Harley Katz
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View article: JWST and ALMA Joint Analysis with [O <scp>II</scp> ] <i>λλ</i> 3726, 3729, [O <scp>III</scp> ] <i>λ</i> 4363, [O <scp>III</scp> ] 88 <i>μ</i> m, and [O <scp>III</scp> ] 52 <i>μ</i> m: Multizone Evolution of Electron Densities at <i>z</i> ∼ 0–14 and its Impact on Metallicity Measurements
JWST and ALMA Joint Analysis with [O <span>II</span> ] <i>λλ</i> 3726, 3729, [O <span>III</span> ] <i>λ</i> 4363, [O <span>III</span> ] 88 <i>μ</i> m, and [O <span>III</span> ] 52 <i>μ</i> m: Multizone Evolution of Electron Densities at <i>z</i> ∼ 0–14 and its Impact on Metallicity Measurements Open
We present a JWST and Atacama Large Millimeter/submillimeter Array (ALMA) detailed study of the interstellar medium properties of high-redshift galaxies. Our JWST/NIRSpec integral field unit spectroscopy targeting three galaxies at z = 6–7…
View article: Cloudy with a chance of starshine: Possible photometric signatures of nebular-dominated emission in $1.5 < z < 8.5$ JADES galaxies
Cloudy with a chance of starshine: Possible photometric signatures of nebular-dominated emission in $1.5 < z < 8.5$ JADES galaxies Open
The discovery of high-redshift galaxies exhibiting a steep spectral UV downturn potentially indicative of two-photon continuum emission marks a turning point in our search for signatures of top-heavy star formation in the early Universe. W…
View article: MEGATRON: The environments of Population III stars at Cosmic Dawn and their connection to present day galaxies
MEGATRON: The environments of Population III stars at Cosmic Dawn and their connection to present day galaxies Open
We present results of Pop. III formation in the MEGATRON suite of simulations, which self-consistently follows radiation and non-equilibrium chemistry, and resolves gas at near-pc resolution of a Milky Way-mass halo at Cosmic Dawn. While t…
View article: What Sets the Metallicity of Ultra-Faint Dwarfs?
What Sets the Metallicity of Ultra-Faint Dwarfs? Open
We use intergalactic medium (IGM) metallicity distributions from several state-of-the-art cosmological simulations of Milky Way analogs and a semi-analytic model of ultra-faint dwarf galaxy (UFD) formation to model the stellar metallicitie…
View article: MEGATRON: Disentangling Physical Processes and Observational Bias in the Multi-Phase ISM of High-Redshift Galaxies
MEGATRON: Disentangling Physical Processes and Observational Bias in the Multi-Phase ISM of High-Redshift Galaxies Open
Now detected out to redshifts of $z\sim 14.5$, the rest-frame ultraviolet and optical spectra of galaxies encode numerous physical properties of the interstellar medium (ISM). Accurately extracting these properties from spectra remains a k…
View article: MEGATRON: the impact of non-equilibrium effects and local radiation fields on the circumgalactic medium at cosmic noon
MEGATRON: the impact of non-equilibrium effects and local radiation fields on the circumgalactic medium at cosmic noon Open
We present three cosmological radiation-hydrodynamic zoom simulations of the progenitor of a Milky Way-mass galaxy from the MEGATRON suite. The simulations combine on-the-fly radiative transfer with a detailed non-equilibrium thermochemica…
View article: Impact of Cosmic Ray-driven Outflows on Ly<i>α</i> Emission in Cosmological Simulations
Impact of Cosmic Ray-driven Outflows on Ly<i>α</i> Emission in Cosmological Simulations Open
Cosmic ray (CR) feedback has been proposed as a powerful mechanism for driving warm gas outflows in galaxies. We use cosmological magnetohydrodynamic simulations to investigate the impact of CR feedback on neutral hydrogen (H i ) in a 10 1…
View article: Declining metallicity and extended HeII in the outflow of an epoch of reionization analogue galaxy
Declining metallicity and extended HeII in the outflow of an epoch of reionization analogue galaxy Open
We present VLT/X-shooter spectroscopy of the extremely metal-poor starburst galaxy SBS 0335-052E, a nearby (D $\sim$54 Mpc) analog of high-redshift systems, probing its outflow up to a distance of $\sim$2.6 kpc. Using direct-method oxygen …
View article: MEGATRON: Reproducing the Diversity of High-Redshift Galaxy Spectra with Cosmological Radiation Hydrodynamics Simulations
MEGATRON: Reproducing the Diversity of High-Redshift Galaxy Spectra with Cosmological Radiation Hydrodynamics Simulations Open
We present the MEGATRON suite of cosmological radiation hydrodynamics simulations following the formation of Milky Way-mass galaxies from the earliest cosmic epochs when Population III stars form to Cosmic Noon. The suite represents the fi…
View article: MEGATRON: how the first stars create an iron metallicity plateau in the smallest dwarf galaxies
