BULGE AND CLUMP EVOLUTION IN HUBBLE ULTRA DEEP FIELD CLUMP CLUSTERS, CHAINS AND SPIRAL GALAXIES

Abstract

Clump clusters and chain galaxies in the Hubble Ultra Deep Field (UDF) are examined for bulges in Near-Infrared Camera Multi-Object Spectrometer images. Approximately 50% of the clump clusters and 30% of the chains have relatively red and massive clumps that could be young bulges. Magnitudes and colors are determined for these bulgelike objects and for the bulges in spiral galaxies, and for all of the prominent star formation clumps in these three galaxy types. The colors are fitted to population evolution models to determine the bulge and clump masses, ages, star formation rate decay times, and extinctions. The results indicate that bulgelike objects in clump clusters and chain galaxies have similar ages and two to five times larger masses compared to the star formation clumps, while the bulges in spirals have roughly six times larger ages and 20 to 30 times larger masses than the clumps. All systems appear to have an underlying red disk population. The masses of star-forming clumps are typically in a range from 10 7 to 10 8 M ☉ ; their ages have a wide range around ∼10 2 Myr. Ages and extinctions both decrease with redshift. Star formation is probably the result of gravitational instabilities in the disk gas, in which case the large clump mass in the UDF is the result of a high gas velocity dispersion, 30 km s −1 or more, combined with a high gas mass column density, ∼100 M ☉ pc −2 . Because clump clusters and chains dominate disk galaxies beyond z ∼ 1, the observations suggest that these types represent an early phase in the formation of modern spiral galaxies, when the bulge and inner disk formed.