arXiv subjects

Justin S. Spilker

Publications and source records attributed to Justin S. Spilker.

At least 19 recordsLinked to original sources

arXiv2026

An extreme ram-pressure stripping event in a protocluster at redshift 4.3

In the nearby Universe, the environment plays a crucial role in suppressing star formation in dense regions. In particular, ram-pressure stripping (RPS) is a major mechanism for removing gas from galaxies in clusters, occurring when galaxies travel through a dense hot atmosphere and leave trailing gaseous wakes. By depleting the cold gas reservoir, RPS can drive outside-in quenching and is therefore thought to be an important route for transforming cluster galaxies. At earlier times, however, the hot atmosphere in protoclusters is expected to be immature, so environmental effects are commonly assumed to be dominated by gravitational interactions. Here we report ALMA and JWST observations of SPT2349$-$56-C26 (hereafter C26), a massive galaxy experiencing an extreme RPS event in the SPT2349$-$56 protocluster at $z\,{=}\,4.3$. More than half of the [CII]-traced cold gas lies outside its stellar body, with the emission peak offset by 6 kpc. These observations show that RPS can remove most of the cold gas from massive galaxies in dense protocluster cores as early as $z\,{=}\,4.3$, providing a direct hydrodynamic pathway for environmental quenching at $z\,{>}\,4$.

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arXiv2026

A large overdensity of LRD-like galaxies discovered by JWST in the SPT2349-56 protocluster at z=4.30

We present 11 newly identified ultra-red galaxies selected with JWST/NIRCam in the core of the z=4.3 protocluster SPT2349-56. Although these sources are not yet spectroscopically confirmed, their extremely red colours (m200-m444 > 2.0 mag), strong spatial concentration, and number density - at least 2000 times that of comparable field populations - make a chance projected association unlikely. Their colours overlap those used to select the widely studied ``little red dot'' (LRD) population, although most of the sources are spatially resolved and are therefore better described as extended red dots (ERDs). The resolved morphologies disfavour continuum emission dominated by an unresolved active nucleus and imply a substantial stellar contribution. Deep ALMA observations put strong limits on obscured star formation, and the ERD population mostly lies significantly below the star-forming main sequence. One object, ERD1/Q1, is substantially brighter and more massive, lying far below the main sequence, making it a strong candidate for a massive quiescent galaxy. These JWST-selected red galaxies reveal a previously inaccessible population extending to lower luminosities and stellar masses than the Dusty Star Forming Galaxy (DSFG) population which characterizes the dense protocluster core. SPT2349-56 may therefore capture an early phase of cluster assembly in which extreme starbursts coexist with galaxies already transitioning toward the passive galaxies characteristic of mature cluster cores.

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arXiv2026

The ALPINE-CRISTAL-JWST Survey: Gas-phase abundance gradients of main sequence star-forming galaxies and their kinematics at $4 < z < 6$

We present gas-phase radial metallicity profiles for 20 main-sequence galaxies at $4 0.05{\rm dexkpc^{-1}}$ at $1σ$, and none have a significant negative gradient. We investigated the correlation between the metallicity gradients and the intrinsic gas velocity dispersion $σ_0$ as well as the $V_{\rm rot}/σ_0$ ratio of the disks. Combining our sample with mass-matched literature samples at $3<z<7$, we found a negative shallow correlation between $V_{\rm rot}/σ_0$ and the metallicity gradients, but no strong relationships with $σ_0$. As $V_{\rm rot}/σ_0$ increases towards later cosmic times, the observed negative trend with $V_{\rm rot}/σ_0$ is consistent with the overall cosmic evolution of metallicity gradients from high to low redshifts. This suggests that disk maturity plays a crucial role in shaping the radial metallicity gradients. [Abridged abstract]

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arXiv2026

Molecular Gas Excitation in z ~ 0.7 Gas-Rich Post-starburst Galaxies from SQuIGGLE

