Photoionization Modeling | Ray Garner

Extreme Nebular Conditions in the Early Universe with JWST

Fri, 09 Oct 2026 00:00:00 +0000

The first galaxies observed by the James Webb Space Telescope (JWST) do not look like simple younger versions of galaxies in the nearby Universe. Their nebular spectra indicate extremely high ionization, low chemical abundances, dense gas, and very young stellar populations. In some galaxies, the spectra also contain emission that conventional stellar populations can’t produce at all.

These observations create exactly the kind of problem that motivates much of my research: When the physical conditions change dramatically, can we still interpret familiar emission-line diagnostics in the same way?

I have begun exploring that question at Cosmic Dawn as a member of the Carbon-3 Plus Oxygen (C3PO) collaboration¹. C3PO uses deep JWST/NIRSpec spectroscopy to measure both the rest-frame ultraviolet and optical spectra of galaxies during the Epoch of Reionization, roughly 6 < z < 9.

This is a powerful combination of lines. The optical spectrum contains familiar diagnostics such as the Balmer lines, [O III], [O II], and the temperature-sensitive [O III] auroral line. The ultraviolet adds transitions from oxygen, silicon, nitrogen, carbon, and helium that probe different ionization zones, densities, abundances, and the shape of the ionizing radiation spectrum. Together, they allow us to investigate the physical properties of these extreme galaxies.

Measuring Properties at Cosmic Dawn

The motivation for C3PO is straightforward: the strongest emission lines already tell us that galaxies near the Epoch of Reionization are unusual, but the weaker lines tell us why.

The rest-frame UV spectrum of a redshift ∼ 7 galaxy observed in C3PO. Strong emission lines are marked, including peculiarly strong He II. The red line is a BPASS fit to the underlying stellar continuum.

Deep NIRSpec spectroscopy gives access to a huge set of physical diagnostics. In the optical, the auroral-to-nebular [O III] ratio provides a measurement of the electron temperature, which can be used with other oxygen lines to estimate the gas-phase abundance without relying on strong-line calibrations (which are increasingly uncertain at high redshift!). Density-sensitive doublets probe the structure of the gas. The [O II] doublet traces relatively low-ionization material, while the C III] doublet probes gas at higher ionization.

In the rest-frame UV, C III], O III], C IV, He II, and other lines add constraints on carbon abundance, and thus the C/O ratio, stellar populations, and the hardness of the ionizing radiation responsible for these emission lines.

This is a natural extension of my research in the nearby universe. Locally, I use spatially resolution to test the assumptions behind nebular diagnostics. At high redshift, I cannot resolve individual H II regions, but I can instead use a much richer set of emission lines to constrain the physics directly.

The He II Problem

My primary C3PO project focuses on a subset of galaxies with unusually strong He II λ1640 emission.

Producing He II requires photons energetic enough to ionize He⁺, above about 54 eV. Standard stellar populations produce relatively few photons at these energies, particularly compared with what is required by several galaxies in the C3PO sample (and others observed by other programs).

Possible explanations include unusual stellar populations, very massive stars, accreting black holes, x-ray binaries, shocks, or combinations of several of these. The challenge is that many different ionizing spectra can be adjusted to reproduce a single He II measurement. Therefore, I am combining measurements from the full NIRSpec spectra with photoionization modeling to address that problem.

The He II line is interesting not only because of its strength, but also because of its kinematics. In the galaxies I am studying, the line widths are not simply identical to either neighboring narrow nebular lines or in the extremely broad stellar-wind features expected from some very-massive star models. That raises the possibility that the highly ionized helium originates in ia physically or dynamically distinct component of the galaxy.

The existing spectra already constrain many of the quantities needed to interpret these sources: electron temperature, electron density, chemical abundance, C/O ratio, and the lower-energy ionizing spectrum. The remaining challenge is determining which physical source produces the photons at the extreme high-energy end.

This project has already motivated me to develop and submit my first JWST observing program, designed as a direct observational follow-up to the questions raised by these C3PO spectra.

Building Models for Galaxies

Interpreting these spectra also requires reconsidering the model grids themselves. A photoionization model can only return physically meaningful answers if the range of conditions explored by the model actually includes the galaxy being observed. That sounds obvious, but JWST is increasingly finding galaxies outside the standard parameter space that was constructed from local Universe systems.

I have been working with graduate student Annabella Yang on her first paper measuring C/O abundance ratios in C3PO galaxies. In particular, these observations motivate extending the models toward low, sub-solar C/O abundance ratios and higher gas densities than are often assumed in standard model grids.

