The Circumgalactic Medium during Cosmic Dawn and Afternoon in Stacked Spectroscopy
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depalma-ddepalma-phd-physics-2026-thesis.pdf
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Author(s)
DePalma, David
Advisor(s)
Simcoe, Robert A.
Date Issued
May 2026
Publisher
Massachusetts Institute of Technology
Abstract
Galaxies are the hubs of star formation in the universe, so galaxies evolve alongside the cosmic star formation rate density (SFRD). Since the history of the cosmic SFRD is singly peaked, the cosmic SFRD’s progression must reflect transformations in or around galaxies. Through the baryon cycle, the circumgalactic medium (CGM)—gas beyond a galaxy’s disk or ISM yet within the dark-matter halo’s virial radius—is crucial for understanding galaxy evolution. As the gas reservoir surrounding the galaxy, the CGM supplies the fuel for galaxies’ star formation while recording feedback from the galaxy. The CGM thereby mediates and archives galaxy evolution.
Observations of the CGM have hitherto targeted either the local universe or the cosmic SFRD’s peak, cosmic noon, out of instrumental and terrestrial considerations. The cosmic SFRD does not significantly change at either cosmic noon or presently, however. My thesis stacks spectra of the CGM at cosmic dawn—between redshifts 5 and 7—and the cosmic afternoon—between redshifts 0.4 and 1.0. As the first investigation of the CGM in these two epochs, my thesis explores the CGM while the SFRD rises and falls.
To probe the CGM, I participated in three surveys, each of which composes a chapter in my thesis. Each survey also represents a distinct method for observing the CGM. Through the Cosmic Ultraviolet Baryon Survey (CUBS), I cataloged ground-based photometry and spectroscopy from galaxies within the cosmic afternoon, then I associated the catalog’s galaxies with absorbers in transverse quasar absorption-line spectroscopy by the Hubble Space Telescope’s Cosmic Origins Spectrograph. Alongside CUBS, I searched for radio hyperfine 21-cm emission by H I around CUBS’ galaxies in the MeerKAT Absorption-Line Survey (MALS). For cosmic dawn, I adapted high-signal-to-noise ratio and high-resolving-power spectra by the MASQUERADE program of the James Webb Space Telescope’s Near-Infrared Spectrograph into down-the-barrel spectroscopy. While analyzing all three surveys, I developed stacking algorithms to boost each survey's signal-to-noise ratio. Together, the three surveys form a multi-band, multi-epoch complement of CGM studies.
My findings for each survey discovered links between the singly peaked history of the cosmic SFRD and the CGM. At cosmic dawn, I measured outflows from star-forming galaxies that were enriching the early CGM. Through detection of high-ionization but not auroral lines, the stacked spectrum evoked an interstellar medium ionized by hot stars rather than a broad-line active galactic nucleus. By the cosmic afternoon, however, star formation was declining, and I measured in MALS a concurrent exhaustion of H I gas that tracked the Kennicutt–Schmidt relation as a potential explanation for the reduction in star formation. CUBS–MALS is the first detection of H I beyond 75 kpc from the cosmic afternoon’s galaxies and the first catalog of galaxies that chronicles H I’s depletion during the cosmic afternoon. Meanwhile, my radial maps for CUBS revealed that, during the cosmic afternoon, the CGM was large, heterogeneous, and depleting its cool gas from which galaxies could form stars. By crosscutting wavelengths and epochs, my thesis contributes toward completing the timeline of coevolution between galaxies and the surrounding gas.
MIT Department
Massachusetts Institute of Technology. Department of Physics
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