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Charles L. Steinhardt

Publications and source records attributed to Charles L. Steinhardt.

At least 19 recordsLinked to original sources
arXiv2026

Performance of Photometric Template Fitting for Ultra-High Redshift Galaxies

The James Webb Space Telescope (JWST) has enabled the discovery of a significant population of galaxies at z > 10. Our understanding of the astrophysical properties of these galaxies relies on fitting templates developed using models predicting the differences between these first galaxies and lower-redshift counterparts. In this work, tests are performed on several of these high-redshift template sets in order to determine how successful they are at predicting both photometric redshifts and full spectral energy distributions (SEDs). Our work shows that the best templates for photometric redshift estimation differ from the best templates for predicting the full SED. Overall, some templates perform adequately at photometric redshift estimation, while all are generally poor predictors of the full SED. A few objects in particular are poorly fit by all the template sets tested. We conclude that although photometric redshifts can be reliable when given a high enough observational depth and adequate filters, models are not yet able to produce robust astrophysical properties for these ultra-high-redshift galaxies.

astro-ph.IM↗
arXiv2026

Direct Evidence for Stellar Initial Mass Function Variation in the Milky Way

Because direct measurements require resolved stellar populations including low-mass stars, determining the stellar initial mass function (IMF) has been a historically difficult problem even within our own Galaxy and impossible everywhere else. As a result, even though it is predicted that the IMF should vary depending upon the properties of each individual star-forming molecular cloud, it is standard to assume a Universal IMF. Using recent observations from {\em Gaia}, it is now possible to test for IMF variation using resolved stellar populations in open clusters and a parameterization that separates properties of the IMF from subsequent dynamical evolution. Here, we show that the IMF is not Universal but instead varies across individual Galactic stellar populations, reflecting evolution in the average conditions of molecular clouds over cosmic time. This evolution is consistent with the predictions of a simple astrophysical model in which the IMF is environmentally-dependent and the Milky Way reflects typical galactic behavior in recent cosmic history. Thus, observational evidence now agrees with long-standing theoretical and numerical predictions.

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arXiv2026

Direct pathway to the Early Supermassive Black Holes: A Red Super-Eddington Quasar in a Massive Starburst Host at $z=7.2$

We present a panchromatic optical-mm characterization of GNz7q, a recently identified X-ray weak, rapidly growing red quasar embedded within a dusty starburst galaxy at $z=7.1899$, using the full suite of JWST/NIRCam, NIRSpec, MIRI, and archival NOEMA observations. Our deep NIRSpec/G395M spectroscopy reveals unambiguous broad Balmer emission (FWHM $=2221\pm20$kms$^{-1}$), confirming a super-Eddington accreting black hole ($λ_{\rm Edd}=2.7\pm0.4$) with a mass of $\log(M_{\rm BH}/M_{\odot})=7.55\pm0.34$, using accretion-rate corrected BH mass estimators. After subtracting the point source, we robustly detect stellar emission from the host galaxy across multiple NIRCam and MIRI filters. Out joint morphological-spectral analysis yields a stellar mass of $\log (M_*/M_\odot)=10.5\pm0.4$ and an intense star formation rate of ${\rm SFR}=330\pm97\,M_\odot\,\rm yr^{-1}$, confirming the host as a massive, dusty starburst galaxy. We find that GNz7q lies on the local $M_{\rm BH}$-$M_*$ relation ($M_{\rm BH}/M_*\simeq 0.001$) and is well positioned to evolve into the locus of massive SDSS quasars with $\log (M_{\rm BH}/M_\odot)\approx 9$ and $M_*\approx 10^{11}\,M_\odot$ at $z\sim 6$, owing to its remarkably rapid growth in both the black hole and its host galaxy. This stands in stark contrast to many recently reported JWST AGN populations at similar redshifts, including the little red dots (LRDs), whose weak or undetected star formation makes it difficult for them to grow into the massive galaxies hosting SDSS-like quasars. These results suggest that GNz7q marks as a rare, pivotal phase of early BH-galaxy co-eolution, plausibly providing a crucial direct pathway to the supermassive black hole systems within the first billion years of the Universe.

