Ann I. Zabludoff

We present a detailed study of the 3D morphology of two extended associations of multiple protoclusters at z = 3.1. These protocluster “complexes,” designated COSMOS-z3.1-A and COSMOS-z3.1-C, are the most prominent overdensities of z = 3.1 Ly α emitters (LAEs) identified in the COSMOS field by the One-hundred-deg ² DECam Imaging in Narrowbands survey. These protocluster complexes have been followed up with extensive spectroscopy from Keck, Gemini, and DESI. Using a probabilistic method that combines photometrically selected and spectroscopically confirmed LAEs, we reconstruct the 3D structure of these complexes on scales of ≈50 cMpc. We validate our reconstruction method using the IllustrisTNG300-1 cosmological hydrodynamical simulation and show that it consistently outperforms approaches relying solely on spectroscopic data. The resulting 3D maps reveal that both complexes are irregular and elongated along a single axis, emphasizing the impact of sightline on our perception of structure morphology. The complexes consist of multiple density peaks, 10 in COSMOS-z3.1-A and 4 in COSMOS-z3.1-C. The former is confirmed to be a proto-supercluster, similar to Hyperion at z = 2.4 but observed at an even earlier epoch. Multiple “tails” connected to the cores of the density peaks are seen, likely representing cosmic filaments feeding into these extremely overdense regions. The 3D reconstructions further provide strong evidence that Ly α blobs preferentially reside in the outskirts of the highest density regions. Descendant mass estimates of the density peaks suggest that COSMOS-z3.1-A and COSMOS-z3.1-C will evolve to become ultramassive structures by z = 0, with total masses log ( M / M ⊙ ) ≳ 15.3 , exceeding that of Coma.

Context . Fast X-ray transients (FXTs) are extragalactic flashes of X-rays with a typical duration of minutes to hours for which a variety of origins has been proposed and observed. Aims . To decipher the origin of FXTs, particularly those lacking multi-wavelength counterparts, we aim to understand their energetics and environments. Methods . We present deep, optical ground-based observations of the positions of eight FXTs in order to try and identify and characterise candidate host galaxies. We used their properties to discriminate between possible progenitor scenarios. Results . We identify candidate host galaxies for Swift J050400.2+673405, XRT140507, XRT040610, XRT151121, XRT191127, and EP240708a. For each candidate, we infer the spectroscopic or photometric redshift, stellar mass, star formation rate, metallicity, and stellar population age by fitting our data with the spectral-energy-distribution fitting code BAGPIPES. We re-identify XRT191223 as a Galactic stellar flare. Conclusions . For several FXTs, there are multiple candidate host galaxies, which complicates deriving constraints on the origin of the FXT. Assuming association with (one of) those candidates, all are consistent with a (non-)relativistic white dwarf–intermediate mass black hole tidal disruption event (WD-IMBH TDE) and a binary neutron star (BNS) merger. Two are consistent with a supernova shock breakout and only EP240708a with cocoon emission from a long γ -ray burst. We also discuss the possibility that the host galaxies remain undetected in our observations. We conclude that FXTs detected by Chandra and XMM- Newton are likely to arise from a variety of origins, and we discuss the fact that part of this population differs from FXTs detected by the Einstein Probe, many of which appear consistent with a collapsar scenario.

The One-hundred-deg ² DECam Imaging in Narrowbands (ODIN) survey is conducting the widest-field deep narrowband (NB) imaging of the equatorial and southern skies. ODIN uses three custom-built NB filters that sample Ly α -emitting galaxies (LAEs) within thin cosmic slices centered at z = 2.4, 3.1, and 4.5. In this work, we utilize extensive DESI spectroscopy of ODIN-selected galaxies in the COSMOS and XMM-LSS fields to validate our LAE selection. Exposures of 2–4 hr with DESI yielded redshift confirmation of 3075 ODIN LAE candidates with NB magnitudes brighter than 26 mag. Restricting to objects that yield high-confidence redshifts, the confirmation rates are (93%, 96%, 92%) at z = (2.4, 3.1, 4.5). The primary contaminants consist of active galactic nuclei at the expected Ly α- redshift range and lower redshifts (C iv , C iii ]), with the remainder being star-forming galaxies ([O ii ] and [O iii ]). We find minimal contamination from [O ii ] emitters in our sample (≲1%), implying that our rest-frame equivalent width (REW) > 20 Å NB excess photometry requirement is sufficient to remove them.

