arXiv subjects

Antony A. Stark

Publications and source records attributed to Antony A. Stark.

At least 19 recordsLinked to original sources

arXiv2026

COOL-LAMPS IX: A Rare Duo of Quasars Each Lensed by a Single Massive Galaxy Cluster

Wide-separation lensed quasars (WSLQs) are rare systems that arise from the chance alignment of two objects: a galaxy cluster and a background quasar. After two decades, only seven WSLQs have been found. Here, we report the discovery of COOLJ1153+0755 by the COOL-LAMPS collaboration in DECaLS imaging and its confirmation with follow-up observations with the Magellan Telescopes and the Nordic Optical Telescope. This system features two multiply-imaged quasars each lensed into four images by the same $z=0.4301$ cluster: a classic broad-line Type I quasar at $z=1.524$ (COOLJ1153A) and a dust-obscured Type II quasar at $z=1.939$ (COOLJ1153B), with maximum image separations of $25.''6$ and $26.''0$, respectively. We construct a lens model to estimate a projected cluster mass of $M(<500\,{\rm kpc})\sim3.3\times10^{14}{\rm M}_{\odot}$ and relative time delays between the three brightest images of each quasar of $Δt_{\rm \,A3,A1}\sim800$, $Δt_{\rm \,A2,A1}\sim1200$, $Δt_{\rm \,B1,B3}\sim800$, and $Δt_{\rm \,B2,B3}\sim1000$ days. COOLJ1153A resides in a dense environment with three nearby galaxies, two of which are also strongly lensed. We identify COOLJ1153+0755 without making a morphological cut in the DECaLS catalog; none of its multiple images are classified as point sources in those data, implying that morphology-based selection would miss such systems. COOLJ1153+0755 expands the WSLQ sample from 7 to 8 systems (9 individual quasars), adding two powerful laboratories for probing black hole-galaxy co-evolution at Cosmic Noon and for time-delay cosmography constraints on the Hubble constant, $H_0$.

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arXiv2024

COOL-LAMPS VIII: Known wide-separation lensed quasars and their host galaxies reveal a lack of evolution in $M_{\rm{BH}}/M_\star$ since $z\sim 3$

Wide-separation lensed quasars (WSLQs) are a rare class of strongly lensed quasars, magnified by foreground massive galaxy clusters, with typically large magnifications of the multiple quasar images. They are a relatively unexplored opportunity for detailed study of quasar host galaxies. The current small sample of known WSLQs has a median redshift of $z\approx 2.1$, larger than most other samples of quasar host galaxies studied to date. Here, we derive precise constraints on the properties of six WSLQs and their host galaxies, using parametric surface brightness fitting, measurements of quasar emission lines, and stellar population synthesis of host galaxies in six WSLQ systems. Our results, with significant uncertainty, indicate that these six hosts are a mixture of star-forming and quiescent galaxies. To probe for co-evolution between AGNs and host galaxies, we model the offset from the `local' ($z=0$) $M_{\rm{BH}}\unicode{x2013}M_\star$ relation as a simple power-law in redshift. Accounting for selection effects, a WSLQ-based model for evolution in the $M_{\rm{BH}}\unicode{x2013}M_\star$ relation has a power-law index of $γ_M=-0.42\pm0.31$, consistent with no evolution. Compared to several literature samples, which mostly probe unlensed quasars at $z<2$, the WSLQ sample shows less evolution from the local relation, at $\sim 4σ$. We find that selection affects and choices of $M_{\rm{BH}}$ calibration are the most important systematics in these comparisons. Given that we resolve host galaxy flux confidently even from the ground in some instances, our work demonstrates that WSLQs and highly magnified AGNs are exceptional systems for future AGN$\unicode{x2013}$host co-evolution studies.

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arXiv2024

Single Aperture Large Telescope for Universe Studies (SALTUS): Science Overview

The SALTUS Probe mission will provide a powerful far-infrared (far-IR) pointed space observatory to explore our cosmic origins and the possibility of life elsewhere. The observatory employs an innovative deployable 14-m aperture, with a sunshield that will radiatively cool the off-axis primary to <45K. This cooled primary reflector works in tandem with cryogenic coherent and incoherent instruments that span the 34 to 660 micron far-IR range at both high and moderate spectral resolutions.