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arXiv2025

Diverse reddening distributions in sight lines to type Ia supernovae

Precise cosmological constraints from type Ia supernovae require adequately accurate corrections for host-galaxy extinction. Modelling these corrections is challenged by the problem of disentangling supernova intrinsic colours from host-galaxy interstellar reddening. The latter is commonly modelled in a probabilistic way assuming an exponential distribution exp(-E(B-V)/τ) as a universal prior which is applied across all types of supernova host galaxies. We test the robustness of the exponential model and its universality against predictions based on simulating dust and type Ia supernova distributions in host galaxies of different morphological types. We find substantial differences between predicted interstellar reddening in late- and early-type host galaxies, primarily driven by the stellar-to-dust mass ratios. The mean simulated reddening in late-type galaxies matches well those derived from type Ia supernova observations, but it is significantly lower for early-type host galaxies. The reddening distributions exhibit an excess of sight lines with vanishing reddening with respect to the exponential model, although the difference is quite mild for late-type galaxies. On the other hand, the distribution may peak at E(B-V)>0 when considering a population of young type Ia supernovae originating from lower heights within the dust disc. We demonstrate that assuming a universal reddening prior distribution for modeling peak magnitude-colour relation, which is currently a common practice, gives rise to a spurious scatter in the derived extinction properties. It may also bias relative distances between supernovae originating from different host-galaxy populations. The discrepancy between the simulated reddening in average early-type host galaxies and the observed occurrence of reddened supernovae suggests that reddening does not originate from interstellar dust expected in these galaxies.

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arXiv2025

Evidence for Multiple Types of Post-Starburst Galaxies

The quenching mechanisms of galaxies are not yet fully understood, but post-starburst galaxies provide one explanation for the rapid transition between star-forming and quiescent galaxies. At low redshift, it is generally thought that the starburst initiating the post-starburst phase is merger-driven, however, not all post-starburst galaxies show evidence of a merger, and recent studies suggested that post-starburst galaxies may be produced by multiple distinct mechanisms. This study examines whether multiple types of post-starburst galaxies actually exist, that is, whether the properties of post-starburst galaxies are multimodal. We used uniform manifold approximation and projection (UMAP) to cluster post-starburst galaxies based on spectroscopic data. The results suggest that there are three types of post-starburst galaxies that have dissimilar stacked spectral energy distributions and are separated by a combination of their H$α$ and [OII]$λ$3727 line strengths with an accuracy of 91%. A comparison of various galaxy properties (e.g., emission line strengths, mass and age distributions, and morphologies) indicates that the grouping is not just an age sequence, but may be correlated to the merger-histories of the galaxies. It suggested that the three post-starburst galaxy types have different origins, some of which may not be merger-driven, and that all typical galaxies go through the post-starburst phase at turnoff.

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arXiv2025

No [CII] or dust detection in two Little Red Dots at z$_{\rm spec}$ > 7

Little Red Dots (LRDs) are compact, point-like sources characterized by their red color and broad Balmer lines, which have been debated to be either dominated by active galactic nuclei (AGN) or dusty star-forming galaxies (DSFGs). Here we report two LRDs (ID9094 and ID2756) at z$_{\rm spec}$>7, recently discovered in the JWST FRESCO GOODS-North field. Both satisfy the "v-shape" colors and compactness criteria for LRDs and are identified as Type-I AGN candidates based on their broad H$β$ emission lines (full width at half maximum: 2280$\pm$490 km/s for ID9094 and 1070$\pm$240 km/s for ID2756) and narrow [OI] lines ($\sim$ 300-400 km/s). To investigate their nature, we conduct deep NOEMA follow-up observations targeting the [CII] 158${\rm μm}$ emission line and the 1.3 mm dust continuum. We do not detect [CII] or 1.3 mm continuum emission for either source. Notably, in the scenario that the two LRDs were DSFGs, we would expect significant detections: $>16σ$ for [CII] and $>3σ$ for the 1.3 mm continuum of ID9094, and $>5σ$ for [CII] of ID2756. Using the 3$σ$ upper limits of [CII] and 1.3 mm, we perform two analyses: (1) UV-to-FIR spectral energy distribution (SED) fitting with and without AGN components, and (2) comparison of their properties with the L$_{[CII]}$-SFR$_{tot}$ empirical relation. Both analyses are consistent with a scenario where AGN activity may contribute to the observed properties, though a dusty star-forming origin cannot be fully ruled out. Our results highlight the importance of far-infrared observations for studying LRDs, a regime that remains largely unexplored.

