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Katherine E. Whitaker

Publications and source records attributed to Katherine E. Whitaker.

At least 37 records · Page 2Linked to original sources
arXiv2026

Resolved Maps of Gas and Dust in a Massive Quiescent Galaxy at z=2 from INQUEST-JWST: Evidence of Accretion and Rejuvenation

Quiescent galaxies in the distant universe exhibit a range of gas content that may indicate a variety of quenching processes are at play. Mapping the distribution and kinematics of the gas can illuminate its origins, but nearly all such observations have been unresolved. We present JWST/NIRSpec IFU observations of MRG-M0138, a gravitationally lensed, massive quiescent galaxy at $z\sim2$ observed as part of the INQUEST-JWST survey. We use Na I D absorption, which we detect in excess of the stellar absorption over most of the galaxy, to trace the kinematics and spatial distribution of the neutral gas in 219 spatial bins. The gas exhibits clear rotation that is kinematically aligned with the stellar disk. Both the gas and dust have a complex spatial structure, including an off-nuclear clump, a dust lane, and patches in the outer disk. The non-equilibrium distribution suggests that the gas was accreted. Analysis of the galaxy's star formation history supports this interpretation by indicating a rejuvenation event 500 Myrs ago. We identify two plausibly associated galaxies and suggest that tidal interactions are a likely source of the accreted gas. Our results indicate that some of the variation in gas content among early quiescent galaxies is not related to differences in gas consumption timescales. The detection of a gas clump at a projected distance of $\sim90$ pc from the known supermassive black hole illustrates a mechanism to fuel the episodic AGN feedback that may maintain quiescence.

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arXiv2026

Winding Back the Clock: Recent Star Formation Histories of Massive Quiescent Galaxies Are Consistent With Their Rapid Number Density Evolution Since $\mathbf{z\sim7}$

Massive quiescent galaxies have been identified out to $z\sim7$ in early JWST data in a substantial excess ($\rm \gtrsim 1\,dex$ at $z>4$) of number densities from most theoretical predictions. We investigate whether the number densities implied by the star formation histories of quiescent galaxies at $2 7$. For this work, we rely on stellar population synthesis modeling of JWST NIRCam photometry (from CEERS and PRIMER) and NIRSpec/PRISM spectra of massive ($\rm M_{*} > 10^{10.5}M_{\odot}$) quiescent galaxies in the RUBIES survey. We infer their star-formation histories through Bayesian spectro-photometric fitting with Prospector, exploring the sensitivity of our results to stellar libraries and SFH priors. For each source, we compute a timescale over which it would be identified as quiescent -- leveraging the recent and most robust SFH timescale -- and deduce the number density of the quiescent population at previous epochs. These reconstructed number densities are then compared to existing observational constraints, including a new measurement from the PANORAMIC pure parallel survey, whose wide-area and independent sightlines reduce sensitivity to cosmic variance. We find striking agreement between reconstructed and observed number densities up to $z\sim7$, a self-consistency that lends credence to stellar population synthesis modeling of distant quiescent galaxies. Furthermore, by connecting the recent ($\rm \sim 1\,Gyr$) star-formation histories and number densities of quiescent galaxies and their implied progenitors, we reinforce the known tension between observations and model predictions at $3<z<7$.

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arXiv2026

Calibrating Photometric Mid-Infrared Star Formation Rates for JWST

The mid-infrared (IR) spectrum of galaxies has a long history as a valuable proxy for the dust-obscured star formation rate (SFR) in massive galaxies. Now, with JWST, we can explore the mid-IR's full potential as a SFR tracer over four orders of magnitude in total infrared luminosity (9<~log LIR/Lo<~13). First, combining the SMILES and FRESCO surveys, we evaluate MIRI photometry against the Pa-alpha emission line - a gold standard SFR indicator - in Main Sequence (MS) galaxies at cosmic noon. We find the rest-frame 6-8um luminosity has a steeply superlinear relation with SFR(Pa-alpha) below ~8 Mo/yr, in contrast with the unity slope seen in coeval massive galaxies. We derive broken power-law SFR indicators from single-band MIRI photometry plus a representative dust template, with a scatter typical of IR SFRs (~0.2-0.3 dex). Despite the break in the mid-IR behavior and our simplifying assumption of a single dust SED, we next successfully formulate a UV+IR composite relation (scatter ~0.15 dex) under the usual assumption of energy balance. This implies that the rest-frame 6-8um primarily tracks the global dust-obscuration fraction - which decreases rapidly at log M*/Mo<~10 - rather than reflecting a deficit in PAH abundances at low mass. Our results thus support MIRI photometry as a robust SFR proxy at log M*/Mo>~9 up to z~3. Finally, extending to local and z>~1 ultraluminous infrared galaxies not represented in SMILES, we examine when Pa-alpha and the IR reliably track SFR in the bright regime.

