PhD Defense by Katriona Mai Landau Gould

PhD Thesis: Link for the PhD Thesis will be updated, when it is uploaded by the NBI-Education team - PhD theses by DAWN – University of Copenhagen (

Title: ULTRA RED GALAXIES IN THE DISTANT UNIVERSE: The first quiescent galaxies and their hidden progenitors

Abstract: Understanding the story of how galaxies formed and evolved over cosmic history is the shared quest of extra-galactic astronomers. If one were to look out into the universe around us, it would become evident that galaxies fall roughly into two groups: those that still actively form stars (blue galaxies), and those that have long since become quiescent (red galaxies). These
populations arise as an natural consequence of the interplay between galaxies, the dark matter halos they live in, and the gas that surrounds these environments, the latter of which is the key ingredient of star formation. But what cannot be explained, is how only 1-2 billion years after the Big Bang, quiescent galaxies began to appear, seemingly having assembled billions of solar masses in a few hundred million years, and quenched their star formation in equally rapid timescales. These are the first massive quiescent galaxies (MQGs). This issue has been the subject of much examination over the past few decades, increasingly so since these galaxies were spectroscopicically confirmed only in the past decade. Despite these studies, several challenges remain, ranging from how best to select these galaxies in our data sets, to understanding their physical origins. This thesis presents the exploration of these first quiescent galaxies and their possible origins via the study of data taken with both ground and spaced-based telescopes, including JWST, using two new codes presented in this work. The first part of this thesis presents a novel selection method designed to find high redshift quiescent galaxies in photometric catalogs, specifically tailored to their unique colours. This method was designed to allow for a wide variety of colours, and therefore allows us to select more recently quenched galaxies, as they are bluer than the typical quiescent galaxy. First tuning the method using cosmological simulations, I found that my method outperforms a classical colour selection at z > 3. I then applied the selection method to the widest and deepest optical/near-infrared photometric catalog of the COSMOS field, COSMOS2020. Therein I explored the number densities to understand the rarity of these galaxies, and compared to similar galaxies in simulations in order to understand their physical origins. I calculated the number density of MQGs from z = 5 to z = 2, confirming this is period of tremendous growth in the early universe. I compared these number densities to those produced from cosmological simulations and found that no simulations can reproduce the number densities of MQGs found observationally at z > 3, implying that new recipes are needed.

In the second part of the thesis, I applied this method to eleven public imaging fields taken in the first three months of JWST observations, finding number densities for massive (log(M∗/Mo)>10,6) (quiescent galaxies at 3 < z < 4 on the order of 10−5 Mpc−1, with field to field variations and cosmic variance playing a huge role in deriving these estimates, highlighting the importance of searching for these galaxies over multiple fields. Tuning the selection allowed for the discovery of lower mass quiescent galaxies, something which was expected, but highly difficult to study prior to the launch of JWST. Finally, a quiescent galaxy candidate at z > 4 selected by the method was later confirmed to be one of the most distant and old quiescent galaxies ever found to date, highlighting the method’s flexibility in finding all types of galaxies. I also used this colour selection to identify quiescent galaxy candidates at z > 2 in the CANUCS survey, and presented a preliminary sample of spectra of quiescent galaxies at z ∼ 2 observed with JWST/NIRSpec, including two galaxies at z > 3. These galaxies exhibit a range of spectra of different apparent ages, further demonstrating the colour selection method as a way to explore galaxies in all stages of quenching.

Finally, in the third part of the thesis, I presented novel observations of an optically invisible, dusty star forming galaxy at z = 3.65 (dubbed “BB”) observed both with JWST/NIRCam and NIRSpec. I developed a new software specifcally to ft the spectrum of this galaxy, and explored its stellar populations and inter-stellar medium (ISM) properties using a combination of photometry and spectroscopy. I found that BB is a massive, star forming, highly dust obscured galaxy, with a strongly ionized ISM. Line ratios based on standard diagnostics as well as significant residuals indicating a broadened HeIλ10830 line imply the galaxy is either hosting an active black hole, is experiencing shocks, or has strong stellar winds from evolved stars. This galaxy is already as small as z ∼ 3 quiescent galaxies, and combined with its stellar mass and fast gas depletion timescale, I hypothesised that BB will join the compact quiescent galaxy population by z ∼ 2. Based on BB’s similarity to other optically dark main sequence galaxies at z > 3, it is possible this population could also be progenitors of the high redshift massive quiescent galaxy population.


  • Gabriel Barnes Brammer, Niels Bohr Institute, University of Copenhagen


  • Francesco Valentino, European Southern Observatory (ESO)

Evaluation Committee:

  • Pascal Oesch, University of Geneva, and the Niels Bohr Institute, University of Copenhagen
  • Mariska Kriek, Leiden University
  • Ross McLure, The University of Edinburgh


There will be a following reception at NBB in the DAWN Section on the 2nd floor (Niels Bohr Building, Rådmandsgade 62-64, 2200 Copenhagen N)

ZOOM-link for participants whom are not able to attend : 

Meeting ID: 613 3396 9355

Passcode: 099435