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arXiv2025

Optimal parameterizations for observational constraints on thawing dark energy

Time-varying dark energy is often modeled in observational analyses through generic parameterizations of its equation of state $w(z)$, which typically use two free parameters $\{w_0, w_a\}$ to span a broad range of behaviors as a function of redshift. However, this broad range of behaviors can only approximately capture the dynamics of any given microphysical theory of dark energy. A complementary approach is to use targeted parameterizations designed to model specific classes of dynamical dark energy with greater precision. Focusing on the class of thawing dark energy, we quantify and compare the precision with which nineteen generic and targeted parameterizations can capture the dynamics of physically motivated thawing quintessence theories. We find that a targeted parameterization derived from a Padé expansion of $w$ is the most reliable of these, producing accurate reconstructions of $w(z)$, the expansion history $H(z)$, and cosmological parameters such as $H_0$ and $Ω_m$ for a broad range of microphysical theories.

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arXiv2025

Dark Energy Constraints and Joint Cosmological Inference from Mutually Inconsistent Observations

Recent results regarding dark energy are mutually inconsistent under the $Λ$CDM cosmological model, hinting at the possibility of undiscovered physics. However, the currently accepted cosmological parameters come from a joint inference between observational data sets, a process that is formally invalid for inconsistent observations. We will show that many problems can arise when using joint inference on disagreeing observations such as significantly overestimated margins of error, high dependencies on priors, and sensitivities to boundary constraints. Because we do not know if these inconsistencies arise due to errors in observation, poor statistical techniques, or an improper model, it is difficult to fix these problems. We will discuss each scenario in which the analysis method breaks and explore an alternative resampling technique, developing methods that may make determining the sources of tensions in cosmological parameters easier.

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arXiv2025

Do Red Galaxies Form More Stars Than Blue Galaxies?

A new model is proposed in which typical galaxies form most of their stellar mass in a phase with an intrinsically red stellar population. In the standard picture, galaxies with intrinsically red stellar populations are believed to have old stellar populations, so that only galaxies with blue stellar populations have significant star formation, and subsequent changes to the stellar population come from predominantly from aging and merging populations which have already formed. However, several observational puzzles have developed which are difficult to reconcile with this standard scenario. The most massive blue star-forming galaxies, presumed to be at the end of their stellar mass growth, are $\sim 1$ dex less massive, have a $\sim 1$ dex lower $M_*/M_{BH}$ ratio, and have a bottom-lighter IMF than local quiescent galaxies. Here, a new solution is proposed: at low temperature and high metallicity, galaxies can continue to form stars efficiently without being able to form O and B stars. These red star-forming galaxies would have many of the same properties of the population currently described as post-starburst galaxies, allowing a new interpretation of their origin. Finally, additional falsifiable observational predictions of this model are also discussed.

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arXiv2025

On the Significance of Rare Objects at High Redshift: The Impact of Cosmic Variance

The discovery of extremely luminous galaxies at ultra-high redshifts ($z\gtrsim 8$) has challenged galaxy formation models. Most analyses of this tension have not accounted for the variance due to field-to-field clustering, which causes the number counts of galaxies to vary greatly in excess of Poisson noise. This super-Poissonian variance is often referred to as cosmic variance. Since cosmic variance increases rapidly as a function of mass, redshift, and smaller observing areas, the most massive objects in deep \textit{JWST} surveys are severely impacted by cosmic variance. We construct a simple model, including cosmic variance, to predict the distribution of the mass of the most massive galaxy for different surveys, which increases the tension with observations. The distributions differ significantly from previous predictions using the Extreme Value Statistics formalism, changing the position and shape of the distributions. We test our model using the \texttt{UniverseMachine} simulations, where the predicted effects of cosmic variance are clearly identifiable. We find that the high skew in the distributions of galaxy counts for typical deep surveys imply a high statistical variance on the cosmic variance itself. This impacts the calibration of the cosmic variance, as well as the expected mass of the most massive galaxy. We also find that the impact of cosmic variance dominates the impact of any realistic scatter in the stellar-to-halo-mass relation at $z\gtrsim 12$. It is therefore crucial to accurately account for the impact of cosmic variance in any analysis of tension between early extreme galaxies and galaxy formation models.

astro-ph.GA↗
arXiv2025

A power spectral study of PHANGS galaxies with JWST MIRI: On the spatial scales of dust and PAHs