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arXiv2024

High-Redshift Extragalactic Science with the Single Aperture Large Telescope for Universe Studies (SALTUS) Space Observatory

This paper presents an overview of the high-redshift extragalactic science case for the Single Aperture Large Telescope for Universe Studies (SALTUS) far-infrared NASA probe-class mission concept. Enabled by its 14m primary reflector, SALTUS offers enormous gains in spatial resolution and spectral sensitivity over previous far-IR missions. SALTUS would be a versatile observatory capable of responding to the scientific needs of the extragalactic community in the 2030s, and a natural follow-on to the near- and mid-IR capabilities of JWST. Key early-universe science goals for SALTUS focus on understanding the role of galactic feedback processes in regulating galaxy growth across cosmic time, and charting the rise of metals and dust from the early universe to the present. This paper summarizes these science cases and the performance metrics most relevant for high-redshift observations.

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arXiv2023

SPT-CL J2215-3537: A Massive Starburst at the Center of the Most Distant Relaxed Galaxy Cluster

We present the discovery of the most distant, dynamically relaxed cool core cluster, SPT-CL J2215-3537 (SPT2215) and its central brightest cluster galaxy (BCG) at z=1.16. Using new X-ray observations, we demonstrate that SPT2215 harbors a strong cool core, with a central cooling time of 200 Myr (at 10 kpc) and a maximal intracluster medium cooling rate of 1900+/-400 Msun/yr. This prodigious cooling may be responsible for fueling extended, star-forming filaments observed in Hubble Space Telescope imaging. Based on new spectrophotometric data, we detect bright [O II] emission in the BCG, implying an unobscured star formation rate (SFR) of 320^{+230}_{-140} Msun/yr. The detection of a weak radio source (2.0+/-0.8 mJy at 0.8 GHz) suggests ongoing feedback from an active galactic nucleus (AGN), though the implied jet power is less than half the cooling luminosity of the hot gas, consistent with cooling overpowering heating. The extreme cooling and SFR of SPT2215 is rare among known cool core clusters, and it is even more remarkable that we observe these at such high redshift, when most clusters are still dynamically disturbed. The high mass of this cluster, coupled with the fact that it is dynamically relaxed with a highly-isolated BCG, suggests that it is an exceptionally rare system that must have formed very rapidly in the early Universe. Combined with the high SFR, SPT2215 may be a high-z analog of the Phoenix cluster, potentially providing insight into the limits of AGN feedback and star formation in the most massive galaxies.

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arXiv2022

Chaotic and Clumpy Galaxy Formation in an Extremely Massive Reionization-Era Halo

The SPT0311-58 system at z=6.900 is an extremely massive structure within the reionization epoch, and offers a chance to understand the formation of galaxies in an extreme peak in the primordial density field. We present 70mas Atacama Large Millimeter/submillimeter Array observations of the dust continuum and CII 158um emission in the central pair of galaxies and reach physical resolution ~100-350pc, among the most detailed views of any reionization-era system to date. The observations resolve the source into at least a dozen kiloparsec-size clumps. The global kinematics and high turbulent velocity dispersion within the galaxies present a striking contrast to recent claims of dynamically cold thin-disk kinematics in some dusty galaxies just 800Myr later at z~4. We speculate that both gravitational interactions and fragmentation from massive parent disks have likely played a role in the overall dynamics and formation of clumps in the system. Each clump individually is comparable in mass to other 6<z<8 galaxies identified in rest-UV/optical deep field surveys, but with star formation rates elevated by ~3-5x. Internally, the clumps themselves bear close resemblance to greatly scaled-up versions of virialized cloud-scale structures identified in low-redshift galaxies. Our observations are qualitatively similar to the chaotic and clumpy assembly within massive halos seen in simulations of high-redshift galaxies.