Many post-starburst galaxies at $z\sim0.7$ have been shown to retain substantial molecular gas reservoirs yet host low ongoing star formation, suggesting that the remaining gas may be inefficient at forming stars during the early post-burst phase. We present new Atacama Large Millimeter/submillimeter Array CO(5-4) observations of nine gas-rich post-starburst galaxies at $z\sim0.7$ from the Studying Quenching in Intermediate-z Galaxies: Gas, angu$\vec{L}$ar momentum, and Evolution (SQuIGG$\vec{L}$E) survey, providing a view of the molecular gas excitation in these systems. Combined with existing CO(2-1) data, we detect CO(5-4) in 8/9 targets and find that most have moderate CO excitation with $r_{52}\equiv L'_{\rm CO(5-4)}/L'_{\rm CO(2-1)}\approx0.1-0.3$. These systems show no clear trend between $r_{52}$ and either total or surface-density of star formation. Specifically, all objects have $Σ_{\mathrm{SFR}} \sim 0.01-1\ \text{M}_\odot\ \text{yr}^{-1}\ \text{kpc}^{-2}$, consistent with compact, modest star formation, even when allowing for buried activity, as these galaxies decline from their peak. One object J1448+1010, which has clear optical, mid-infrared, and radio indicators of an active galactic nucleus, is an outlier with $r_{52}\approx0.6$; its elevated excitation likely requires significant non-stellar heating, with a contribution from potentially obscured star formation. Together, most gas-rich SQuIGG$\vec{L}$E post-starbursts have moderately excited molecular gas alongside little to modest star-forming activity, indicating that the remaining gas hosts relatively suppressed star formation efficiencies instead of strong buried starburst activity.

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arXiv2026

A Possible Protocluster of Galaxies Serendipitously Discovered in the Field of an Intermediate-Redshift Post-starburst Galaxy

We present the serendipitous discovery of an overdensity of submillimeter galaxies (SMGs) in the field of SDSSJ0909-0108, a massive z~0.7 post-starburst galaxy from the SQuIGGLE survey. ALMA observations at 870um and 2mm reveal six galaxies within a 35'' region with flux ratios consistent with emission from dust. Given the rarity of 870um sources and the small field-of-view of ALMA, we speculate that some of these sources are physically associated. None of the sources are at the same redshift as the post-starburst, and four do not have spectroscopic redshifts. We suggest that follow-up optical and/or ALMA observations be carried out to measure redshifts for the galaxies in this potential protocluster environment.

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arXiv2026

Joint Optical and Infrared Observations of N and O Reveal the Dust-Obscured Gas in Haro 3

Accurate chemical compositions of star-forming regions are a critical diagnostic tool to characterize the star formation history and gas flows which regulate galaxy formation. However, the abundance discrepancy factor (ADF) between measurements from the "direct" optical electron temperature ($T_e$) method and from the recombination lines (RL) represents $\sim0.2$ dex systematic uncertainty in oxygen abundance. The degree of uncertainty for other elements is unknown. We conduct a comprehensive analysis of O$^{++}$ and N$^+$ ion abundances using optical and far-infrared spectra of a star-forming region within the nearby dwarf galaxy Haro 3, which exhibits a typical ADF. Assuming homogeneous conditions, the far-IR emission indicates an O abundance which is higher than the $T_e$ method and consistent with the RL value, as would be expected from temperature fluctuations, whereas the far-IR N abundance is too large to be explained by temperature fluctuations. A two-phase analytical model reveals that differential dust obscuration associated with temperature inhomogeneity is likely required to explain all the emission line ratios, and that the total oxygen metallicity of two phases is consistent with the RL metallicity. Our findings underscore the critical importance of resolving the cause of abundance discrepancies and understanding the biases between different metallicity methods. This work represents a promising methodology, and we identify further approaches to address the current dominant uncertainties.

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arXiv2026

A Multiwavelength Evaluation of AGN in the Post-Starburst Phase

The quenching of star formation is a crucial phase in galaxy evolution. Although active galactic nuclei (AGN) feedback has been proposed as a key driver of this transition, the lack of strong AGN in nearby quenching galaxies raises questions about its effectiveness. In this study, we investigate AGN activity in post-starburst galaxies (PSBs), star-forming galaxies (SFGs), and quiescent galaxies (QGs) at $z<$ 0.2, using multiwavelength data from eROSITA/eFEDS (X-ray), WISE (mid-infrared), and FIRST (radio). We assess AGN incidence and strength across different stages and apply stacking techniques to undetected galaxies to recover average AGN properties. Comparisons between observed luminosity and that expected from star formation (L$_{\rm obs}$/L$_{\rm SF}$) show that PSBs are consistent with star formation dominating their radio and X-ray emission. Although PSBs exhibit a MIR AGN incidence rate twice that of SFGs, their estimated AGN luminosities are small compared to those of MIR AGN in the literature. PSBs overall do not display significantly enhanced AGN emission relative to mass- and redshift-matched SFGs and QGs. While the presence of obscured, low-luminosity AGN in PSBs cannot be excluded, such AGN, if present, could be fueled by residual gas from the preceding starburst and may not play a dominant role in quenching. Our findings suggest that AGN's role in quenching at low redshift is more subtle than violently removing the gas -- the feedback is likely more "preventive" than "ejective".

