Bulge and Clump Evolution in Hubble Ultra Deep Field Clump Clusters, Chains and Spiral Galaxies

TL;DR

This study investigates the properties and evolution of bulges and star-forming clumps in high-redshift galaxies from the Hubble Ultra Deep Field, revealing their masses, ages, and formation processes, and suggesting they are early phases of spiral galaxy development.

Contribution

It provides new insights into the ages, masses, and formation mechanisms of bulges and clumps in early galaxies, linking them to the formation of modern spiral structures.

Findings

Bulge-like objects in clump and chain galaxies are similar in age and more massive than star-forming clumps.

Bulges in spiral galaxies are older and significantly more massive than the clumps.

Clump masses range from 10^7 to 10^8 solar masses, with ages around 100 Myr.

Abstract

Clump clusters and chain galaxies in the Hubble Ultra Deep Field are examined for bulges in the NICMOS 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 bulge-like 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 bulge-like objects in clump cluster and chain galaxies have similar ages and 2 to 5 times larger masses compared to the star-formation clumps, while the bulges in spirals have ~6 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 Msun; 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 or more, combined with a high gas mass column density, ~100 Msun/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.