The Hubble Ultra-Deep Field, 15 Years Later

Fifteen years ago, the Hubble Space Telescope gazed intently at the infrared glow of galaxies in a tiny fraction of the sky. New research shows how this patch of space has changed since then.

Ultra-Deep and Ultra-Famous

The Hubble Ultra-Deep Field is perhaps one of the most recognizable images of our universe. Assembled from observations made in 2003–2004, the visible-light Ultra-Deep Field showcases 10,000 galaxies that stretch back to less than a billion years after the Big Bang.

In 2008–2009 and 2012, Hubble revisited the region, this time piecing together infrared portraits that revealed even more distant galaxies that were absent from the visible-light view. The most distant galaxies in the infrared images appear as they were when the universe was just 2–5% of its current age. What can we learn from comparing these archival images to new images of the same patch of sky?

Paying Another Visit

With Hubble happily still operational (knock on wood!), Matthew Hayes (Stockholm University) and collaborators turned the telescope’s infrared camera toward the Ultra-Deep Field once again in 2023, aiming to find differences between the new images and the original infrared deep-field images from 2008–2009 and 2012.

In particular, the team hoped to find evidence for faint active galactic nuclei: accreting supermassive black holes at the centers of galaxies. As an active galactic nucleus gulps down varying amounts of gas from its surroundings, its brightness changes like a flickering flame — and these brightness changes are potentially detectable in the set of Ultra-Deep Field images. By cataloging active galactic nuclei in the early universe, researchers hope to pin down how the number of supermassive black holes has changed with cosmic time. This information can help determine how black holes form and evolve.

Spotted: Black Holes

Hayes’s team used two methods to search for sources with varying brightness — potential active galactic nuclei — in the new and archival Hubble images:

1. Subtracting one image from another to identify objects that appear in only one image
2. Comparing the brightness of the centers of galaxies between images

In total, they spotted 71 objects whose brightness varied significantly over the time period. Of the eight objects of interest presented in this work, two are active galactic nuclei at redshifts of z = 2.0 and 3.2 (about 2 to 3 billion years after the Big Bang). Three other objects are likely active galactic nuclei at redshifts beyond z = 6 (less than a billion years after the Big Bang) that couldn’t be definitively cataloged. A further three objects appear to be supernovae, one of which is perched on the edge of a disk galaxy and two of which have no apparent host galaxy.

Hayes’s team used these results to place a lower limit on the number density of black holes during the epoch of reionization, when radiation from the first stars and galaxies transformed the universe from opaque to transparent. They found that the number density of black holes in this time period is similar to the present day value, providing a critical test of black hole formation models. These are just the first results from this campaign, so you can look forward to more ultra-deep-field findings in the future!

Citation

“Glimmers in the Cosmic Dawn: A Census of the Youngest Supermassive Black Holes by Photometric Variability,” Matthew J. Hayes et al 2024 ApJL 971 L16. doi:10.3847/2041-8213/ad63a7