The recent candidate gravitational-wave (GW) alert from a compact object merger involving at least one subsolar mass (SSM) object has prompted questions about their origins. S251112cm is reported by LIGO/Virgo with a false-alarm rate of 1 per 6.2 yr, nearby luminosity distance of 93 ± 27 Mpc, and probability of containing an SSM object of 100%. Such a system, if astrophysical, likely did not involve the supersolar neutron stars or black holes invoked to explain kilonovae. One must then also invoke hitherto unobserved and speculative models to produce SSM mergers that may have electromagnetic (EM) counterparts. We introduce a framework that vets and scores candidate counterparts to SSM GW events to inform follow-up in search of any among the zoo of potential EM transients: kilonovae, kilonovae-within-supernovae, superkilonovae, or active galactic nucleus (AGN) flares from binary black hole mergers. We use a suite of telescopes to perform tiling, galaxy-targeted observations, and photometric/spectroscopic follow-up of promising candidates. In near-real time, we ingest candidates reported by the community, including some of the first observations reported by the Vera C. Rubin Observatory. We vet and score a total of 456 candidates, including 67 from Rubin, but find no likely counterpart. We nonetheless highlight candidates that demonstrate the ability of our framework to distinguish between different transient types and describe strategies to maximize the chances of detecting a counterpart to the next SSM event. Our framework will be implemented in the forthcoming Multimessenger Tool for Rapid Object Vetting and Examination.

We present high-resolution (beam size 0 . ″ 076 × 0 . ″ 040 ) Atacama Large Millimeter/submillimeter Array observations of the far-infrared ( λ _rest = 162.7 μ m) dust continuum of J0439+1634, a gravitationally lensed quasar at z = 6.52. We perform pixelated lens modeling for the visibility data, finding that J0439+1634 is well-described by a singular isothermal ellipsoid plus an external shear lensing model. The best-fit lensing potential exhibits a naked-cusp configuration, confirming the finding in X. Fan et al. The reconstructed source plane continuum emission shows a compact bright core, with size ≲200 pc and peak brightness ∼0.6 Jy arcsec ⁻² . The total continuum flux at 245 GHz is 3.36 ± 0.02 mJy. The flux magnification is 4.63 ± 0.03, indicating an average source-plane resolution of 0 . ″ 019 (equivalent to 104 pc). The spatial resolution around the supermassive black hole reaches ∼36 pc. Leveraging the exceptional source-plane resolution, we build a radiative transfer model to describe the observed dust emission profile. The best-fit model indicates that heated dust from the active galactic nucleus (AGN) dominates the submillimeter emission at r ≲ 100 pc and that star-heated dust dominates the outer region of the host galaxy. AGN heating contributes ∼13% to the observed submillimeter flux. Therefore, previous far-infrared-based star formation rate measurements for most high-redshift quasars are likely mildly overestimated.

We investigated Lyman-continuum (LyC) emission from Lyman-alpha emitters (LAEs) at z = 4.5, identified in the One-hundred-deg ² DECam Imaging in Narrowbands (ODIN) survey. Of the 7498 LAEs (4101 in COSMOS and 3397 in XMM-LSS), we excluded LAEs that are either likely low-z objects or contaminated by neighboring sources. Additional background modeling process with thorough quality assessments leaves a final sample of 851 galaxies. We then performed forced photometry on u / u *-band images from the CFHT large area u -band deep survey to measure their LyC fluxes. This represents the largest sample of z = 4.5 LAEs searched for such a purpose. Within this sample, we identified 12 “gold” and 39 “silver” LyC-emitting candidates, with LyC fluxes detected of >3 σ and between 2 σ and 3 σ , respectively, in the range of 5.16–55.29 nJy. No LyC signal is detected in the weighted mean stack of the final sample (0.20 ± 0.37 nJy). Given the UVC magnitudes of LAEs in our sample, the expected LyC emission is likely below the detection limit even when stacking the full sample of ODIN LAEs. Nevertheless, having a large sample of LAEs remains valuable for identifying individual LyC leaker candidates. Among the gold and silver candidates, the LyC flux appears to correlate positively with UVC flux and negatively with Ly α equivalent width, although the correlations are weak. A larger sample of LyC leakers will allow a more robust confirmation of these trends and provide better insights into their physical origins.

Multiwavelength analyses of astrophysical transients are essential for understanding the physics of these events. To make such analyses more efficient and effective, we present the Open mulTiwavelength Transient Event Repository (OTTER), a publicly available catalog of published transient event metadata and photometry. Unlike previous efforts, our data schema is optimized for the storage of multiwavelength photometric datasets spanning the entire electromagnetic spectrum from multiple published sources. Open-source software, including an application programming interface (API) and web application, are available for viewing, accessing, and analyzing the dataset. For the initial release of OTTER, we present the largest ever photometric archive of tidal disruption event (TDE) candidates, including ≳118,000 observations of 240 TDE candidates spanning from radio to X-ray wavelengths. We demonstrate the power of this infrastructure through four example analyses of the TDE population. We plan to maintain this dataset as more TDE candidates are proposed in the future and encourage other users to contribute by uploading newly published data via our web application. The infrastructure was built with the goal of archiving additional transient data (supernovae, gamma-ray bursts, fast blue optical transients, fast radio bursts, etc.) in the future. The web application is available at https://otter.idies.jhu.edu and the API documentation is available at https://astro-otter.readthedocs.io .