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arXiv2026

Discovery of Ancient Globular Cluster Candidates in the Relic, a Quiescent Galaxy at $z=2.5$

Globular clusters (GCs) are some of the oldest bound structures in the Universe, holding clues to the earliest epochs of star formation and galaxy assembly. However, accurate age measurements of ancient clusters are challenging due to the age-metallicity degeneracy. Here, we report the discovery of 36 compact stellar systems within the 'Relic', a massive, quiescent galaxy at $z=2.53$. The Relic resides in an overdensity behind the Abell~2744 cluster, with a prominent tidal tail extending towards two low-mass companions. Using deep data from the UNCOVER/MegaScience JWST Surveys, we find that clusters formed in age intervals ranging from 8~Myr up to $\sim2$~Gyr, suggesting a rich formation history starting at $z\sim10$. While the cluster-based star formation history is broadly consistent with the high past star formation rates derived from the diffuse host galaxy light, one potential discrepancy is a tentative $\sim2-3\times$ higher rate in the cluster population for the past Gyr. Taken together with the spatial distribution and low inferred metallicities of these young-to-intermediate age clusters, we may be seeing direct evidence for the accretion of star clusters in addition to their early in situ formation. The cluster masses are high, $\sim10^6-10^7~M_{\odot}$, which may explain why we are able to detect them around this likely post-merger galaxy. Overall, the Relic clusters are consistent with being precursors of the most-massive present-day GCs. This unique laboratory enables the first connection between long-lived, high-redshift clusters and local stellar populations, offering insights into the early stages of GC evolution and the broader processes of galaxy assembly.

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arXiv2026

A PANORAMIC of UV-optical morphologies of "Little Red Dots": Two groups of LRDs distinguished by UV half-light radius

Among the most remarkable results from JWST is the discovery of abundant, compact, and very red sources in the early Universe known as "Little Red Dots" (LRDs). The relative degree to which starlight and active galactic nuclei (AGN) drive the rest-frame UV and optical emission from LRDs remains unclear. With a large sample of LRDs selected photometrically from the pure-parallel PANORAMIC survey, we study their morphology as a function of rest-wavelength and find that the rest-UV light is typically more extended than the rest-optical. This result holds both when measuring LRD sizes with a single Sérsic profile and when comparing the fraction of light from a point source via joint PSF+Sérsic modeling. A shift occurs at the Balmer break, with LRDs becoming highly compact and unresolved ($R_{50,\rm{opt}}\lesssim100\;\rm{pc}$) in the rest-optical relative to the rest-UV. When splitting the sample at the Balmer break into those that are resolved and unresolved, a stacking analysis demonstrates that the latter are compact ($R_{50}\lesssim100\;\rm{pc}$) on average across the full rest-UV-optical spectrum. Conversely, those LRDs resolved at the break show extended UV emission ($R_{50,\rm{UV}}>200\;\rm{pc}$) on average. We find a similar dichotomy when repeating with a spectroscopic sample. Altogether, these results are consistent with the rest-UV emission driven by a combination of emission from starlight and a dense, dust-poor cloud of hydrogen gas enveloping an AGN. Differences between LRDs in the relative contribution from the AGN and starlight could reflect an ensemble of black hole seed masses, where a heavier seed produces an LRD of smaller $R_{50,\rm{UV}}$.

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arXiv2026

JWST+ALMA reveal the build up of stellar mass in the cores of dusty star-forming galaxies at Cosmic Noon

