galaxies have generally higher characteristic ellipticities, and boxy elliptical galaxies have larger half-light radii and are brighter in the rest-frame B band. In this respect then, no significant structural differences are seen for elliptical galaxies between z = 0 and 1. Exception can be possibly made for the a₃/a parameter, which is larger at z ∼ 1 than usually measured at z = 0. The a₃/a parameter measures the deviations from a pure elliptical isophote, which are not symmetric with respect to the galaxy center, as in the case of dust features and most notably of clumps. Blue clumps have been detected in nearly 50% of the z ∼ 1 early-type galaxies; their photometry is suggestive of young star clusters or dwarf irregulars if they are assumed to be at the same redshift as their host galaxies. We speculate that these clumps may represent recent accretion episodes and that they could be a way to produce blue cores if their dynamical time is such for them to rapidly sink to the galaxy center.