First-Author Publications

Identification of High-Redshift Galaxy Overdensities in GOODS-N and GOODS-S
     Abstract: We conduct a systematic search for high-redshift galaxy overdensities at 4.9 < z_spec < 8.9 in both the GOODS-N and GOODS-S fields using JWST/NIRCam imaging from JADES and JEMS in addition to JWST/NIRCam wide field slitless spectroscopy from FRESCO. High-redshift galaxy candidates are identified using HST+JWST photometry spanning λ = 0.4−5.0 μm. We confirmed the redshifts for roughly a third of these galaxies using JWST/FRESCO spectroscopy over λ = 3.9−5.0 μm through identification of either Hα or [OIII]λ5008 around the best-fit photometric redshift. The rest-UV magnitudes and continuum slopes of these galaxies were inferred from the photometry: the brightest and reddest objects appear in more dense environments and thus are surrounded by more galaxy neighbors than their fainter and bluer counterparts, suggesting accelerated galaxy evolution within overdense environments. We find 17 significant (δ_gal ≥ 3.04, N_gal ≥ 4) galaxy overdensities across both fields (7 in GOODS-N and 10 in GOODS-S), including the two highest redshift spectroscopically confirmed galaxy overdensities to date at ⟨z_spec⟩ = 7.955 and ⟨z_spec⟩ = 8.222 (representing densities around ≈ 6 and ≈ 12 times that of a random volume). We estimate the total halo mass of these large-scale structures to be 11.5 ≤ log(M_halo/M_⊙) ≤ 13.4 using an empirical stellar mass to halo mass relation, which are likely underestimates as a result of incompleteness. These protocluster candidates are expected to evolve into massive galaxy clusters with log(M_halo/M_⊙) ≥ 14 by z = 0. ADS Link. arXiv Link.

The JWST Advanced Deep Extragalactic Survey: Discovery of an Extreme Galaxy Overdensity at z = 5.4 with JWST/NIRCam in GOODS-S
     Abstract: We report the discovery of an extreme galaxy overdensity at z = 5.4 in the GOODS-S field using JWST/NIRCam imaging from JADES and JEMS alongside JWST/NIRCam wide field slitless spectroscopy from FRESCO. We identified potential members of the overdensity using HST+JWST photometry spanning λ = 0.4−5.0 μm. These data provide accurate and well-constrained photometric redshifts down to m ≈ 29−30 mag. We subsequently confirmed N = 81 potential members of the overdensity using JWST slitless spectroscopy over λ = 3.9−5.0 μm through a targeted line search for Hα around the best-fit photometric redshift. We verified that N = 42 galaxies reside in the field at z = 5.2−5.5 while N = 39 galaxies reside in an overdensity at z = 5.4 around ≈ 10 times that of a random volume. Stellar populations for these galaxies were inferred from the photometry and used to construct the star-forming main sequence, where protocluster members appeared more massive and exhibited earlier star formation (and thus older stellar populations) when compared to their field galaxy counterparts. We estimate the total halo mass of this large-scale structure to be 12.6 < log(M_halo/M_⊙) < 12.8 using an empirical stellar mass to halo mass relation, although this is likely an underestimate as a result of incompleteness. Our discovery demonstrates the power of JWST at constraining dark matter halo assembly and galaxy formation at very early cosmic times. ADS Link. arXiv Link.

The nebular properties of star-forming galaxies at intermediate redshift from the Large Early Galaxy Astrophysics Census
     Abstract: We present a detailed study of the partial rest-optical (λ_obs ≈ 3600-5600 Å) spectra of N = 328 star-forming galaxies at 0.6 < z < 1.0 from the Large Early Galaxy Astrophysics Census (LEGA-C). We compare this sample with low-redshift (z = 0) galaxies from the Sloan Digital Sky Survey (SDSS), intermediate-redshift (z = 1.6) galaxies from the Fiber Multi-Object Spectrograph (FMOS)-COSMOS Survey, and high-redshift (z = 2) galaxies from the Keck Baryonic Structure Survey (KBSS). At a lookback time of 6-8 Gyr, galaxies with stellar masses 10.25 < log(M_*/M_⊙) < 11.00 appear remarkably similar to z = 0 galaxies in terms of their nebular excitation, as measured using [OIII]λ5008/Hβ. There is some evidence that 0.6 < z < 1.0 galaxies with lower M_* have higher [OIII]λ5008/Hβ than z = 0 galaxies and are more similar to less evolved z = 1.6 and z = 2 galaxies, which are offset from the z = 0 locus at all M_*. We explore the impact selection effects, contributions from active galactic nuclei, and variations in physical conditions (ionization parameter and gas-phase oxygen abundance) have on the apparent distribution of [OIII]λ5008/Hβ and find somewhat higher ionization and lower enrichment in 0.6 < z < 1.0 galaxies with lower M_* relative to z = 0 galaxies. We use new near-infrared spectroscopic observations of N = 53 LEGA-C galaxies to investigate other probes of enrichment and excitation. Our analysis demonstrates the importance of obtaining complete rest-optical spectra of galaxies in order to disentangle these effects. ADS Link. arXiv Link.