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arXiv2022

Synthesizing Stellar Populations in South Pole Telescope Galaxy Clusters: I. Ages of Quiescent Member Galaxies at 0.3 < z < 1.4

Using stellar population synthesis models to infer star formation histories (SFHs), we analyse photometry and spectroscopy of a large sample of quiescent galaxies which are members of Sunyaev-Zel'dovich (SZ)-selected galaxy clusters across a wide range of redshifts. We calculate stellar masses and mass-weighted ages for 837 quiescent cluster members at 0.3 < z < 1.4 using rest-frame optical spectra and the Python-based Prospector framework, from 61 clusters in the SPT-GMOS Spectroscopic Survey (0.3 < z < 0.9) and 3 clusters in the SPT Hi-z cluster sample (1.25 < z < 1.4). We analyse spectra of subpopulations divided into bins of redshift, stellar mass, cluster mass, and velocity-radius phase-space location, as well as by creating composite spectra of quiescent member galaxies. We find that quiescent galaxies in our dataset sample a diversity of SFHs, with a median formation redshift (corresponding to the lookback time from the redshift of observation to when a galaxy forms 50% of its mass, t$_{50}$) of $z=2.8\pm0.5$, which is similar to or marginally higher than that of massive quiescent field and cluster galaxy studies. We also report median age-stellar mass relations for the full sample (age of the Universe at $t_{50}$ (Gyr) = $2.52 (\pm0.04) - 1.66 (\pm0.11)$ log$_{10}(M/10^{11} M\odot))$ and recover downsizing trends across stellar mass; we find that massive galaxies in our cluster sample form on aggregate $\sim0.75$ Gyr earlier than lower mass galaxies. We also find marginally steeper age-mass relations at high redshifts, and report a bigger difference in formation redshifts across stellar mass for fixed environment, relative to formation redshifts across environment for fixed stellar mass.

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arXiv2021

COOL-LAMPS I. An Extraordinarily Bright Lensed Galaxy at Redshift 5.04

We report the discovery of COOL J1241+2219, a strongly-lensed galaxy at redshift $z$=5.043$\pm$0.002 with observed magnitude $z_{AB}=20.47$, lensed by a moderate-mass galaxy cluster at $z$=1.001$\pm$0.001. COOL J1241+2219 is the brightest lensed galaxy currently known at optical and near-infrared wavelengths at $z$ $\gtrsim$ 5; it is $\sim$5 times brighter than the prior record-holder lensed galaxy, and several magnitudes brighter than the brightest unlensed galaxies known at these redshifts. It was discovered as part of COOL-LAMPS, a collaboration initiated to find strongly lensed systems in recent public optical imaging data. We characterise the lensed galaxy, as well as the central galaxy of the lensing cluster using ground-based $griz$JH imaging and optical spectroscopy. We report model-based magnitudes, and derive stellar masses, dust content, metallicity and star-formation rates via stellar-population synthesis modeling. Our lens mass modeling, based on ground-based imaging, implies a median source magnification of $\sim$30, which puts the stellar mass and star formation rate (in the youngest age bin, closest to the epoch of observation) at logM$_{*}$ = $10.11^{+0.21}_{-0.26}$ and SFR = $27^{+13}_{-9}$ M$_{\odot}$/yr, respectively. We constrain a star formation history for COOL J1241+2219 consistent with constant star formation across $\sim$1 Gyr of cosmic time, and that places this galaxy on the high-mass end of the star-forming main sequence. COOL J1241+2219 is 2-4 times more luminous than a galaxy with the characteristic UV luminosity at these redshifts. The UV continuum slope $β$= -2.2$\pm$0.2 places this galaxy on the blue side of the observed distribution of galaxies at $z$=5, although the lack of Ly$α$ emission indicates dust sufficient to suppress this emission.

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arXiv2021

The Clusters Hiding in Plain Sight (CHiPS) survey: Complete sample of extreme BCG clusters

