Central Blue Clumps in Elliptical Galaxies of the Hubble Ultra Deep Field

Abstract

Elliptical galaxies larger than 10 pixels in the Hubble Ultra Deep Field (UDF) were surveyed for internal structure; 30 out of 100 in a sample of 884 morphologically classified galaxies exhibit large blue clumps near their centers. Unsharp-masked images of the clearest cases are presented. The distributions of the clumps on color-color and color-magnitude diagrams are about the same as the distributions of isolated objects in the UDF with the same size, suggesting a possible accretion origin. In the few cases for which redshifts have been published, the clump masses and star formation ages were determined from stellar evolution models, as were the galaxy masses. The clump mass scales with galaxy mass, probably because of selection effects, and ranges from 106 to 108 Msolar for galaxies with masses from 109 to 1011 Msolar. The clump star formation age ranges between 107 and 2×108 yr. With partial evaporation and core contraction in the intervening years, some of these clumps could resemble globular clusters today. Stars that evaporate will contribute to the field population in the elliptical galaxies.