This is important because carbon and oxygen are produced on different stellar timescales and through different pathways. Their ratio therefore constrains information about how rapidly a galaxy has enriched, the populations of stars responsible for that enrichment, and potentially the very short star formation histories of galaxies observed only a few hundred million years after the Big Bang.

When Familiar AGN Diagnostics Stop Working

My beginnings in the high-redshift universe actually began before my current C3PO project. I am a co-author on Cleri et al. (2025), which explored whether familiar optical emission line diagnostics can still distinguish stellar populations from accreting black holes under the physical conditions seen at high redshift.

A characteristic optical diagnostic diagram. Models for stellar populations are gold circles, while those for accreting black holes are blue crosses. Each panel shows a particular cross section of metallicity and ionization parameter. At nearly every combination, black holes and stellar populations overlap in the diagnostic space. Figure 8 of Cleri et al. (2025).

Unfortunately, at high ionization parameter and low metallicity, the model predictions for stellar populations and accreting black holes overlap substantially in standard diagnostic diagrams. The line ratios become more sensitive to the gas metallicity and ionization parameter than to the identity of the ionizing source itself.

This means that placing a galaxy on a familiar BPT diagram does not necessarily answer whether stars or black hole accretion dominate the ionizing radiation. Diagnostics based largely on photons below the He⁺ ionization edge may simply not probe the part of the spectrum where the competeing ionizing sources differ most strongly.

Little Red Dots

I have also contributed to C3PO work led by Casey Papovich studying two Little Red Dots in our sample. These sources truly illustrate how unfamiliar the nebular conditions of early galaxies can be.

The rest-frame optical spectrum of a galaxy at a redshift of 8.35. Important emission lines are indicated. Note the broad [O III] λ4363 and Hβ emission. Figure 2 from Papovich et al. (2026).

Their spectra contain broad Balmer emission and broad auroral [O III] emission, while several of the stronger forbidden metal lines remain narrow. The inferred densities differ dramatically between these components, suggesting that the emitting gas is strongly stratified rather than described by a single set of physical conditions.

These objects are not the central focus of my own research, but participating in this project broadened my experience with the range of physical conditions present in the C3PO dataset and the high-redshift universe more broadly.

The Broad Picture

At first glance, an H II region a few hundred parsecs across in a nearby spiral galaxy and an unresolved galaxy at the Epoch of Reionization seem like very different research problems.

However, they’re really the same problem at different scales. In nearby galaxies, spatial resolution lets me see that emission line ratios depend on geometry, stellar populations, feedback, and where within an H II region they are measured. At high redshift, those structures are compressed into a single spectrum. At the same time, the physical conditions themselves can be very different from those on which many familiar calibrations were built. The question becomes Which lessons from the nearby Universe remain valid when the nebula is unresolved and the physical regime itself has changed? C3PO gives me the chance to test that directly.

Methods and Expertise

My high-redshift work has expanded my experience across both observational spectroscopy and physical modeling.

JWST spectroscopy NIRSpec spectroscopy · rest-frame UV and optical emission lines · line profile fitting · weak-line measurements

Nebular conditions Electron temperatures · electron densities · direct-method oxygen abundance · multi-zone structure

Ionizing sources Hard ionizing spectra · stellar populations · very massive stars · AGN

Photoionization modeling Cloudy · BPASS · Bayesian model comparison

Research development JWST proposals · mentoring · collaborations

Selected Publications

First Author Work

He II Emitters in C3PO - in preparation:
Analyzing galaxies with strong He II λ1640 emission that can’t be reproduced by standard stellar populations.

Collaborative Work

Yang et al. - in preparation: C/O abundances in Epoch of Reionization galaxies.

Olivier et al. - in preparation: Spatially resolved direct-method abundances in a strongly lensed galaxy.

Papovich et al. (2026):
Deep NIRSpec spectroscopy reveals strongly stratified gas conditions and evidence for an additional ionizing source in two compact systems.

Cleri et al. (2025):
Photoionization models demonstrate that familiar rest-optical diagnostics become degenerate under the extreme physical conditions common at high redshift.

Where This Work Is Going

JWST is showing us galaxies with densities, abundance patterns, ionization states, and ionizing spectra that frequently sits at the edges of the models we have traditionally used to interpret nebular emission. Simply extending familiar calibration curves to higher redshift is therefore not enough.

In nearby galaxies, I can resolve the physics. At the Epoch of Reionization, I have only the integrated spectrum, but JWST gives me enough diagnostic lines to interpret that spectrum in unprecedented detail. Together, those regimes let me explore what physical information is encoded in a nebular spectrum, and how do we recover it when the Universe no longer resembles the systems on which our assumptions were built?