Discovery and origins of giant optical nebulae surrounding quasar PKS 0454-22
     Abstract: We report optical integral-field spectroscopy in the field of one of the most luminous quasars in the z < 1 Universe, PKS 0454-22, with the Multi-Unit Spectroscopic Explorer. These data enable the discovery of three large ionized nebulae emitting in [OII], Hβ, and [OIII] with projected areas of 1720, 1520, and 130 pkpc², which we refer to as N1, N2, and N3, respectively. N1 spatially and kinematically surrounds the quasar host and five nearby galaxies. The morphology and kinematics of N1 are most consistent with stripped interstellar medium resulting from ongoing interactions. Its ionization properties can be explained by quasar photoionization. N2 spatially and kinematically surrounds two galaxies that are at projected distances of d ≈ 90 pkpc and line-of-sight velocities of δv ≈ +1410 km s^-1 from the quasar. The morphology and kinematics of N2 are also consistent with stripped interstellar medium. However, its ionization state requires additional ionization sources beyond the quasar, likely from fast shocks as it moves through the hot halo associated with a galaxy overdensity around the quasar. N3 is not coincident with any galaxies with secure redshifts, and may arise from a cool gas structure in the intragroup medium or a dwarf galaxy. These large ionized nebulae demonstrate that interactions can produce cool gas structures on halo scales, while also possibly facilitating quasar fueling. The growing availability of wide-area integral field spectroscopic data will continue to reveal the morphologies, kinematics, and conditions of the gas flows, which may fuel galaxy and black hole growth. ADS Link. arXiv Link.

Contributing-Author Publications

The FLAMINGO simulation view of cluster progenitors observed in the epoch of reionization with JWST
     Abstract: Motivated by the recent JWST discovery of galaxy overdensities during the Epoch of Reionzation, we examine the physical properties of high-z protoclusters and their evolution using the FLAMINGO simulation suite. We investigate the impact of the apertures used to define protoclusters, because the heterogeneous apertures used in the literature have limited our understanding of the population. Our results are insensitive to the uncertainties of the subgrid models at a given resolution, whereas further investigation into the dependence on numerical resolution is needed. When considering galaxies more massive than log(M_*/M_⊙) ≈ 8.0, the FLAMINGO simulations predict a dominant contribution from progenitors similar to those of the Coma cluster to the cosmic star-formation rate density during the reionization epoch. Our results indicate the onset of suppression of star formation in the protocluster environments as early as z ≈ 5. The galaxy number density profiles are similar to NFW at z < 1 while showing a steeper slope at earlier times before the formation of the core. Different from most previous simulations, the predicted star-formation history for individual protoclusters is in good agreement with observations. We demonstrate that, depending on the aperture, the integrated physical properties including the total (dark matter and baryonic) mass can be biased by a factor of 2 to 5 at z = 5.5-7.0, and by an order of magnitude at z < 4. This correction suffices to remove the ≈ 3σ tensions with the number density of structures found in recent JWST observations. ADS Link. arXiv Link.