The interstellar medium (ISM) consists of a diversity of structures across a range of spatial scales, intimately tied to galactic evolution. In this work, Fourier analysis is used to characterize the spatial structures of dust and Polycyclic Aromatic Hydrocarbons (PAHs) in the ISM of PHANGS-JWST galaxies, observed in the four photometric mid-infrared (MIR) filters from F770W to F2100W (i.e., 7.7 to 21$μ$m)). We quantify the abundance of structure on different spatial scales using the power-law slope, $α$, of the spatial power spectra. The distribution of $α$ across all length scales differs significantly between filters, with steeper slopes for PAH-dominated filters ($α_{F770W} = 2.19^{+0.16}_{-0.15}$, $α_{F1130W} = 1.88^{+0.25}_{-0.37}$) and shallower for the dust-continuum ($α_{F1000W} = 1.48^{+0.33}_{-0.47}$, $α_{F2100W} = 0.94^{+0.23}_{-0.28}$). The distribution of $α$ across galaxies is narrower for PAH-dominated than for thermal dust-dominated bands, highlighting that PAHs trace photo-dissociation regions dominated by similar physical processes, whereas dust structures are an integrated property over the diverse evolutionary histories of their host galaxies. Unlike dust structures, PAH-sensitive bands display a break in the power spectrum: below a characteristic scale, $\ell_0=160\mathrm{pc}^{+110\mathrm{pc}}_{-50\mathrm{pc}}$, PAH structures are suppressed.

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arXiv2025

Identification of $>$40 gravitationally magnified stars in a galaxy at redshift of 0.725

Strong gravitational magnifications enable to detect faint background sources, resolve their internal structures, and even identify individual stars in distant galaxies. Highly magnified individual stars allow various applications, including studies of stellar populations in distant galaxies and constraining dark matter structures in the lensing plane. However, these applications have been hampered by the small number of individual stars observed, as typically one or a few stars are identified from each distant galaxy. Here, we report the discovery of more than 40 microlensed stars in a single galaxy behind Abell 370 at redshift of 0.725 when the Universe was half of its current age (dubbed ``the Dragon arc''), using James Webb Space Telescope (JWST) observations with the time-domain technique. These events are found near the expected lensing critical curves, suggesting that these are magnified stars that appear as transients from intracluster stellar microlenses. Through multi-wavelength photometry, we constrain stellar types and find that many of them are consistent with red giants/supergiants magnified by factors of hundreds. This finding reveals an unprecedented high occurrence of microlensing events in the Dragon arc, and proves that {\it JWST}'s time-domain observations open up the possibility of conducting statistical studies of high-redshift stars.

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arXiv2024

BUFFALO Wild Wings: A High Precision Free-Form Lens Model of MACSJ0416 with Constraints on Dark Matter from Substructure and Highly Magnified Arcs

We present new free-form and hybrid mass reconstructions of the galaxy cluster lens MACS J0416.1$-$2403 at $z=0.396$ using the lens inversion method GRALE. The reconstructions use 237 spectroscopically confirmed multiple images from Bergamini et. al. 2023 as the main input. Our primary model reconstructs images to a positional accuracy of 0.191", thus representing one of the most precise reconstructions of this lens to date. Our models find broad agreement with previous reconstructions, and identify two $\sim 10^{12} M_{\odot}$ light-unaffiliated substructures. We focus on two highly magnified arcs: Spock and Mothra. Our model features a unique critical curve structure around the Spock arc with 2 crossings. This structure enables sufficient magnification across this arc to potentially explain the large number of transients as microlensing events of supergiant stars. Additionally, we develop a model of the millilens substructure expected to be magnifying Mothra, which may be a binary pair of supergiants with $μ\sim 6000$. This model accounts for flexibility in the millilens position while preserving the observed flux and minimizing image position displacements along the Mothra arc. We constrain the millilens mass and core radius to $\lesssim 10^6 M_{\odot}$ and $\lesssim 17$ pc, respectively, which would render it one of the smallest and most compact substructures constrained by lensing. If the millilens is dominated by wave dark matter, the axion mass is constrained to be $\lesssim 3.0 \times 10^{-21}$ eV. Further monitoring of this lens with JWST will uncover more transients, permitting tighter constraints on the structure surrounding these two arcs.

astro-ph.GA↗
arXiv2024

Reverberation mapping of high-mass and high-redshift quasars using gravitational time delays