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arXiv2026

A census of star-formation and gas mass tracers in two lensed $z \sim 4$ dusty star-forming galaxies

We present new and archival Atacama Large Millimeter/submillimeter Array (ALMA) observations of two strongly lensed dusty star-forming galaxies (DSFGs) selected from the South Pole Telescope survey, SPT0418-47 $(z = 4.225)$ and SPT2147-50 $(z = 3.760)$. We study the [C II], CO(7-6), [C I](2-1), and, in SPT0418-47, $p$-H$_2$O emission, which along with the underlying continuum (rest-frame 160 $μ$m and 380 $μ$m) are routinely used as tracers of gas mass and/or star-formation rate (SFR). We perform a pixel-by-pixel analysis of both sources in the image plane to study the resolved Kennicutt-Schmidt relation, finding generally good agreement between the slopes of the SFR versus gas mass surface density using the different tracers. Using lens modeling methods, we find that the dust emission is more compact than the line emission in both sources, with CO(7-6) and [C I](2-1) similar in extent and [C II] the most extended, reminiscent of recent findings of extended [C II] spatial distributions in galaxies at similar cosmic epochs. We develop the [C I](2-1) / CO(7-6) flux density ratio as an observable proxy for gas depletion timescale ($τ_{\rm dep}$), which can be applied to large samples of DSFGs, in lieu of more detailed inferences of this timescale which require analysis of observations at multiple wavelengths. Furthermore, the extended [C II] emission in both sources, compared to the total continuum and line emission, suggests that [C II], used in recent years as a molecular gas mass and SFR tracer in high-$z$ galaxies, may not always be a suitable tracer of these physical quantities.

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arXiv2025

Unusually High Gas-to-Dust Ratios Observed in High-Redshift Quiescent Galaxies

Tracking the cold molecular gas contents of galaxies is critical to understand the interplay between star formation and galaxy growth across cosmic time. Observations of the long-wavelength dust continuum, a proxy for the cold gas, are widely used in the high-redshift community because of their ease and efficiency. These measurements rely on the assumption of a molecular gas-to-dust mass ratio, typically taken to be GDR ~ 100 in massive, metal-rich systems. We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of the 870um dust continuum in a sample of five massive quiescent galaxies at z ~ 1 with existing detections of CO(2-1). We find surprisingly weak dust emission, falling a factor of >~0.4-0.8 dex below the typical correlation between CO and continuum luminosity. We interpret this dust deficiency as evidence for unusually high GDR in these galaxies, which we calculate to range from 300 to at least 1200. Our results and other observations from the literature are generally compatible with predictions from the SIMBA cosmological simulation that dust is preferentially destroyed in quiescent galaxies. Ultimately, we conclude that the dust continuum is a highly unreliable tracer of the molecular gas in high-redshift quiescent galaxies. As a consequence we may know much less about the cold gas contents of this population than previously thought.

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arXiv2025

Meet the Neighbors: Gas Rich "Buddy Galaxies" are Common Around Recently Quenched Massive Galaxies in the SQuIGG$\vec{L}$E Survey

In this work, we characterize the environments of massive ($\log(M_\odot/M_\star)\sim11.2$) $z\sim0.7$ post-starburst galaxies (PSBs) by studying serendipitously-detected CO(2-1) emitters found in targeted observations of the SQuIGG$\vec{L}$E sample. We report $31\pm6\%$ of the galaxies from this survey host nearby gas-rich ``buddies'' with stellar masses $\geq 10^{10},M_\odot$ and molecular gas comparable to their central PSBs ($M_{H_{2}} \sim 10^{10} M_\odot$), but $\sim0.8$ dex lower stellar mass ($\sim 10^{10.4} M_\odot$). Based on their location in position-velocity space, each buddy is consistent with being bound to the haloes of their SQuIGG$\vec{L}$E host galaxies. We compare to the UniverseMachine model and find that SQuIGG$\vec{L}$E galaxies host a typical number of neighbors for their stellar mass, suggesting that PSBs live in environments typical of co-eval similarly-massive galaxies.