As a huge Star Wars fan, it gives me endless happiness that they named the collaboration this way. ↩︎

Resolved HII Regions with SIGNALS

Thu, 08 Oct 2026 00:00:00 +0000

Nebular emission lines are some of our most powerful tools for studying galaxies. From a handful of bright lines, we routinely estimate properties of star-forming regions (and whole galaxies) such as chemical abundances, ionization parameters, densities, temperatures, dust extinctions, and the source of ionizing radiation. But every one of those estimations depends on assumptions.

We often assume a geometry for the ionized gas, a relationship between the gas and its ionizing stars, a chemical abundance pattern, or that a spectrum integrated over an H II region adequately represents everything inside it. Those assumptions become especially important when the same diagnostics are applied to galaxies too distant to resolve.

My work with the Star-formation, Ionized Gas, and Nebular Abundances Legacy Survey (SIGNALS) attempts to answer the question: What physical information do these lines provide, and when do our assumptions about these diagnostics break down?

SIGNALS uses an instrument called SITELLE, a wide-field imaging Fourier transform spectrometer at the Canada-France-Hawaii Telescope (CFHT) to obtain spatially resolved spectroscopy across nearby galaxies. In my mind, SITELLE is the halfway point between narrowband photometry and true integral-field spectroscopy. We take images in particular wavelength ranges (usually targeting bright emission lines), but inside each of those images, we have spectral information. SITELLE’s combination of a large field of view and its high spatial resolution makes it possible to study multiple things at once: individual H II regions, populations of H II regions across a galaxy, and the larger galactic environments in which those regions form.

An example of a SITELLE spectrum. SITELLE collects data in three filters, targeting the strong optical emission lines. Because it is an imaging Fourier transform spectrometer, SITELLE spectra have a characteristic shape described by a sincgauss function rather than a standard Gaussian. Figure 2 in Garner et al. (2026).

My involvement in SIGNALS has likewise spanned several scales. I have led projects on H II region diagnostics and giant H II region structure, contributed methods and analyses to studies as a co-author, obtained new SITELLE observations to pursue my own questions, and participated in the broader scientific organization of the collaboration.

Testing the Assumptions Behind Nebular Diagnostics

Metallicity and ionization parameter are two of the most important quantities used to describe an H II region. Both strongly affect its emission-line spectrum, and they are often thought to be anti-correlated: as the oxygen abundance increases, the ionization parameter decreases. Yet, numerous investigations have found either no correlation or a positive correlation, sparking renewed interest in how these properties are related. As if to make things worse, published calibrations of the ionization parameter can disagree substantially because they are derived from photoionization models containing different assumptions about stellar populations, chemical abundances, dust, density, and nebular geometry.

Rather than adopting one of these calibrations, I constructed a custom grid of Cloudy photoionization models and used NebulaBayes to infer oxygen abundance and ionization parameter simultaneously for more than 1,500 H II regions across the nearby face-on spiral galaxy NGC 628.

I recovered the expected anti-correlation between oxygen abundance and ionization parameter, but the more important result was how strongly the inferred relationship depended on the assumptions built into the models. The correlation also showed substantial intrinsic scatter, with dust extinction emerging as one secondary variable related to both quantities.

The ionization parameter (log U) - oxygen abundance (12 + log(O/H)) relation. The left panel shows all H II regions regardless of signal-to-noise. The middle panel selects only those with a signal-to-noise of at least 15. The right panel shows how the signal-to-noise varies across the plot. The small points are the raw data analyzed through NebulaBayes, which are binned into the larger black points. The black line is a fit to the binned data. Other correlations from the literature are also plotted. Figure 9 in Garner et al. (2025).

The larger lesson was that a commonly-used diagnostic should not be treated as a black box simply because it has become standard in the literature. Understanding how a calibration was constructed, and whether its assumptions match the observations, is part of making the measurement itself.

Developing a Methodology

This combination of a custom Cloudy grid and Bayesian inference was subsequently adapted within SIGNALS to address very different scientific questions.

In NGC 6946, Bresolin[+Garner] et al. (2025) used this methodology to derive oxygen and nitrogen abundances for hundreds of H II regions and investigate azimuthal abundance structures across the galaxy. We found an azimuthal variation across one of its spiral arms, together with comparable spatial correlation scales for oxygen and nitrogen.

Oxygen abundance variations across the spiral arms of NGC 6946. Along the top left arm, the oxygen abundances grow more metal poor as you move from one side of the arm to the other. Figure 7 in Bresolin et al. (2025).