We present optical follow-up observations for candidate clusters in the Clusters Hiding in Plain Sight (CHiPS) survey, which is designed to find new galaxy clusters with extreme central galaxies that were misidentified as bright isolated sources in the ROSAT All-Sky Survey catalog. We identify 11 cluster candidates around X-ray, radio, and mid-IR bright sources, including six well-known clusters, two false associations of foreground and background clusters, and three new candidates which are observed further with Chandra. Of the three new candidates, we confirm two newly discovered galaxy clusters: CHIPS1356-3421 and CHIPS1911+4455. Both clusters are luminous enough to be detected in the ROSAT All Sky-Survey data if not because of their bright central cores. CHIPS1911+4455 is similar in many ways to the Phoenix cluster, but with a highly-disturbed X-ray morphology on large scales. We find the occurrence rate for clusters that would appear to be X-ray bright point sources in the ROSAT All-Sky Survey (and any surveys with similar angular resolution) to be 2+/-1%, and the occurrence rate of clusters with runaway cooling in their cores to be <1%, consistent with predictions of Chaotic Cold Accretion. With the number of new groups and clusters predicted to be found with eROSITA, the population of clusters that appear to be point sources (due to a central QSO or a dense cool core) could be around 2000. Finally, this survey demonstrates that the Phoenix cluster is likely the strongest cool core at z<0.7 -- anything more extreme would have been found in this survey.

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arXiv2020

The Most Rapidly Declining Type I Supernova 2019bkc/ATLAS19dqr

We report observations of the hydrogen-deficient supernova (SN) 2019bkc/ATLAS19dqr. With B- and r-band decline between peak and 10 days post peak of Delta m_10(B)=5.24+/-0.07 mag and Delta m_10(r)=3.85+/-0.10$ mag, respectively, SN 2019bkc is the most rapidly declining SN I discovered so far. While its closest matches are the rapidly declining SN 2005ek and SN 2010X, the light curves and spectra of SN 2019bkc show some unprecedented characteristics. SN 2019bkc appears "hostless," with no identifiable host galaxy near its location, although it may be associated with the galaxy cluster MKW1 at z = 0.02. We evaluate a number of existing models of fast-evolving SNe, and we find that none of them can satisfactorily explain all aspects of SN 2019bkc observations.

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arXiv2019

Strong Lensing Model of SPT-CLJ0356-5337, a Major Merger Candidate at Redshift 1.0359

We present an analysis of the mass distribution inferred from strong lensing by SPT-CL J0356-5337, a cluster of galaxies at redshift z = 1.0359 revealed in the follow-up of the SPT-SZ clusters. The cluster has an Einstein radius of Erad=14 for a source at z = 3 and a mass within 500 kpc of M_500kpc = 4.0+-0.8x10^14Msol. Our spectroscopic identification of three multiply-imaged systems (z = 2.363, z = 2.364, and z = 3.048), combined with HST F606W-band imaging allows us to build a strong lensing model for this cluster with an rms of <0.3'' between the predicted and measured positions of the multiple images. Our modeling reveals a two-component mass distribution in the cluster. One mass component is dominated by the brightest cluster galaxy and the other component, separated by ~170 kpc, contains a group of eight red elliptical galaxies confined in a ~9'' (~70 kpc) diameter circle. We estimate the mass ratio between the two components to be between 1:1.25 and 1:1.58. In addition, spectroscopic data reveal that these two near-equal mass cores have only a small velocity difference of 300 km/s between the two components. This small radial velocity difference suggests that most of the relative velocity takes place in the plane of the sky, and implies that SPT-CL J0356-5337 is a major merger with a small impact parameter seen face-on. We also assess the relative contributions of galaxy-scale halos to the overall mass of the core of the cluster and find that within 800 kpc from the brightest cluster galaxy about 27% of the total mass can be attributed to visible and dark matter associated with galaxies, whereas only 73% of the total mass in the core comes from cluster-scale dark matter halos.

