research
how the universe's most massive galaxies formed, and the projects that get me there
I study how the universe’s most massive galaxies formed in its first billion years—an era now in tension with cosmological models. By combining wide-area near-infrared surveys with spatially-resolved spectroscopy, I identify and characterize rare, ultra-luminous galaxies at cosmic dawn. My work leverages custom-built catalogs, advanced statistical tools, and cutting-edge observations to uncover how these galaxies assembled so rapidly in the opening act of cosmic history.
| PhD Thesis | MPhys Thesis | Approved observing programs | Professional Talks | Group |
Beasts in the Bubbles
Where do today's most massive galaxies come from — what did their earliest progenitors look like?
My JWST Cycle 1 PI program, Beasts in the Bubbles, targets five of the most massive galaxy candidates known at z ~ 9 — plausible progenitors of the massive galaxies we see later in cosmic history. Using deep, spatially-resolved NIRSpec IFU spectroscopy, we measure their stellar masses, ages, dust content, and ionizing photon output. Results are currently in preparation from the group (Nezhad, Manesh, Taamoli, Henson).
The growth of massive galaxies
How did the population of massive galaxies assemble and grow across cosmic time?
Once you have candidate progenitors, the next step is charting the build-up of the population they belong to — how the number density and typical mass of galaxies evolves with cosmic time. This thread measures stellar mass functions across COSMOS2020, Euclid's early data releases, and the Cosmic Dawn Survey, tracing the assembly history of massive galaxies from cosmic dawn onward.
- Zalesky, Weaver et al. 2026 — Euclid preparation. LXXXVI. Cosmic Dawn Survey: Evolution of the galaxy stellar mass function across 0.2 < z ≤ 6.5 measured over 10 square degrees. ADS
- Weaver et al. 2023b — COSMOS2020: The galaxy stellar mass function. The assembly and star formation cessation of galaxies at 0.2 < z ≤ 7.5. ADS
Characterizing luminous, actively star-forming galaxies
What powers the extreme luminosity of the brightest galaxies at cosmic dawn — star formation, AGN, or both?
For the most luminous candidates, the next question is what's actually driving the light: a burst of star formation, an active black hole, or some mix of both. This thread uses spectroscopy to pin down the ionizing sources and ISM conditions in these galaxies.
- Algera, Weaver et al. 2026 — A first systematic study of [OIII] 88μm at z > 8: two luminous oxygen lines and a powerful ionized outflow in the first 600 million years. ADS
- Treiber, Greene, Weaver et al. 2025 — UNCOVERing the High-redshift AGN Population among Extreme UV Line Emitters. ADS
- Weaver et al. 2025 — Euclid: Early Release Observations — NISP-only sources and the search for luminous z = 6─8 galaxies. ADS | video summary
- Fujimoto, Wang, Weaver et al. 2024 — UNCOVER: A NIRSpec Census of Lensed Galaxies at z = 8.50─13.08 Probing a High-AGN Fraction and Ionized Bubbles in the Shadow. ADS
The origins of bimodality
Why do galaxies split into two populations — star-forming and quiescent — and what drives a galaxy from one to the other?
This question runs through my whole career, from resolved spectroscopy of a single nearby merger to statistical samples of massive galaxies across cosmic time. My Masters research at the University of St Andrews asked it directly, tracing stellar age gradients in nearby post-starburst galaxies; my work on NGC 7252 used integral field spectroscopy to catch a merger remnant mid-transition. The same question now drives my work on quiescent galaxies at cosmic dawn and cosmic noon — including a hint of early environmental quenching in a low-mass system, a different mechanism than the internal processes usually invoked for the most massive quiescent galaxies.