This methodology has now also been extended again by González-Koda[+Garner] et al. to investigate the N/O-O/H relation for galaxies occupying different parts of the cosmic large-scale structure.

Spatially-Resolved H II Regions

My next lead SIGNALS project approached the same problem from the opposite direction: rather than study thousands of H II regions as individual objects, I asked what happened when we resolve one H II region internally.

NGC 604 in M33 is the most luminous H II region in that galaxy and one of the largest star-forming complexes in the Local Group. At the distance of M33, the SIGNALS observations provide a physical resolution of only a few parsecs. At that scale, the assumptions behind integrated nebular diagnostics can be examined directly.

The giant H II region NGC 604 as seen through SITELLE. The left panel shows the deep image formed by summing all three filters together. The middle panel shows an RGB image of the gas emission lines Hα, [O III], and [O II]. The right panel is the same as the middle, but with particular features annotated, including four known bubbles (A-D), and the locations of a supernova remnant and planetary nebula. Figure 1 in Garner et al. (2026).

I mapped the principal optical emission-line rations across the entire nebula and found enormous spatial variations in diagnostics normally used to infer ionization parameter and chemical abundance. Those variations do not primarily trace chemical inhomogeneities. Instead, they closely follow the positions of the massive stellar populations and the shells, cavities, and filaments that those stars have carved into the surrounding gas.

In other words, applying an integrated H II region calibration pixel-by-pixel can return apparently dramatic changes in “metallicity” that are actually changes in ionization structure and geometry.

The resolved data also revealed sources that effectively disappear when the nebula is integrated into one spectrum, including a known supernova remnant and a previously unidentified planetary nebula.

The basic problem is therefore not simply spatial resolution for its own sake. It is understanding what information survives when complex physical structure is compressed into the single spectrum we usually observe.

Stellar Feedback Kinematics

The original SITELLE data were excellent for mapping excitation and ionization, but the Hα profiles showed evidence that multiple kinematic components were unresolved. That immediately suggested a next observation: observe the same nebula at substantially higher spectral resolution.

I led a successful CFHT proposal for new SITELLE SN4 observations at R ∼ 10,000, designed specifically to resolve the ionized gas kinematics of NGC 604.

The giant H II region NGC 604 in high-resolution. This is a Doppler image, meaning the blue colors correspond to blueshifted velocities (i.e., gas moving toward us), and the orange colors correspond to redshifted velocities (i.e., gas moving away from us). Courtesy of Laurent Drissen.

The new spectra allow me to fit multiple velocity components across the nebula and construct maps of velocity, velocity dispersion, and kinematic complexity. After removing the large-scale rotation of M33, the residual velocity field contains coherent structures associated with the nebula itself. I am now asking how those gas motions relate to the ionization structure measured in the original SIGNALS observations.

To do this, I have used structure functions, spatial correlation measurements, and cross-comparisons of the resolved maps to quantify characterstic spatial scales. The velocity residuals and velocity dispersion show structure on smaller scales than several ionization-sensitive line ratios, suggesting that the morphology of the radiation field and the motions of the gas do not simply trace one another.

This work has developed into a collaboration with researchers at Aix-Marseille Université, where we can compare NGC 604 with SITELLE observations of Milky Way H II regions at still finer physical resolution. We are attempting to answer the question: What happens to the physics we infer as an H II region moves from sub-parsec resolution to a few parsecs?

Galactic Dynamics

I have also led a successful SITELLE proposal targeting NGC 5474, the peculiar dwarf satellite of M101.

NGC 5474 is scientifically quite different from NGC 604. Instead of resolving one giant H II region, these observations allow me to investigate ionized gas across a galaxy with an unusual dynamical history, asymmetric structure, and an extended low surface brightness disk.

The peculiar dwarf galaxy NGC 5474. Reddish light indicates star-forming regions. Courtesy of Laurent Drissen.

My goal is to use the ionized gas kinematics and H II region population to ask how the gas responded to both the peculiar offset bulge of NGC 5474 and its interaction history with M101. The wide SITELLE field is also particularly valuable for tracing emission into the galaxy’s low surface brightness outskirts.

Science in a Collaboration

SIGNALS has also been important to me because it is my first experience with a large collaborative group.

The survey brings together people studying H II regions, chemical evolution, diffuse ionized gas, stellar feedback, supernova remnants, planetary nebulae, galactic dynamics, and other features we can trace with ionized gas in nearby galaxies. Working within that environment has allowed me to grow as a researcher.