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arXiv2019

Probing IGM accretion onto faint Lyα emitters at z~2.8

(abridged) Observing the signature of accretion from the intergalactic medium (IGM) onto galaxies at z~3 requires the detection of faint (L<<L*) galaxies embedded in a filamentary matrix of low-density, metal-poor gas coherent over hundreds of kpc. We study the gaseous environment of three Lyman$α$ emitters (LAEs) at z=2.7-2.8, found to be aligned in projection with a background QSO over ~250 kpc along the slit of a long-slit spectrum. The lack of detection of the LAEs in deep continuum images and the low inferred Ly$α$ luminosities show the LAEs to be intrinsically faint, low-mass galaxies (L<0.1 L*, M_star< 0.1 M*). An echelle spectrum of the QSO reveals strong Ly-alpha absorption within $\pm200$ km/s from the LAEs. Our absorption line analysis leads to HI column densities in the range of log N(HI) =16-18. Associated absorption from ionic metal species CIV and SiIV constrains the gas metallicities to ~0.01 solar if the gas is optically thin, and possibly as low as ~0.001 solar if the gas is optically thick, assuming photoionization equilibrium. While the inferred metallicities are at least a factor of ten lower than expected metallicities in the interstellar medium (ISM) of these LAEs, they are consistent with the observed chemical enrichment level in the IGM at the same epoch. Total metal abundances and kinematic arguments suggest that these faint galaxies have not been able to affect the properties of their surrounding gas. The projected spatial alignment of the LAEs, together with the kinematic quiescence and correspondence between the LAEs and absorbing gas in velocity space suggests that these observations probe a possible filamentary structure. Taken together with the blue-dominant Ly$α$ emission line profile of one of the objects, the evidence suggests that the absorbing gas is part of an accretion stream of low-metallicity gas in the IGM.

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arXiv2019

Source structure and molecular gas properties from high-resolution CO imaging of SPT-selected dusty star-forming galaxies

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of high-J CO lines ($J_\mathrm{up}=6$, 7, 8) and associated dust continuum towards five strongly lensed, dusty, star-forming galaxies (DSFGs) at redshift $z = 2.7$-5.7. These galaxies, discovered in the South Pole Telescope survey, are observed at $0.2''$-$0.4''$ resolution with ALMA. Our high-resolution imaging coupled with the lensing magnification provides a measurement of the structure and kinematics of molecular gas in the background galaxies with spatial resolutions down to kiloparsec scales. We derive visibility-based lens models for each galaxy, accurately reproducing observations of four of the galaxies. Of these four targets, three show clear velocity gradients, of which two are likely rotating disks. We find that the reconstructed region of CO emission is less concentrated than the region emitting dust continuum even for the moderate-excitation CO lines, similar to what has been seen in the literature for lower-excitation transitions. We find that the lensing magnification of a given source can vary by 20-50% across the line profile, between the continuum and line, and between different CO transitions. We apply Large Velocity Gradient (LVG) modeling using apparent and intrinsic line ratios between lower-J and high-J CO lines. Ignoring these magnification variations can bias the estimate of physical properties of interstellar medium of the galaxies. The magnitude of the bias varies from galaxy to galaxy and is not necessarily predictable without high resolution observations.

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arXiv2018

SPT-3G secondary mirror geometry

SPT-3G is a detector system for the 10m diameter South Pole Telescope, comprising 16,000 millimeter-wave bolometers. It is used for a deep Cosmic Microwave Background survey of the Southern sky. This paper describes the geometry of the secondary mirror, which is a section of a prolate spheroid, in several useful coordinate systems. There is application to off-axis mirrors in general. A geometric theorem is proven, relating to the Dragone condition: the intersection of a prolate spheroid and any plane is an ellipse; the lines connecting points on that ellipse to either focus compose a right circular cone; the central axes of the two cones from the two foci intersect outside the interior of the spheroid.

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arXiv2018

A VLBI receiving system for the South Pole Telescope

The Event Horizon Telescope (EHT) is a very-long-baseline interferometry (VLBI) experiment that aims to observe supermassive black holes with an angular resolution that is comparable to the event horizon scale. The South Pole occupies an important position in the array, greatly increasing its north-south extent and therefore its resolution. The South Pole Telescope (SPT) is a 10-meter diameter, millimeter-wavelength telescope equipped for bolometric observations of the cosmic microwave background. To enable VLBI observations with the SPT we have constructed a coherent signal chain suitable for the South Pole environment. The dual-frequency receiver incorporates state-of-the-art SIS mixers and is installed in the SPT receiver cabin. The VLBI signal chain also includes a recording system and reference frequency generator tied to a hydrogen maser. Here we describe the SPT VLBI system design in detail and present both the lab measurements and on-sky results.