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arXiv2025

Bright [CII]158$μ$m Streamers as a Beacon for Giant Galaxy Formation in SPT2349$-$56 at $z=4.3$

Observations of extreme starbursts, often located in the cores of protoclusters, challenge the classical bottom-up galaxy formation paradigm. Giant elliptical galaxies at $z=0$ must have assembled rapidly, possibly within few 100 Myr through an extreme growth phase at high-redshift, characterized by elevated star-formation rates of several thousand solar masses per year distributed over concurrent, gas-rich mergers. We present a novel view of the $z=4.3$ protocluster core SPT2349$-$56 from sensitive multi-cycle ALMA dust continuum and [CII]158$μ$m line observations. Distributed across 60 kpc, a highly structured gas reservoir with a line luminosity of $L_\mathrm{[CII]}=3.0\pm0.2\times10^9$ $L_\odot$ and an inferred cold gas mass of $M_{gas}= 8.9\pm0.7\times10^{9}$ $M_\odot$ is found surrounding the central massive galaxy triplet. Like ``beads on a string'', the newly-discovered [CII] streamers fragment into a few kpc-spaced and turbulent clumps that have a similar column density as local Universe spiral galaxy arms at $Σ_{gas}=20$--$60$ $M_\odot$ pc$^{-2}$. For a dust temperature of 30 K, the [CII] emission from the ejected clumps carry $\gtrsim$3% of the FIR luminosity, translating into an exceptionally low mass-to-light ratio of $α_\mathrm{[CII]}=2.95\pm0.3$ $M_\odot$ $L_\odot^{-1}$, indicative of shock-heated molecular gas. In phase space, about half of the galaxies in the protocluster core populate the same caustic as the [CII] streamers ($r/r_{vir}\times|Δv|/σ_{vir}\approx0.1$), suggesting angular momentum dissipation via tidal ejection while the brightest cluster galaxy (BCG) is assembling. Our findings provide new evidence for the importance of tidal ejections of [CII]-bright, shocked material following multiple major mergers that might represent a landmark phase in the $z\gtrsim4$ co-evolution of BCGs with their hot, metal enriched atmospheres.

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arXiv2025

What you see is what you get: empirically measured bolometric luminosities of Little Red Dots

New populations of red active galactic nuclei (known as ``Little Red Dots'') discovered by JWST exhibit remarkable spectral energy distributions. Leveraging X-ray through far-infrared observations of two of the most luminous known Little Red Dots, we directly their bolometric luminosities. We find evidence that more than half of the bolometric luminosity likely emerges in the rest-frame optical, with $L_{\rm bol}/L_{5100} = 5$, roughly half the value for ``standard'' Active Galactic Nuclei. Meanwhile, the X-ray emitting corona, UV-emitting black-body, and reprocessed mid to far-infrared emission are all considerably sub-dominant, assuming that the far-infrared luminosity is well below current measured limits. We present new bolometric corrections that dramatically lower inferred bolometric luminosities by a factor of ten compared to published values in the literature. These bolometric corrections are in accord with expectations from models in which gas absorption and reprocessing are responsible for the red rest-frame optical colors of Little Red Dots. We discuss how this lowered luminosity scale suggests a lower mass scale for the population by at least an order of magnitude {\bf (e.g., $\sim 10^5-10^7~{\rm M_{\odot}}$ black holes, and $\sim 10^8~{\rm M_{\odot}}$ galaxies)}, alleviating tensions with clustering, overmassive black holes, and the integrated black hole mass density in the Universe.