Ly α blobs (LABs) are large, spatially extended Ly α -emitting objects whose nature remains unclear. Their statistical properties, such as number densities and luminosity functions, are still uncertain because of small sample sizes and large cosmic variance. The One-hundred-deg ² DECam Imaging in Narrowbands (ODIN) survey, with its large volume, offers an opportunity to overcome these limitations. We describe our LAB selection method and present 112 new LABs in the 9 deg ² E-COSMOS field. We begin with the conventional LAB selection approach, crossmatching Ly α -emitting galaxies with extended Ly α sources, yielding 89 LAB candidates. To obtain a more complete LAB sample, we introduce a new selection pipeline that models all galaxies detected in deep broadband imaging, subtracts them from the narrowband image, and then directly detects extended Ly α emission. This method successfully identifies 23 additional low-SB LABs, which could otherwise be missed by the conventional method. The number density of ODIN LABs near an ODIN protocluster ( n = 7.5 × 10 ⁻⁵ cMpc ⁻³ ) is comparable to that found in the SSA22 protocluster and is 4 times higher than the average across the field. The cumulative Ly α luminosity function within the protocluster regions is similar to that measured for the LABs in the SSA22 protocluster, suggesting a large excess of luminous LABs relative to the average field. These findings suggest the Ly α luminosities and number densities of LABs are environment-dependent. ODIN will provide an expansive LAB and protocluster samples across six additional fields and two more redshifts, allowing us to investigate the nature of LABs in relation to their environments.

Aims. We investigate if systems of multiple Lyman-alpha emitters (LAEs) can serve as a proxy for dark matter halo mass, assess how their radiative properties relate to the underlying halo conditions, and explore the physics of star formation activity in LAEs and its relation to possible physically related companions. Methods. We used data from the One-hundred-deg ² DECam Imaging in Narrowbands (ODIN) survey, which targets LAEs in three narrow redshift slices. We identified physically associated LAE multiples in the COSMOS field at z = 2.4, z = 3.1, and z = 4.5, and we used a mock catalog from the IllustrisTNG100 simulation to assess the completeness and contamination affecting the resulting sample of LAE multiples. We then studied their statistical and radiative properties as a function of multiplicity, for which we adopted the term “multiplicity” to refer to the number of physically associated LAEs. Results. We find a strong correlation between LAE multiplicity and host halo mass in the mocks, with higher multiplicity systems preferentially occupying more massive halos. In both the ODIN and the mock sample, we find indications that the mean Ly α luminosity and UV magnitude of LAEs in multiples increase with multiplicity. The halo-wide LAE surface brightness densities in Ly α and UV increase with multiplicity, reflecting more compact and actively star-forming environments. The close agreement between the model and ODIN-COSMOS observations supports the validity of the Ly α emission model in capturing key physical processes in LAE environments. Finally, a subhalo-based perturbation-induced star formation model reproduces the minimum subhalo mass distribution in simulations at z = 2.4, suggesting that local perturbations–rather than the presence of LAE companions–drive star formation activity in these systems. For the higher redshift samples, neighbor perturbations do not seem to be the main driver that triggers star formation.

Using Hubble Space Telescope/Space Telescope Imaging Spectrograph observations, we present the highest-spatial-resolution spectroscopic study to date of four tidal disruption event (TDE) host galaxies, with the best observed being the post-starburst host of ASASSN-14li. The stellar population of ASASSN-14li’s host, within 44 pc of the nucleus, reveals a younger recent starburst (∼340 Myr) compared to the population at an offset radius of 88 pc that excludes the nucleus (∼550 Myr), a radial age gradient suggesting gas inflows from a minor merger. We estimate a stellar density of ∼5900 ± 800 M _⊙ pc ⁻³ within 30 pc of the nucleus of ASASSN-14li’s host, exceeding densities expected for nuclear star clusters. High-ionization “coronal” emission lines, [Fe vi ] λ 5677, [Fe vii ] λ 6087, and [Fe x ] λ 6375, are also detected within the nuclear spectra of the hosts of ASASSN-14li and PTF09ge, importantly alongside the nondetection of [O iii ] at the same scale. We similarly do not detect [O iii ] in the nuclear region of ASASSN-14ae’s host despite its presence in the Sloan Digital Sky Survey spectrum. The different ionization radiation levels detected at various radii from TDE host nuclei may indicate echoes of earlier accretion episodes, including, potentially, a prior TDE. We posit that a minor merger driving gas inflow to the nucleus could drive the enhanced TDE rates in post-starburst galaxies, inducing variation in nuclear gas properties and star formation history on <150 pc scales in TDE hosts.