To high redshift and low mass: exploring the emergence of quenched galaxies and their environments at 3 < z < 6 in the ultra-deep JADES MIRI F770W parallel
     Abstract: We present the robust selection of quiescent (QG) and post-starburst (PSB) galaxies using ultra-deep NIRCam and MIRI imaging from the JWST Advanced Deep Extragalactic Survey (JADES). Key to this is MIRI 7.7 μm imaging which breaks the degeneracy between old stellar populations and dust attenuation at 3 < z < 6 by providing rest-frame J-band. Using this, we identify 23 passively evolving galaxies in UVJ color space in a mass-limited (log(M_*/M_⊙) > 8.5) sample over 8.8 square arcminutes. Evaluation of this selection with and without 7.7 μm shows that dense wavelength coverage with NIRCam (8−11 bands including 1−4 medium-bands) can compensate for lacking the J−band anchor, meaning that robust selection of high-redshift QGs is possible with NIRCam alone. Our sample is characterized by rapid quenching timescales (≈ 100−600 Myr) with formation redshifts z_f < 8.5 and includes a potential record-holding massive QG at z_phot = 5.33^+0.16_-0.17 and two QGs with evidence for significant residual dust content (A_V ≈ 1−2). In addition, we present a large sample of 12 log(M_*/M_⊙) = 8.5-9.5 PSBs, demonstrating that UVJ selection can be extended to low mass. Analysis of the environment of our sample reveals that the group known as the Cosmic Rose contains a massive QG and a dust-obscured star-forming galaxy (a so-called Jekyll and Hyde pair) plus three additional QGs within ≈ 20 kpc. Moreover, the Cosmic Rose is part of a larger overdensity at z ≈ 3.7 which contains 7/12 of our low-mass PSBs. Another 4 low-mass PSBs are members of an overdensity at z ≈ 3.4; this result strongly indicates low-mass PSBs are preferentially associated with overdense environments at z > 3. ADS Link. arXiv Link.

JADES: deep spectroscopy of a low-mass galaxy at redshift 2.3 quenched by environment
     Abstract: We report the discovery of a quiescent galaxy at z = 2.34 with a stellar mass of only M_* ≈ 9.5 × 10⁸ M_⊙, based on deep JWST/NIRSpec spectroscopy. This is the least massive quiescent galaxy found so far at high redshift. We use a Bayesian approach to model the spectrum and photometry, and find the target to have been quiescent for 0.6 Gyr with a mass-weighted average stellar age of 0.8-1.7 Gyr (dominated by systematics). The galaxy displays an inverse colour gradient with radius, consistent with environment-driven quenching. Based on a combination of spectroscopic and robust (medium- and broad-band) photometric redshifts, we identify a galaxy overdensity near the location of the target (5σ above the background level at this redshift). We stress that had we been specifically targetting galaxies within overdensities, the main target would not have been selected on photometry alone; therefore, environment studies based on photometric redshifts are biased against low-mass quiescent galaxies. The overdensity contains three spectroscopically confirmed, massive, old galaxies (M_* = 8−17 × 10¹⁰ M_⊙). The presence of these evolved systems points to accelerated galaxy evolution in overdensities at redshifts z > 2, in agreement with previous works. In projection, our target lies only 35 pkpc away from the most massive galaxy in this overdensity (spectroscopic redshift z = 2.349) which is located close to overdensity's centre. This suggests the low-mass galaxy was quenched by environment, making it possibly the earliest evidence for environment-driven quenching to date. ADS Link. arXiv Link.

Evolution of Gas Flows along the Starburst to Post-Starburst to Quiescent Galaxy Sequence
     Abstract: We measure velocity offsets in the NaI λλ5890,5896 (Na D) interstellar medium absorption lines to track how neutral galactic winds change as their host galaxies evolve. Our sample of ∼80,000 SDSS spectra at 0.010 < z < 0.325 includes starburst, post-starburst, and quiescent galaxies, forming an evolutionary sequence of declining star formation rate (SFR). We detect bulk flows across this sequence, mostly at higher host stellar masses (log[M_*/M_⊙] > 10). Along this sequence, the fraction of outflows decreases (76±2% to 65±4% to a 3σ upper limit of 34%), and the mean velocity offset changes from outflowing to inflowing (−84.6±5.9 to −71.6±11.4 to 76.6±2.3 km/s). Even within the post-starburst sample, wind speed decreases with time elapsed since the starburst ended. These results reveal that outflows diminish as galaxies age. For post-starbursts, there is evidence for an AGN contribution, especially to the speediest outflows: 1) SFR declines faster in time than outflow velocity, a decoupling arguing against massive stellar feedback; 2) of the few outflows strong enough to escape the interstellar medium (9/105), three of the four hosts with measured emission lines are Seyfert galaxies. For disky starburst galaxies, however, the trends suggest flows out of the stellar disk plane (with outflow 1/2-opening angle > 45 degrees) instead of from the nucleus: the wind velocity decreases as the disk becomes more edge-on, and the outflow fraction, constant at ≈ 90% for disk inclinations i < 45 degrees, steadily decreases from ≈ 90% to 20% for i > 45 degrees. ADS Link. arXiv Link.

