PhD defense by John R. Weaver

Title: COSMOS2020: Insights into Galaxy Assembly and Evolution over the First 10 Billion Years


Long before the invention of the telescope, early humans looked up and marvelled at the beauty of the night sky. Our inescapable curiosity has led us to chart those distant points of light, to measure their properties, and understand them through physical law. This cosmic cartography was revolutionized in the 16th century with the first scientific, and in many senses modern, star catalogs built by Tycho Brahe from his observatory in the middle of the windswept Danish straits. Then almost four centuries later in 1995, the Hubble Space Telescope stared deep into the abyss and captured the light of thousands of galaxies seen as they were in their infancy, when our universe was only a fraction of its current age. This was a watershed moment, and since then, astronomers have mapped successively larger and deeper regions of our universe. Such is the COSMOS survey, observed over a region of the sky a bit larger than the full moon by some of the foremost telescopes and studied by a worldwide collaboration of astronomers. Through its discoveries, COSMOS has earned its place as a cornerstone of modern galaxy evolution studies. This dissertation is centered around the COSMOS2020 catalogs, our latest cartographic effort to measure these increasingly deeper and complex images. New innovations in measurement techniques developed for this task have proven their worth: instead of measuring the light of galaxies in circular apertures, whole models are fitted that produce more accurate measures of their brightness, masses, and distances than ever before. We have used these new catalogs to measure how galaxies assemble and transform over 75% of cosmic history. To our surprise, we have discovered a handful of ultra-luminous galaxies seen only 600 million years after the big bang — cosmic beasts — whose incredible mass and maturity defies our understanding of how galaxies form and evolve. Soon, their mysterious origins will be revealed by some of the first observations by the James Webb Space Telescope.


  • Sune Toft, Niels Bohr Institute, University of Copenhagen

Evaluation Committee:

  • Karina Caputi, Faculty of Science and Engineering, University of Groningen
  • Danilo Marchesini, Physics & Astronomy Department, Tufts University
  • Pascal Oesch, Niels Bohr Institute, University of Copenhagen


There will be a following reception in Aud. C (across the hallway of Aud. A)


ZOOM-link for participants whom are not able to attend at Aud. A :

Meeting ID: 699 4274 1037

Passcode: 860981