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arXiv2025

SQuIGG$\vec{L}$E: Buried star formation cannot explain the rapidly fading CO(2-1) luminosity in massive, $z\sim0.7$ post-starburst galaxies

Observational and theoretical studies have long held that rapid gas consumption in starbursts is responsible for the formation of quiescent galaxies. However, studies of recently quenched ``post-starburst" galaxies have discovered that a number of them are surprisingly luminous in CO, challenging this assumption. We present deep ALMA CO(2-1) observations of 50 massive ($\log(M_\star/M_\odot)\sim11.2$) post-starburst galaxies from the SQuIGG$\vec{L}$E sample at $z\sim0.7$. We detect a large fraction (27/50) of the galaxies in CO(2-1). Furthermore, we find that the CO luminosity correlates with the age of the recent starburst, suggesting a gas-removal timescale of $\lesssim140$ Myr, an order of magnitude shorter than is implied by their rest optical star formation rates. We perform new spectral energy distribution fits incorporating mid- and far-IR photometry to test whether dust-obscured star formation can explain this trend. We find that while allowing for buried star formation can raise star formation rates by $\sim0.5$ dex, for almost all galaxies it is neither required to fit the observed IR SED, nor is it sufficient to explain the observed depletion trend. Even the combination of significant buried star formation and ULIRG-like $α_{CO}$ is not enough to explain this decay in CO luminosity. Furthermore, there is no strong evidence to support either of those modifications to the depletion time. Therefore, it remains a distinct possibility that the age-CO luminosity trend should not be interpreted as an evolutionary sequence, and that gas-rich SQuIGG$\vec{L}$E galaxies will soon rejuvenate.

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arXiv2025

An Image-Plane Approach to Gravitational Lens Modeling of Interferometric Data

Strong gravitational lensing acts as a cosmic telescope, enabling the study of the high-redshift universe. Astronomical interferometers, such as the Atacama Large Millimeter/submillimeter Array (ALMA), have provided high-resolution images of strongly lensed sources at millimeter and submillimeter wavelengths. To model the mass and light distributions of lensing and source galaxies from strongly lensed images, strong lens modeling for interferometric observations is conventionally performed in the visibility space, which is computationally expensive. In this paper, we implement an image-plane lens modeling methodology for interferometric dirty images by accounting for noise correlations. We show that the image-plane likelihood function produces accurate model values when tested on simulated ALMA observations with an ensemble of noise realizations. We also apply our technique to ALMA observations of two sources selected from the South Pole Telescope survey, comparing our results with previous visibility-based models. Our model results are consistent with previous models for both parametric and pixelated source-plane reconstructions. We implement this methodology for interferometric lens modeling in the open-source software package lenstronomy.

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arXiv2025

Quenching Through Tidal Gas Removal: Molecular Gas and Star Formation in Tidal Tails of z ~ 0.7 Post-Starburst Galaxies

The active suppression of star formation in galaxies is critical in preventing the growth of overly massive systems and explaining the formation of present-day elliptical galaxies. We present a high-resolution, spatially-resolved multiwavelength study of two z ~ 0.7 massive post-starburst galaxies, SDSS J1448+1010 and SDSS J2258+2313, from the SQuIGGLE survey (Studying Quenching in Intermediate-z Galaxies: Gas, anguLar momentum, and Evolution), providing new insights into the role of mergers in driving quenching. ALMA CO(2-1) observations show that both galaxies removed ~50% of their molecular gas into extended tidal tails, spanning up to 65 kpc, following recent mergers. HST WFC3 imaging and grism spectroscopy show that while SDSS J1448+1010 exhibits Halpha emission in its northern tidal tail consistent with ongoing star formation, SDSS J2258+2313 lacks detectable star-forming activity outside the central galaxy. VLA 6 GHz continuum data reveal compact radio emission in SDSS J2258+2313, while SDSS J1448+1010 hosts small radio jets indicative of AGN activity. Both galaxies retain substantial molecular gas reservoirs in their central regions that appear more turbulent than 'normal' star-forming galaxies, likely contributing to the observed low star formation rates in the hosts. Despite similarities in their cold gas content and tidal features the galaxies are distinct from each other in their star formation, gas-star alignment, and radio morphology, highlighting the complexity of tidal gas removal as a quenching mechanism at intermediate redshifts.