In addition to the NGC 6946 and large-scale environment abundance studies, I am a co-author on SIGNALS work examining AGN-ISM interactions in NGC 4258, where spatially resolved excitation and kinematics reveal how the galaxy’s jet interacts with the surrounding gas.

Several additionall projects are currently moving through the publication process, including:

a large H II region catalog for M33 (led by Emma Jarvis);
a comparison of H II region structure using PHANGS-MUSE observations (led by Lorena Aragüete Riesco);
and work on the filling factor and dust-filament structure of H II regions in NGC 628 (also led by Lorena Aragüete Riesco).

These projects span substantially different science questions, but they share a common need: understanding how resolved nebular structure affects the physical quantities we infer from emission lines.

The Broad Picture

The individual projects above address different objects and different physical questions, but they have increasingly converged on the same underlying theme.

In NGC 628, I found that even measurements of integrated H II regions depend strongly on the assumptions used to translate emission-line ratios into physical properties.
In NGC 604, I showed that diagnostics that are meaningful for an integrated nebula can become physically misleading when applied to individual pixels.
The new NGC 604 observations push this one step further by asking how ionization and gas kinematics are related across physical scales.
The broader SIGNALS collaboration then provides populations of H II regions across many galactic environments with which those ideas can be tested.

Together, these projects allow me to answer What physical structure produced this line ratio, and what information survives in the way I have observed it?

Methods and Expertise

My work within SIGNALS has developed and demonstrated expertise across several interconnected areas:

Nebular astrophysics H II region physics · chemical abundances · ionization parameter · excitation diagnostics · stellar feedback

Physical modeling Cloudy · NebulaBayes · PyNeb

Spectroscopy CFHT/SITELLE · ORCS · multi-component kinematics

Spatial analysis Resolved emission-line maps · velocity fields · structure functions · correlation lengths

Statistical methods Bayesian inference · Monte Carlo uncertainty propagation

Research development Observing proposal design · collaboration across institutions · mentoring and collaborative analysis

Selected Publications

First Author Work

Garner et al. (2026):
Resolved excitation, ionization, abundance diagnostics, and gas kinematics across one of the Local Group’s most luminous H II regions.
Garner et al. (2025):
A self-consistent photoionization model analysis of more than 1,500 H II regions, testing both the physical O/H-logU relation and the assumptions behind common calibrations.

Collaborative Work

González-Koda et al. (2026):
N/O-O/H across cluster, filament, and void environments, extending my photoionization methodology to galaxy environments.
Aragüete Riesco et al. (2026):
Investigating the internal gas and dust structure of H II regions.
Fernández-Arenas et al. (2025):
SIGNALS study of the spatially resolved excitation and kinematics associated with the galaxy’s anomalous arms and jet-ISM interaction.
Bresolin et al. (2025):
The Cloudy + NebulaBayes methodology adapted to study azimuthal abundance variations and metal mixing across NGC 6946.

Where This Work Is Going

My long-term goal is to turn the extraordinary spatial information available in nearby H II regions into a better physical foundation for interpreting nebular spectra everywhere else.

High-resolution kinematics can determine how stellar feedback structures the gas and whether the characteristic scales of motion correspond to those seen in ionization diagnostics. Comparisons among Milky Way, Local Group, and more distant H II regions can quantify exactly what information disappears as spatial resolution worsens.

Broader-wavelength spectroscopy can provide a particularly important test. Direct measurements of temperature-sensitive auroral lines across resolved giant H II regions would allow the apparent abundance variations predicted by strong-line diagnostics to be compared against the actual temperature and abundance structure of the gas.

At the same time, the larger SIGNALS sample makes it possible to ask how all of these relationships change with galactic environment: spiral structure, metallicity, star formation rates, interaction history, and location within the host galaxy.

Photoionization Modeling | Ray Garner

Extreme Nebular Conditions in the Early Universe with JWST

Measuring Properties at Cosmic Dawn

The He II Problem

Building Models for Galaxies

When Familiar AGN Diagnostics Stop Working

Little Red Dots

The Broad Picture

Methods and Expertise

Selected Publications

First Author Work

Collaborative Work

Where This Work Is Going

Resolved HII Regions with SIGNALS

Testing the Assumptions Behind Nebular Diagnostics

Developing a Methodology

Related Publications

Spatially-Resolved H II Regions

Related Publications

Stellar Feedback Kinematics

Galactic Dynamics

Science in a Collaboration

The Broad Picture

Methods and Expertise

Selected Publications

First Author Work

Collaborative Work

Where This Work Is Going