The Cosmos in its Infancy: JADES Galaxy Candidates at z > 8 in GOODS-S and GOODS-N
     Abstract: We present a catalog of 717 candidate galaxies at z > 8 selected from 125 square arcminutes of NIRCam imaging as part of the JWST Advanced Deep Extragalactic Survey (JADES). We combine the full JADES imaging dataset with data from the JEMS and FRESCO JWST surveys along with extremely deep existing observations from HST/ACS for a final filter set that includes fifteen JWST/NIRCam filters and five HST/ACS filters. The high-redshift galaxy candidates were selected from their estimated photometric redshifts calculated using a template fitting approach, followed by visual inspection from seven independent reviewers. We explore these candidates in detail, highlighting interesting resolved or extended sources, sources with very red long-wavelength slopes, and our highest redshift candidates, which extend to z_phot = 18. Over 93% of the sources are newly identified from our deep JADES imaging, including 31 new galaxy candidates at z_phot > 12. We also investigate potential contamination by stellar objects, and do not find strong evidence from SED fitting that these faint high-redshift galaxy candidates are low-mass stars. Using 42 sources in our sample with measured spectroscopic redshifts from NIRSpec and FRESCO, we find excellent agreement to our photometric redshift estimates, with no catastrophic outliers and an average difference of ⟨δz = z_phot − z_spec⟩ = 0.26. These sources comprise one of the most robust samples for probing the early buildup of galaxies within the first few hundred million years of the Universe's history. ADS Link. arXiv Link.

Inside the bubble: exploring the environments of reionisation-era Lyman-α emitting galaxies with JADES and FRESCO
     Abstract: We present a study of the environments of 16 Lyman-α emitting galaxies (LAEs) in the reionisation era (5.8 < z < 8.0) identified by JWST/NIRSpec as part of the JWST Advanced Deep Extragalactic Survey (JADES). Unless situated in sufficiently (re)ionised regions, Lyman-α emission from these galaxies would be strongly absorbed by neutral gas in the intergalactic medium (IGM). We conservatively estimate sizes of the ionised regions required to reconcile the relatively low Lyman-α velocity offsets (δv_{esc, Ly-α} < 300 km/s) with moderately high Lyman-α escape fractions (f_{esc, Ly-α} > 5%) observed in our sample of LAEs, indicating the presence of ionised "bubbles" with physical sizes of the order of 0.1 < R_ion/pMpc > 1.0 in a patchy reionisation scenario where the bubbles are embedded in a fully neutral IGM. Around half of the LAEs in our sample are found to coincide with large-scale galaxy overdensities seen in FRESCO at z ≈ 5.8-5.9 and z ≈ 7.3, suggesting Lyman-α transmission is strongly enhanced in such overdense regions, and underlining the importance of LAEs as tracers of the first large-scale ionised bubbles. Considering only spectroscopically confirmed galaxies, we find our sample of UV-faint LAEs (M_UV > −20 mag) and their direct neighbours are generally not able to produce the required ionised regions based on the Lyman-α transmission properties, suggesting lower-luminosity sources likely play an important role in carving out these bubbles. These observations demonstrate the combined power of JWST multi-object and slitless spectroscopy in acquiring a unique view of the early stages of Cosmic Reionisation via the most distant LAEs. ADS Link. arXiv Link.