Dusty star-forming galaxies have long been suspected to serve as the missing evolutionary bridge between the star-forming and quiescent phases of massive galaxy evolution. With the combined power of JWST and ALMA, it is now possible to use high resolution imaging at rest-frame ultraviolet (UV), optical, near-infrared (NIR), and sub-mm wavelengths to study the multi-wavelength morphologies tracing both the stellar populations and dust during this key phase. We present the joint analysis of JWST/NIRCam imaging in GOODS-S and mm dust emission traced by ALMA for a sample of 33 galaxies at $z=1.5$ to $z=5.5$ selected from the 1.1mm GOODS-ALMA 2.0 survey, and compare the morphologies of this population to mass- and redshift-selected samples of field star-forming and quiescent galaxies. The 1.1mm-selected sample is morphologically distinct from other similarly massive star-forming galaxies; we find a steeper size-wavelength gradient from 1.5-4.4$μ$m, with a more dramatic decrease in size towards longer wavelengths. While the rest-NIR surface brightness profiles of the 1.1mm-selected galaxies are brighter in the inner regions relative to the field star-forming population, they are remarkably similar to the quiescent population. These morphological differences could suggest that dusty star-forming galaxies, unlike more typical star-forming galaxies, have already built up stellar mass in a severely dust-obscured core, leading to extended and clumpy morphologies at rest-UV and rest-optical wavelengths and more compact emission in the rest-NIR that is co-spatial with dust. If the bulge is already established, we speculate that mm-selected galaxies may imminently evolve to join their quiescent descendants.

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arXiv2026

A Spectral Atlas for Quiescent Galaxies

Key absorption features in the spectra of massive quiescent galaxies reveal a wealth of information about their stellar populations and, in some cases, the properties of gas within and around them. With the spectroscopic capabilities of \textit{JWST}, we are now able to probe deeper and farther into the near-infrared than ever before. It is therefore crucial that we fully understand the origins of observed spectral absorption and emission features. The goal of this document is to collate important rest-frame optical to near-infrared (NIR) spectral features of quiescent galaxies in the context of their physical origin (e.g., stellar photospheres, the interstellar medium, or a combination thereof). We present a look-up table summarizing key information, including a ``diagnostic'' column indicating whether lines are most sensitive to age, metallicity, surface gravity (IMF sensitivity), $α$-enhancement, electron temperature or gas density. This compilation is intended to serve as a practical reference for interpreting rest-optical-NIR spectral features in quiescent galaxies, particularly as \textit{JWST} spectroscopy enables increasingly detailed studies of their stellar populations and surrounding gas.

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arXiv2026

Taking a Break at Cosmic Noon: Continuum-selected Low-mass Galaxies Require Long Burst Cycles

While bursty star formation in low-mass galaxies has been observed in local populations and reproduced in simulations, the dormant phase of the burst cycle has not been well studied beyond the local Universe due to observational limitations. We present a unique sample of 43 JWST PRISM spectra of low-mass galaxies ($M_\star < 10^{9.5}\,M_\odot$) at cosmic noon ($1<z<3$), uniformly selected on F200W magnitude and precise photometric redshifts enabled by 20-band JWST photometry from the UNCOVER and MegaScience surveys. The spectra reveal numerous strong Balmer breaks, which are negatively correlated with the galaxies' H$α$ equivalent width. By comparing these observations to synthetic samples of spectra generated using a simple parametrization of bursty star formation histories, we show that star formation in low-mass galaxies at cosmic noon is likely dominated by burst cycles with long timescales ($\gtrsim 100$ Myr) and large deviations below the star-forming main sequence ($\gtrsim 0.8$ dex). Our results suggest that galaxies in this population--at least those within our detection limits--should not be classified solely by their current star formation rates, but instead viewed as a unified population undergoing dynamic movement above and below the star-forming main sequence. The derived constraints demonstrate that long-timescale fluctuations are important for this class of galaxies, indicating that galaxy-scale gas cycles--rather than molecular-cloud-scale stochasticity--are the primary regulators of star formation variability in low-mass galaxies at cosmic noon.

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arXiv2026

Everything Every Band All at Once II: The Relationship Between Optical Size and Stellar Mass Over Eight Billion Years of Cosmic History

While the size-mass relation provides insight into the structural evolution of galaxies, the data available and methods employed have hindered our ability to study a detailed and comprehensive description of this key relation across cosmic history. The first paper in this series presents a morphology catalog based on 20 band JWST data in the field of Abell 2744. In this paper we utilize this catalog to measure the size-mass relation from $0.5 10$, we discuss three distinct phases: Rapid growth at $z>5$, growth that mimics dark matter halos at $5< z <1$ and a late plateau at $0.5 1$. Our results imply that quiescent galaxies are smaller than their star-forming counterparts only at around $\log M_*/M_\odot = 10$; the two populations have similar sizes at lower and higher masses.