MEGATRON: how the first stars create an iron metallicity plateau in the smallest dwarf galaxies Open
We study the stellar mass-iron metallicity relation of dwarf galaxies in the new high-resolution MEGATRON cosmological radiation-hydrodynamics simulations. These simulations model galaxy formation up to $z\approx8$ in a region that will co…
View article: Uncertainties in high-$z$ galaxy properties inferred from SED fitting using JWST NIRCam photometry
Uncertainties in high-$z$ galaxy properties inferred from SED fitting using JWST NIRCam photometry Open
Numerous high-$z$ galaxies have recently been observed with the James Webb Space Telescope (JWST), providing new insights into early galaxy evolution. Their physical properties are typically derived through spectral energy distribution (SE…
View article: JWST/NIRSpec Observations of High-ionization Emission Lines in Galaxies at High Redshift
JWST/NIRSpec Observations of High-ionization Emission Lines in Galaxies at High Redshift Open
JWST spectroscopy has built large emission line samples at z ≳ 4, but it has yet to confidently reveal many galaxies with the hard radiation fields commonly associated with active galactic nucleus photoionization. While this may indicate a…
View article: Hydrogen-Alpha as a Tracer of Star Formation in the SPHINX Cosmological Simulations
Hydrogen-Alpha as a Tracer of Star Formation in the SPHINX Cosmological Simulations Open
The Hydrogen-alpha (Ha) emission line in galaxies is a powerful tracer of their recent star formation activity. With the advent of JWST, we are now able to routinely observe Ha in galaxies at high redshifts (z > 3) and thus measure their s…
View article: A remarkable ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a little red dot at <i>z</i> = 3.5
A remarkable ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a little red dot at <i>z</i> = 3.5 Open
The origin of the rest-optical emission of compact, red, high-redshift sources known as little red dots (LRDs) poses a major puzzle. If interpreted as starlight, it would imply that LRDs constitute the densest stellar systems in the Univer…
View article: 21 Balmer Jump Street: The Nebular Continuum at High Redshift and Implications for the Bright Galaxy Problem, UV Continuum Slopes, and Early Stellar Populations
21 Balmer Jump Street: The Nebular Continuum at High Redshift and Implications for the Bright Galaxy Problem, UV Continuum Slopes, and Early Stellar Populations Open
We study, from both a theoretical and observational perspective, the physical origin and spectroscopic impact of extreme nebular emission in high-redshift galaxies. The nebular continuum, which can appear during an extreme starburst, is of…
View article: Impact of Cosmic Ray-driven Outflows on Lyman-$α$ Emission in Cosmological Simulations
Impact of Cosmic Ray-driven Outflows on Lyman-$α$ Emission in Cosmological Simulations Open
Cosmic ray (CR) feedback has been proposed as a powerful mechanism for driving warm gas outflows in galaxies. We use cosmological magnetohydrodynamic simulations to investigate the impact of CR feedback on neutral hydrogen (HI) in a $10^{1…
View article: An Extremely Metal-poor Ly<i>α</i> Emitter Candidate at <i>z</i> = 6 Revealed through Absorption Spectroscopy
An Extremely Metal-poor Ly<i>α</i> Emitter Candidate at <i>z</i> = 6 Revealed through Absorption Spectroscopy Open
We report the discovery of a Ly α emitter (LAE) candidate in the immediate foreground of the quasar PSO J158-14 at z QSO = 6.0685 at a projected distance ∼29 pkpc that is associated with an extremely metal-poor absorption system. This syst…
View article: Breaking Through the Cosmic Fog: JWST/NIRSpec Constraints on Ionizing Photon Escape in Reionization-Era Galaxies
Breaking Through the Cosmic Fog: JWST/NIRSpec Constraints on Ionizing Photon Escape in Reionization-Era Galaxies Open
The escape fraction of Lyman continuum photons (fesc(LyC)) is the last key unknown in our understanding of cosmic reionization. Directly estimating the escape fraction (fesc) of ionizing photons in the epoch of reionization (EoR) is imposs…
View article: A black hole in a near-pristine galaxy 700 million years after the Big Bang
A black hole in a near-pristine galaxy 700 million years after the Big Bang Open
The recent discovery of a large number of massive black holes within the first two billion years after the Big Bang, as well as their peculiar properties, have been largely unexpected based on the extrapolation of the properties of luminou…
View article: JWST & ALMA Joint Analysis with [OII]$λλ$3726,3729, [OIII]$λ$4363, [OIII]88$μ$m, and [OIII]52$μ$m: Multi-Zone Evolution of Electron Densities at $\mathbf{z\sim0-14}$ and Its Impact on Metallicity Measurements