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arXiv2017

Velocity Segregation and Systematic Biases In Velocity Dispersion Estimates With the SPT-GMOS Spectroscopic Survey

The velocity distribution of galaxies in clusters is not universal; rather, galaxies are segregated according to their spectral type and relative luminosity. We examine the velocity distributions of different populations of galaxies within 89 Sunyaev Zel'dovich (SZ) selected galaxy clusters spanning $ 0.28 < z < 1.08$. Our sample is primarily draw from the SPT-GMOS spectroscopic survey, supplemented by additional published spectroscopy, resulting in a final spectroscopic sample of 4148 galaxy spectra---2868 cluster members. The velocity dispersion of star-forming cluster galaxies is $17\pm4$% greater than that of passive cluster galaxies, and the velocity dispersion of bright ($m < m^{*}-0.5$) cluster galaxies is $11\pm4$% lower than the velocity dispersion of our total member population. We find good agreement with simulations regarding the shape of the relationship between the measured velocity dispersion and the fraction of passive vs. star-forming galaxies used to measure it, but we find a small offset between this relationship as measured in data and simulations in which suggests that our dispersions are systematically low by as much as 3\% relative to simulations. We argue that this offset could be interpreted as a measurement of the effective velocity bias that describes the ratio of our observed velocity dispersions and the intrinsic velocity dispersion of dark matter particles in a published simulation result. Measuring velocity bias in this way suggests that large spectroscopic surveys can improve dispersion-based mass-observable scaling relations for cosmology even in the face of velocity biases, by quantifying and ultimately calibrating them out.

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arXiv2015

A rocky planet transiting a nearby low-mass star

M-dwarf stars -- hydrogen-burning stars that are smaller than 60 per cent of the size of the Sun -- are the most common class of star in our Galaxy and outnumber Sun-like stars by a ratio of 12:1. Recent results have shown that M dwarfs host Earth-sized planets in great numbers: the average number of M-dwarf planets that are between 0.5 to 1.5 times the size of Earth is at least 1.4 per star. The nearest such planets known to transit their star are 39 parsecs away, too distant for detailed follow-up observations to measure the planetary masses or to study their atmospheres. Here we report observations of GJ 1132b, a planet with a size of 1.2 Earth radii that is transiting a small star 12 parsecs away. Our Doppler mass measurement of GJ 1132b yields a density consistent with an Earth-like bulk composition, similar to the compositions of the six known exoplanets with masses less than six times that of the Earth and precisely measured densities. Receiving 19 times more stellar radiation than the Earth, the planet is too hot to be habitable but is cool enough to support a substantial atmosphere, one that has probably been considerably depleted of hydrogen. Because the host star is nearby and only 21 per cent the radius of the Sun, existing and upcoming telescopes will be able to observe the composition and dynamics of the planetary atmosphere.

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arXiv2008

Cosmology from Antarctica

Observation of the CMB is central to observational cosmology, and the Antarctic Plateau is an exceptionally good site for this work. The first attempt at CMB observations from the Plateau was an expedition to the South Pole in December 1986 by the Radio Physics Research group at Bell Laboratories. Sky noise and opacity were measured. The results were sufficiently encouraging that in the Austral summer of 1988-1989, three CMB groups participated in the "Cucumber" campaign, where a temporary site dedicated to CMB anisotropy measurements was set up 2 km from South Pole Station. Winter-time observations became possible with the establishment in 1990 of the Center for Astrophysical Research in Antarctica (CARA), a National Science Foundation Science and Technology Center. CARA developed year-round observing facilities in the "Dark Sector", a section of Amundsen-Scott South Pole Station dedicated to astronomical observations. CARA scientists fielded several astronomical instruments: AST/RO, SPIREX, White Dish, Python, Viper, ACBAR, and DASI. By 2001, data from CARA, together with BOOMERANG, a CMB experiment on a long-duration balloon launched from McMurdo Station on the coast of Antarctica, showed clear evidence that the overall geometry of the Universe is flat, as opposed to being positively or negatively curved. In 2002, the DASI group reported the detection of polarization in the CMB. These observations strongly support the concordance model of cosmology, where the dynamics of a flat Universe are dominated by forces exerted by the Dark Energy and Dark Matter. The South Pole Telescope (SPT) is a newly-operational 10 m diameter offset telescope designed to rapidly measure anisotropies on scales much smaller than 1 degree.

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