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arXiv2026

Evidence for Shallow Nebular Attenuation Curves and Patchy Dust Geometry at z~2 with Pa-beta/H-alpha Measurements from JWST-MegaScience Medium Band Photometry

We constrain the nebular attenuation curve and investigate dust geometry in star-forming galaxies at cosmic noon using photometric medium-band emission line measurements. We measure H-alpha emission line fluxes for a sample of 209 star-forming galaxies at 1.2<z<2.4 in MegaScience/UNCOVER with stellar masses spanning $7.85<\log_{10}(M_*/M_\odot)<11.0$. For 66 of these galaxies, we also measure a Pa-beta flux. We find that the Pa-beta/H-alpha line ratio increases strongly with stellar mass and star-formation rate (SFR) across our full mass range, indicating that more massive galaxies are dustier. We compare our results with a mass-, SFR-, and redshift-matched sample of galaxies from the MOSDEF survey with spectroscopic measurements of H-alpha/H-beta, finding that a shallow Reddy et al. (2025) nebular attenuation curve is more consistent with our observations than the typically assumed Cardelli et al. (1989) attenuation curve, especially for massive galaxies. This shallow attenuation curve could be explained by low dust covering fractions in star-forming regions. Through comparison to other studies, we show that assuming this shallower attenuation curve can increase the inferred A_Halpha,neb by up to 1 magnitude at high masses. We observe no trend between A_Halpha,neb and axis ratio, indicating that nebular attenuation is likely localized to small clumps. Altogether, our results strongly suggest that dust geometry is patchy and non-uniform, especially in massive galaxies. Our results highlight the ability of JWST medium bands to probe emission lines for large samples of galaxies, and statistically constrain dust properties in upcoming large programs.

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arXiv2026

Water absorption confirms cool atmospheres in two little red dots

Little red dots (LRDs) are an abundant population of compact high-redshift sources with red rest-frame optical continua, discovered by the James Webb Space Telescope (JWST). Their red colors and power sources have been attributed either to dust reddening of standard hot accretion disks or to intrinsically cool thermal emission from dense hydrogen envelopes, in both cases surrounding accreting supermassive black holes. These scenarios predict order-of-magnitude differences in emission temperature but have lacked decisive temperature diagnostics. Here we report a prominent absorption feature at rest-frame $\sim 1.4 \, μ\mathrm{m}$ in two out of four LRDs at $z \sim 2$ with high signal-to-noise JWST spectra, among the coolest from a large LRD sample. The feature matches the shape and wavelength of the water absorption band seen in cool stars. Atmosphere models require $T \lesssim 3000\, \mathrm{K}$ to reproduce it, confirming unambiguously the presence of a cool, dense gas component contributing $20-30\%$ to the emergent continuum. A composite model reproduces both the absorption and the rest-frame optical-to-infrared continuum shape and suggests a temperature range ($\sim2000\, \mathrm{K} - 4000 \, \mathrm{K}$) rather than a single blackbody predicted by some gas envelope models. Molecular absorption demonstrates that the red continua of some LRDs are intrinsic rather than dust-reddened, implying order-of-magnitude lower bolometric luminosities and black-hole masses, and providing a new diagnostic of the emitting gas.

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arXiv2026

A Cosmic Miracle: A Remarkably Luminous Galaxy at $z_{\rm{spec}}=14.44$ Confirmed with JWST

JWST has revealed a stunning population of bright galaxies at surprisingly early epochs, $z>10$, where few such sources were expected. Here we present the most distant example of this class yet -- MoM-z14, a luminous ($M_{\rm{UV}}=-20.2$) source in the COSMOS field at $z_{\rm{spec}}=14.44^{+0.02}_{-0.02}$ that expands the observational frontier to a mere 280 million years after the Big Bang. The redshift is confirmed with NIRSpec/PRISM spectroscopy through a sharp Lyman-$α$ break and $\sim3σ$ detections of five rest-UV emission lines. The number density of bright $z_{\rm{spec}}\sim14-15$ sources implied by our "Mirage or Miracle" survey spanning $\sim350$ arcmin$^{2}$ is $>100\times$ larger ($182^{+329}_{-105}\times$) than pre-JWST consensus models. The high EWs of UV lines ($\sim15{-}35$ Å) signal a rising star-formation history, with a $\sim10\times$ increase in the last 5 Myr ($\rm{SFR_{\rm{5Myr}}}/\rm{SFR_{\rm{50Myr}}}=9.9^{+3.0}_{-5.8}$). The source is extremely compact (circularized $r_{\rm{e}} = 74^{+15}_{-12}$ pc), and yet elongated ($b/a=0.25^{+0.11}_{-0.06}$), suggesting an AGN is not the dominant source of UV light. The steep UV slope ($β=-2.5^{+0.2}_{-0.2}$) implies negligible dust attenuation and a young stellar population. The absence of a strong damping wing provides tentative evidence that the immediate surroundings of MoM-z14 may be partially ionized at a redshift where virtually every reionization model predicts a $\sim100\%$ neutral fraction. The nitrogen emission and highly super-solar [N/C]$>1$ hint at an abundance pattern similar to local globular clusters that may have once hosted luminous supermassive stars. Since this abundance pattern is also common among the most ancient stars born in the Milky Way, we may be directly witnessing the formation of such stars in dense clusters, connecting galaxy evolution across the entire sweep of cosmic time.