- Cutler, Weaver et al. 2025 — The Structure and Formation Histories of Low-mass Quiescent Galaxies in the A2744 Cluster Environment. ADS
- Wright, Whitaker, Weaver et al. 2024 — Remarkably Compact Quiescent Candidates at 3 < z < 5 in JWST-CEERS. ADS
- Weaver et al. 2018 — History and destiny of an emerging early-type galaxy: New IFU insights on the major-merger remnant NGC 7252. ADS | ESO Image of the Week
- MPhys Thesis — The Origins of Bimodality: Post-starburst Galaxies at z < 0.1. PDF (MPhys Project Prize 2018)
UNCOVER
What can you find in the deepest combined JWST+HST imaging of a single lensing cluster?
UNCOVER/MegaScience combines the deepest HST and JWST imaging yet obtained of the Abell 2744 cluster field, using gravitational lensing to push to extremely faint magnitudes. Beyond the catalog itself, the survey's spectroscopic follow-up revealed a startling abundance of compact, red, likely AGN-dominated sources — the "Little Red Dots" — reshaping how common actively accreting black holes are thought to be at cosmic dawn. In 2027, the survey team was awarded the AAS's Berkeley Prize.
- Labbe et al. 2025 — UNCOVER: Candidate Red Active Galactic Nuclei at 3 < z < 7 with JWST and ALMA. ADS
- Suess, Weaver et al. 2024 — Medium Bands, Mega Science: A JWST/NIRCam Medium-band Imaging Survey of A2744. ADS
- Greene et al. 2024 — UNCOVER Spectroscopy Confirms the Surprising Ubiquity of Active Galactic Nuclei in Red Sources at z > 5. ADS
- Weaver et al. 2024 — The UNCOVER Survey: A First-look HST + JWST Catalog of 60,000 Galaxies near A2744 and beyond. ADS | data
- Kokorev et al. 2023 — UNCOVER: A NIRSpec Identification of a Broad-line AGN at z = 8.50. ADS
- Goulding et al. 2023 — UNCOVER: The Growth of the First Massive Black Holes from JWST/NIRSpec-Spectroscopic Redshift Confirmation of an X-Ray Luminous AGN at z = 10.1. ADS
COSMOS2020
What can you study once you have the largest near-infrared survey at this depth?
COSMOS2020 combined data from over 30 observatories into two complementary multi-wavelength catalogs spanning the full COSMOS field. Beyond the catalogs themselves, the team has used it to study everything from protoclusters and quenching to AGN activity and machine-learning-based parameter estimation.
- Weaver et al. 2022 — COSMOS2020: A Panchromatic View of the Universe to z∼10 from Two Complementary Catalogs. ADS | catalogs
- Gould et al. 2023 — COSMOS2020: Exploring the Dawn of Quenching for Massive Galaxies at 3 < z < 5 with a New Color-selection Method. ADS
- Brinch, Greve, Weaver et al. 2023 — COSMOS2020: Identification of High-z Protocluster Candidates in COSMOS. ADS
- Kauffmann, Ilbert, Weaver et al. 2022 — COSMOS2020: UV-selected galaxies at z ≥ 7.5. ADS
- Shuntov et al. 2022 — COSMOS2020: Cosmic evolution of the stellar-to-halo mass relation for central and satellite galaxies up to z ~ 5. ADS
- Ito et al. 2022 — COSMOS2020: Ubiquitous AGN Activity of Massive Quiescent Galaxies at 0 < z < 5 Revealed by X-Ray and Radio Stacking. ADS
Methods & tools
What statistical and software tools make this kind of survey science possible?
Behind every catalog is a pipeline. I built The Farmer, an open-source profile-fitting photometry package now used across several JWST programs, and have worked on statistical and machine-learning methods for galaxy classification. See also the repositories page.
AGN & black holes
How do active black holes vary, and how common are they in the early universe?
A shorter-running thread: using photometric variability to probe accretion physics around supermassive black holes, and more recently, how common active galactic nuclei are among the highest-redshift galaxy populations.
- Weaver & Horne 2022 — Dust and the intrinsic spectral index of quasar variations: hints of finite stress at the innermost stable circular orbit. ADS