Mass estimates of black holes (BHs) in the centers of Active Galactic Nuclei (AGN) often rely on the radius-luminosity relation. However, this relation, usually probed by reverberation mapping (RM), is poorly constrained in the high-luminosity and high-redshift ends due to the very long expected RM lag times. Multiply imaged AGN may offer a unique opportunity to explore the radius-luminosity relation at these ends. In addition to comprising several magnified images enabling a more efficient light-curve sampling, the time delay between multiple images of strongly lensed quasars can also aid in making such RM measurements feasible on reasonable timescales: If the strong-lensing time delay is, for example, of the order of the expected RM time lag, changes in the emission lines in the leading image can be observed around the same time as the changes in the continuum in the trailing image. In this work we probe the typical time-delay distribution in galaxy-cluster lenses and estimate the number of both high-mass ($\sim10^9-10^{10}$ M$_{\odot}$), and high-redshift ($z\gtrsim4-12$) quasars that are expected to be strongly lensed by clusters. We find that up to several dozen thousand M$_{BH}\sim10^{6}$-$10^{8}$ M$_{\odot}$ broad-line AGN at $z>4$ should be multiply imaged by galaxy clusters and detectable with JWST, hundreds with Euclid and several thousands with the Roman Space Telescope, across the whole sky. These could supply an important calibration for the BH mass scaling in the early Universe.

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arXiv2023

Strong gravitational lensing's 'external shear' is not shear

The distribution of mass in galaxy-scale strong gravitational lenses is often modelled as an elliptical power law plus 'external shear', which notionally accounts for neighbouring galaxies and cosmic shear. We show that it does not. Except in a handful of rare systems, the best-fit values of external shear do not correlate with independent measurements of shear: from weak lensing in 45 Hubble Space Telescope images, or in 50 mock images of lenses with complex distributions of mass. Instead, the best-fit shear is aligned with the major or minor axis of 88% of lens galaxies; and the amplitude of the external shear increases if that galaxy is disky. We conclude that 'external shear' attached to a power law model is not physically meaningful, but a fudge to compensate for lack of model complexity. Since it biases other model parameters that are interpreted as physically meaningful in several science analyses (e.g. measuring galaxy evolution, dark matter physics or cosmological parameters), we recommend that future studies of galaxy-scale strong lensing should employ more flexible mass models.

astro-ph.CO↗
arXiv2023

Efficient survey design for finding high-redshift galaxies with JWST

Several large JWST blank field observing programs have not yet discovered the first galaxies expected to form at $15 \leq z \leq 20$. This has motivated the search for more effective survey strategies that will be able to effectively probe this redshift range. Here, we explore the use of gravitationally lensed cluster fields, that have historically been the most effective discovery tool with HST. In this paper, we analyze the effectiveness of the most massive galaxy clusters that provide the highest median magnification factor within a single JWST NIRCam module in uncovering this population. The results of exploiting these lensing clusters to break the $z > 15$ barrier are compared against the results from large area, blank field surveys such as JADES and CEERS in order to determine the most effective survey strategy for JWST. We report that the fields containing massive foreground galaxy clusters specifically chosen to occupy the largest fraction of a single NIRCam module with high magnification factors in the source plane, whilst containing all multiple images in the image plane within a single module provide the highest probability of both probing the $15 \leq z \leq 20$ regime, as well as discovering the highest redshift galaxy possible with JWST. We also find that using multiple massive clusters in exchange for shallower survey depths is a more time efficient method of probing the $z > 15$ regime.

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arXiv2023

Self-consistent Combined HST, K-band, and Spitzer Photometric Catalogs of the BUFFALO Survey Fields

This manuscript presents new astronomical source catalogs using data from the BUFFALO Survey. These catalogs contain detailed information for over 100,000 astronomical sources in the 6 BUFFALO clusters: Abell 370, Abell 2744, Abell S1063, MACS 0416, MACS 0717, and MACS 1149 spanning a total 240 arcmin^2. The catalogs include positions and forced photometry measurements of these objects in the F275W, F336W, F435W, F606W, F814W, F105W, F125W, F140W, and F160W HST -bands, Keck-NIRC2/VLT-HAWKI Ks band, and IRAC Channel 1 and 2 bands. Additionally, we include photometry measurements in the F475W, F625W, and F110W bands for Abell 370. This catalog also includes photometric redshift estimates computed via template fitting using LePhare. When comparing to spectroscopic reference, we obtain an outlier fraction of 9.2% and scatter, normalized median absolute deviation (NMAD), of 0.062. The catalogs are publicly available for their use by the community.

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