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arXiv2025

The ALMA-CRISTAL survey: Resolved kinematic studies of main sequence star-forming galaxies at 4<z<6

We present a detailed kinematic study of a sample of 32 massive ($9.5\leqslant\log(M_*/{\rm M_{\odot}})\leqslant10.9$) main-sequence star-forming galaxies (MS SFGs) at $4<z<6$ from the ALMA-CRISTAL program. The data consist of deep (up to 15hr observing time per target), high-resolution ($\sim1$kpc) ALMA observations of the [CII]158$μ$m line emission. This data set enables the first systematic kpc-scale characterisation of the kinematics nature of typical massive SFGs at these epochs. We find that $\sim50\%$ of the sample are disk-like, with a number of galaxies located in systems of multiple components. Kinematic modelling reveals these main sequence disks exhibit high-velocity dispersions ($σ_0$), with a median disk velocity dispersion of $\sim70{\rm kms^{-1}}$ and $V_{\rm rot}/σ_0\sim2$, and consistent with dominant gravity driving. The elevated disk dispersions are in line with the predicted evolution based on Toomre theory and the extrapolated trends from $z\sim0$-$2.5$ MS star-forming disks. The inferred dark matter (DM) mass fraction within the effective radius $f_{\rm DM}(<R_{\rm e})$ for the disk systems decreases with the central baryonic mass surface density, and is consistent with the trend reported by kinematic studies at $z\lesssim3$; roughly half the disks have $f_{\rm DM}(<R_{\rm e})\lesssim30\%$. The CRISTAL sample of massive MS SFGs provides a reference of the kinematics of a representative population and extends the view onto typical galaxies beyond previous kpc-scale studies at $z\lesssim3$.

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arXiv2025

Cold gas in a post-starburst pair at z ~ 1.4: major mergers as a pathway to quenching in the HeavyMetal survey

Recent observations at low redshift have revealed that some post-starburst galaxies retain significant molecular gas reservoirs despite low ongoing star formation rates, challenging theoretical predictions for galaxy quenching. To test whether this finding holds during the peak epoch of quenching, here we present ALMA CO(2-1) observations of five spectroscopically confirmed post-starburst galaxies at z ~ 1.4 from the HeavyMetal survey. While four galaxies are undetected in CO emission, we detect M_H2 ~ 10^9.7 Msun of molecular gas in one system. The detected system is a close pair of massive (M* = 10^(11.1-11.2) Msun) post-starburst galaxies with no clear tidal features, likely caught in the early stages of a major merger. These results suggest that mergers may be a key factor in retaining molecular gas while simultaneously suppressing star formation in quenched galaxies at high redshift, possibly by driving increased turbulence that decreases star formation efficiency. Unlike previous studies at z < 1, we find no correlation between molecular gas mass and time since quenching. This may be explained by the fact that -- despite having similar UVJ colors -- all galaxies in our sample have post-burst ages older than typical gas-rich quenched systems at low redshift. Our results highlight the importance of major mergers in shaping the cold gas content of quiescent galaxies during the peak epoch of quenching.

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arXiv2025

The ALMA-CRISTAL survey: weak evidence for star-formation driven outflows in $z\sim5$ main-sequence galaxies

There is a broad consensus from theory that stellar feedback in galaxies at high redshifts is essential to their evolution, alongside conflicting evidence in the observational literature about its prevalence and efficacy. To this end, we utilize deep, high-resolution [CII] emission line data taken as part of the [CII] resolved ISM in star-forming galaxies with ALMA (CRISTAL) survey. Excluding sources with kinematic evidence for gravitational interactions, we perform a rigorous stacking analysis of the remaining 15 galaxies to search for broad emission features that are too weak to detect in the individual spectra, finding only weak evidence that a broad component is needed to explain the composite spectrum. Additionally, such evidence is mostly driven by CRISTAL-02, which is already known to exhibit strong outflows in multiple ISM phases. Interpreting modest residuals in the stack at $v\sim300$kms$^{-1}$ as an outflow, we derive a mass outflow rate of $\dot{M}_{\rm out}=26\pm11$M$_\odot$yr$^{-1}$ and a cold outflow mass-loading factor of $η_m=0.49\pm0.20$. This result holds for the subsample with the highest star-formation rate surface density $(Σ_{\rm{SFR}}>1.93$M$_\odot$yr$^{-1}$kpc$^{-2}$) but no such broad component is present in the composite of the lower-star-formation rate density subsample. Our results imply that the process of star-formation-driven feedback may already be in place in typical galaxies at $z=5$, but on average not strong enough to completely quench ongoing star formation.

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