JADES Imaging of GN-z11: Revealing the Morphology and Environment of a Luminous Galaxy 430 Myr After the Big Bang
     Abstract: We present JWST NIRCam nine-band near-infrared imaging of the luminous z = 10.6 galaxy GN-z11 from the JWST Advanced Deep Extragalactic Survey of the GOODS-N field. We find a spectral energy distribution (SED) entirely consistent with the expected form of a high-redshift galaxy: a clear blue continuum from 1.5 to 4.0 microns with a complete dropout in F115W. The core of GN-z11 is extremely compact in JWST imaging. We analyze the image with a two-component model, using a point source and a Sérsic profile that fits to a half-light radius of 200 pc and an index n = 0.9. We find a low-surface brightness haze about 0.4 arcseconds to the northeast of the galaxy, which is most likely a foreground object but might be a more extended component of GN-z11. At a spectroscopic redshift of 10.60 (Bunker et al. 2023), the comparison of the NIRCam F410M and F444W images spans the Balmer jump. From population synthesis modeling, here assuming no light from an active galactic nucleus, we reproduce the SED of GN-z11, finding a stellar mass of log(M_*/M_⊙) ≈ 9, a star-formation rate of ≈ 20 M_⊙/yr and a young stellar age of ≈ 20 Myr. As massive galaxies at high redshift are likely to be highly clustered, we search for faint neighbors of GN-z11, finding nine galaxies out to ≈ 5 comoving Mpc transverse with photometric redshifts consistent with z = 10.6, and a 10th more tentative dropout only 3 arcseconds away. This is consistent with GN-z11 being hosted by a massive dark-matter halo (≈ 8 × 10¹⁰ M_⊙), though lower halo masses cannot be ruled out. ADS Link. arXiv Link.

Identification and properties of intense star-forming galaxies at redshifts z > 10
     Abstract: Surveys with the James Webb Space Telescope (JWST) have discovered candidate galaxies in the first 400 Myr of cosmic time. Preliminary indications have suggested these candidate galaxies may be more massive and abundant than previously thought. However, without confirmed distances, their inferred properties remain uncertain. Here we identify four galaxies located in the JWST Advanced Deep Extragalactic Survey Near-Infrared Camera imaging with photometric redshifts z of roughly 10-13. These galaxies include the first redshift z > 12 systems discovered with distances spectroscopically confirmed by JWST in a companion paper. Using stellar population modelling, we find the galaxies typically contain 100 million solar masses in stars, in stellar populations that are less than 100 million years old. The moderate star-formation rates and compact sizes suggest elevated star-formation rate surface densities, a key indicator of their formation pathways. Taken together, these measurements show that the first galaxies contributing to cosmic reionization formed rapidly and with intense internal radiation fields. ADS Link. arXiv Link.

The Atacama Cosmology Telescope: DR4 maps and cosmological parameters
     Abstract: We present new arcminute-resolution maps of the Cosmic Microwave Background temperature and polarization anisotropy from the Atacama Cosmology Telescope, using data taken from 2013-2016 at 98 and 150 GHz. The maps cover more than 17,000 deg², the deepest 600 deg² with noise levels below 10μK-arcmin. We use the power spectrum derived from almost 6,000 deg² of these maps to constrain cosmology. The ACT data enable a measurement of the angular scale of features in both the divergence-like polarization and the temperature anisotropy, tracing both the velocity and density at last-scattering. From these one can derive the distance to the last-scattering surface and thus infer the local expansion rate, H₀. By combining ACT data with large-scale information from WMAP we measure H₀ = 67.6 ± 1.1 km/s/Mpc, at 68% confidence, in excellent agreement with the independently-measured Planck satellite estimate (from ACT alone we find H₀ = 67.9 ± 1.5 km/s/Mpc). The ΛCDM model provides a good fit to the ACT data, and we find no evidence for deviations: both the spatial curvature, and the departure from the standard lensing signal in the spectrum, are zero to within 1σ; the number of relativistic species, the primordial Helium fraction, and the running of the spectral index are consistent with ΛCDM predictions to within 1.5-2.2σ. We compare ACT, WMAP, and Planck at the parameter level and find good consistency; we investigate how the constraints on the correlated spectral index and baryon density parameters readjust when adding CMB large-scale information that ACT does not measure. The DR4 products presented here will be publicly released on the NASA Legacy Archive for Microwave Background Data Analysis. ADS Link. arXiv Link.

Portfolios

My photography book
     Abstract: The American Midwest // The American Southwest. A two-part photography book shot on 35mm film.

My digital art portfolio
     Abstract: The first (second) set of images were made using Deep Haze (Adobe Fresco).

My digital photography portfolio
     Abstract: These images were shot on digital cameras, edited with Adobe Lightroom.

My analog photography portfolio
     Abstract: These images were shot on 35mm film, developed and scanned by The Darkroom Lab.

My home movies
     Abstract: These movies were shot with a JVC Everio GZ-MG330 and edited with Adobe Premiere Pro.

Untitled
     Abstract: Unabstracted.

Miscellaneous

CV.
Resume.
ADS Link.
ORCID Link.
JADES Website.