JWST & ALMA Joint Analysis with [OII]$λλ$3726,3729, [OIII]$λ$4363, [OIII]88$μ$m, and [OIII]52$μ$m: Multi-Zone Evolution of Electron Densities at $\mathbf{z\sim0-14}$ and Its Impact on Metallicity Measurements Open
We present a JWST and ALMA detailed study of the ISM properties of high-redshift galaxies. Our JWST/NIRSpec IFU spectroscopy targeting three galaxies at $z=6-7$ detects key rest-frame optical emission lines, allowing us to derive [OII]$λλ$…
View article: JWST/NIRSpec Observations of High Ionization Emission Lines in Galaxies at High Redshift
JWST/NIRSpec Observations of High Ionization Emission Lines in Galaxies at High Redshift Open
JWST spectroscopy has built large emission line samples at $z\gtrsim4$, but it has yet to confidently reveal many galaxies with the hard radiation fields commonly associated with AGN photoionization. While this may indicate a weaker UV ion…
View article: RUBIES: JWST/NIRSpec Confirmation of an Infrared-luminous, Broad-line Little Red Dot with an Ionized Outflow
RUBIES: JWST/NIRSpec Confirmation of an Infrared-luminous, Broad-line Little Red Dot with an Ionized Outflow Open
The JWST discovery of “little red dots” (LRDs) is reshaping our picture of the early Universe, yet the physical mechanisms driving their compact size and UV-optical colors remain elusive. Here, we report an unusually bright LRD ( z spec = …
View article: RUBIES: A complete census of the bright and red distant Universe with JWST/NIRSpec
RUBIES: A complete census of the bright and red distant Universe with JWST/NIRSpec Open
We present the Red Unknowns: Bright Infrared Extragalactic Survey (RUBIES) providing JWST/NIRSpec spectroscopy of red sources selected across ∼150 arcmin 2 from public JWST/NIRCam imaging in the UDS and EGS fields. The novel observing stra…
View article: A "Black Hole Star" Reveals the Remarkable Gas-Enshrouded Hearts of the Little Red Dots
A "Black Hole Star" Reveals the Remarkable Gas-Enshrouded Hearts of the Little Red Dots Open
The physical processes that led to the formation of billion solar mass black holes within the first 700 million years of cosmic time remain a puzzle. Several theoretical scenarios have been proposed to seed and rapidly grow black holes, bu…
View article: Inferring the ionizing photon contributions of high-redshift galaxies to reionization with <i>JWST</i> NIRCam photometry
Inferring the ionizing photon contributions of high-redshift galaxies to reionization with <i>JWST</i> NIRCam photometry Open
JWST observations are providing unprecedented constraints on the history of reionization owing to the ability to detect faint galaxies at $z\gg 6$. Modelling this history requires understanding both the ionizing photon production rate ($\x…
View article: High-z Stellar Masses Can Be Recovered Robustly with JWST Photometry
High-z Stellar Masses Can Be Recovered Robustly with JWST Photometry Open
Robust inference of galaxy stellar masses from photometry is crucial for constraints on galaxy assembly across cosmic time. Here, we test a commonly used spectral energy distribution (SED) fitting code using simulated galaxies from the S p…
View article: Rising from the ashes: evidence of old stellar populations and rejuvenation events in the very early Universe
Rising from the ashes: evidence of old stellar populations and rejuvenation events in the very early Universe Open
While JWST has observed galaxies assembling as early as $z\sim 14$, evidence of galaxies with significant old stellar populations in the Epoch of Reionization (EoR) – the descendants of these earliest galaxies – are few and far between. Bu…
View article: Impact of star formation models on the growth of simulated galaxies at high redshifts
Impact of star formation models on the growth of simulated galaxies at high redshifts Open
Star formation is a key process that governs the baryon cycle within galaxies, however, the question of how it controls their growth remains elusive due to modeling uncertainties. To understand the impact of star formation models on galaxy…
View article: High-z stellar masses can be recovered robustly with JWST photometry
High-z stellar masses can be recovered robustly with JWST photometry Open
Robust inference of galaxy stellar masses from photometry is crucial for constraints on galaxy assembly across cosmic time. Here, we test a commonly-used Spectral Energy Distribution (SED) fitting code, using simulated galaxies from the SP…
View article: Hitting the slopes: A spectroscopic view of UV continuum slopes of galaxies reveals a reddening at z > 9.5
Hitting the slopes: A spectroscopic view of UV continuum slopes of galaxies reveals a reddening at z > 9.5 Open
The UV continuum slope of galaxies, $β$, is a powerful diagnostic. Understanding the redshift evolution of $β$ and its dependence on key galaxy properties can shed light on the evolution of galaxy physical properties over cosmic time. In t…