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arXiv2026

UNCOVER/MegaScience Finds Uniform and Highly Bursty Star Formation at 3 < z < 9, consistent with the High-Redshift UV Luminosity Function

Star formation timescales are key to understanding fundamental physics like feedback mechanisms, as well as the abundance of bright galaxies at $z>10$. We investigate galaxy star formation histories (SFHs) and their evolution across $z\sim3$--9 by measuring the line-to-UV ratio (\rline) and line equivalent width (EW) of \hanii\ and \oiiihb\ directly from UNCOVER/MegaScience spectro-photometry without relying on a specific SFH or nebular line modeling. Our photometric measurements recover \rline\ and EW to $<10\%$ systematic accuracy compared to spectroscopy. This allows us to construct a large mass- (and flux-) complete sample and quantitatively examine how \rline\ evolves with redshift and stellar mass. We find that the intrinsic scatter in \rline\ does not significantly evolve with redshift across $3 10$. This suggests that no significant evolution in star formation burstiness is required to explain the abundance of UV-bright galaxies at high redshift.

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arXiv2025

Picture an Astronomer: Best Practices for Retaining Talent in Astrophysics

Women are consistently underrepresented in astrophysics yet are simultaneously subject to disproportionate attrition at every career stage. This disparity between demonstrated efficacy in job performance and ultimate career outcome was the primary motivation for the Picture an Astronomer series, which included both targeted public outreach to increase representation of women in astrophysics and high-level, solution-oriented discussions among professional astronomers. In March 2025, more than 200 astronomers came together in a hybrid-format symposium focused on the state of the field for female scientists, combining scientific exchange with discussions of policies and practices to strengthen retention of talent in the field. This white paper is the result of those discussions, offering a wide range of recommendations developed in the context of gendered attrition in astrophysics but which ultimately support a healthier climate for all scientists alike.

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arXiv2025

The Mass-Metallicity Relation and its Observational Effects at z~3-6

The correlation between galaxy stellar mass and gas-phase metallicity, known as the mass-metallicity relation (MZR), gives key insights into the processes that govern galaxy evolution. However, unquantified observational and selection biases can result in systematic errors in attempts to recover the intrinsic MZR, particularly at higher redshifts. We characterize the MZR at z~3-6 within a fully Bayesian framework using JWST NIRSpec spectra of 193 galaxies from the RUBIES survey. We forward model the observed mass-metallicity surface using prospector-generated spectra to account for two selection biases: the survey selection function and success in observing high signal-to-noise emission lines. We demonstrate that the RUBIES selection function, based on F444W magnitude and F150W-F444W color, has a negligible effect on our measured MZR. A correct treatment of the non-Gaussian metallicity uncertainties from strong-line calibrations lowers the derived MZR normalization by 0.2 dex and flattens the slope by ~20%; forward-modeling the effect of emission line observability steepens the slope by ~15%. Both of these biases must be taken into account in order to properly measure the intrinsic MZR. This novel forward modeling process motivates careful consideration of selection functions in future surveys, and paves the way for robust, high-redshift chemical enrichment studies that trace the evolution of the mass-metallicity relation across cosmic time.

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arXiv2025

Where Galaxies Go to Die: The Environments of Massive Quiescent Galaxies at $3<z<5$

At low redshift, massive quiescent galaxies (MQGs) are most frequently found in massive, rich galaxy clusters, but at high redshift the trend is less clear. Here, we present spectroscopic evidence of the effects of environment on the formation and assembly of high-redshift MQGs. We identify 25 (5) $\log (M_*/\mathrm{M_\odot}\geq10.5$ ($10.0\leq\log (M_*/\mathrm{M_\odot}<10.5$) spectroscopically-confirmed quiescent galaxies in the UDS and EGS fields at $3 5$, finding that the masses from the inferred MQG SFHs regularly exceed either observed or simulated high-redshift galaxies, which suggests indicates that mergers and ex-situ star formation play a key role in the mass assembly of MQGs in overdense environments.

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