Robin Ciardullo

Post-asymptotic giant branch (PAGB) stars share many advantages with the tip of the red giant branch (TRGB) for Population II distance measurements, including minimal internal reddening, easy identification, a solid theoretical foundation, and an expected weak metallicity dependence. At visual wavelengths, PAGB stars are the brightest members in old populations, appearing up to ∼2 mag brighter than the TRGB and ∼4 mag brighter than RR Lyrae stars. Stellar evolution theory predicts that these objects occupy a narrow luminosity strip across the top of the Hertzsprung–Russell (HR) diagram, making them excellent standard candles. Observations in Galactic globular clusters (GCs) support this prediction, but a comprehensive survey is required to define their luminosity function and test for systematic errors. In this work, we construct a multiwavelength catalog, spanning the ultraviolet (UV) to infrared (IR), for PAGB stars in 39 Galactic GCs. We validate cluster membership with Gaia astrometry, identify 21 objects across 16 GCs using ground-based uBVI and space-based UV photometry, and perform Bayesian spectral energy distribution (SED) fitting with stellar atmosphere models to estimate their effective temperatures, luminosities, and radii. We find that PAGB stars occupy a narrow luminosity range centered at log ( L bol / L ⊙ ) = 3.25 ± 0.03 , with dispersion σ = 0.12 dex, supporting their use as reliable Population II distance indicators. Comparisons with theoretical tracks show excellent agreement with evolutionary models. We confirm that PAGB stars occur preferentially in metal-poor clusters with blue horizontal branches. Our results demonstrate that PAGB stars provide a powerful probe of late stellar evolution and a promising Population II standard candle.

Population III (Pop III) stars are expected to produce extremely hard ionizing spectra, yet direct evidence for their signature—strong narrow He ii λ 1640 emission with weak or absent metal lines—remains elusive. Although Pop III formation peaks at z ≳ 10, models predict that nearly pristine gas pockets can survive to later times, making intermediate-redshift searches a probe of metal mixing across cosmic time. We search 109,545 high-confidence Ly α -emitting galaxies at 1.9 < z < 2.3 in the Hobby–Eberly Telescope Dark Energy Experiment’s database for systems with strong He ii λ 1640 and no detected UV metal lines, recovering eight candidates and a first-order, tentative comoving number density of ∼30 Gpc ⁻³ . The eight-object stack yields He ii equivalent width (EW) EW _rest = 28.7 ± 6.7 Å, He ii /Ly α = 0.354 ± 0.094, and a resolution-corrected He ii FWHM = 449 ± 105 km s ⁻¹ . We find no significant N v , C iv , or O iii ] emission; using the He ii width to set the scale, we derive 3 σ upper limits of N v /He ii < 0.167, C iv /He ii < 0.128, and O iii ]/He ii < 0.140. This combination of strong He ii , elevated He ii /Ly α , moderate line width, and weak metal lines is difficult to reproduce with metal-enriched stellar populations, active galactic nucleus (AGN) narrow-line regions, shocks, or classical Wolf–Rayet features. We interpret the sample as candidates for very metal-poor or Pop III–like ionizing sources at z ∼ 2. Definitive confirmation requires deeper rest-frame UV spectroscopy to measure He ii and metal-line limits in individual objects, followed by rest-frame optical spectroscopy of H β , [O iii ] λλ 4959, 5007, [O ii ] λ 3727, and H α to constrain gas-phase metallicity, ionization conditions, and AGN activity.

We analyze the rest-frame optical (∼8000 Å) morphologies and star formation activity of Ly α emitters (LAEs) at redshifts 2.4, 3.1, and 4.5, identified in the One-hundred-deg ² DECam Imaging in Narrowband (ODIN) survey. To compare their physical properties with those of other galaxies, we construct a comparison sample of typical star-forming galaxies (SFGs) at similar redshifts from the COSMOS2025 catalog. Using James Webb Space Telescope NIRCam images from the COSMOS-Web survey, we measure the rest-frame optical sizes and Sérsic indices. We first examine their size–mass relations and find that LAEs at all three redshifts have smaller sizes than typical SFGs, with the size difference decreasing at higher redshifts. We also find that LAEs tend to have larger Sérsic indices at z = 2.4 and 3.1 than typical SFGs, but the difference becomes weaker at z = 4.5. These trends are qualitatively reproduced in the Horizon Run 5 cosmological hydrodynamical simulation. We then investigate star formation activity and find that LAEs exhibit higher star formation rates (SFRs) than typical SFGs at all redshifts considered. Finally, we examine the connection between Ly α emission and galaxy structure, finding that the rest-frame equivalent width (REW) of the Ly α emission line is negatively and positively correlated with size and Sérsic index, respectively. In addition, we find a strong positive correlation between Ly α REW and the ratio of the instantaneous SFR to that averaged over the last 100 Myr (i.e., SFR _inst /SFR _100Myr ). These results suggest the compact and starbursting nature of LAEs, and provide important constraints on the physical mechanisms responsible for Ly α photon escape from galaxies.

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.

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.

Spatially extended Ly α nebulae, known as Ly α blobs (LABs), are a rare population at z > 2 that are thought to trace protogroups or the progenitors of massive galaxies in the present-day Universe. However, their dark matter halo properties (e.g., halo mass) are still uncertain, due to their rarity and strong field-to-field variation. The One-hundred-deg ² DECam Imaging in Narrowbands (ODIN) survey has discovered 103 and 112 LABs in the extended (∼9 deg ² ) COSMOS field at z ∼ 2.4 and 3.1, respectively, enabling the estimation of their bias and host halo masses through clustering analysis. We measure the angular autocorrelation functions (ACFs) of the LABs and derive galaxy bias factors of b = 4.0 ± 0.8 and 3.8 ± 0.7, corresponding to minimum halo masses of 2 . 8 − 1.8 + 3.0 and 0 . 7 − 0.5 + 0.8 × 1 0 12 M _⊙ and median halo masses of 4 . 2 − 2.5 + 3.8 and 1 . 1 − 0.7 + 1.1 × 1 0 12 M _⊙ at z ∼ 2.4 and 3.1, respectively. LABs occupy ∼11 − 8 + 39 % and ∼3 − 2 + 9 % of all dark matter halos above these minimum halo masses. These findings suggest that LABs inhabit massive dark matter halos, likely tracing protogroup environments that evolve into present-day massive halos (∼10 ¹³ M _⊙ ), where massive elliptical galaxies or galaxy groups reside, by z = 0.

The Hobby–Eberly Telescope Dark Energy Experiment (HETDEX) is a wide-field, integral-field spectroscopic survey designed to map the large-scale distribution of Ly α -emitting galaxies (LAEs) at 1.88 < z < 3.52 and constrain dark energy at cosmic noon. Using the 10 m Hobby–Eberly Telescope and the Visible Integral-Field Replicable Unit (IFU) Spectrograph, HETDEX obtains >35,000 spectra per exposure over 3500–5500 Å at R ∼ 800 with ∼ 1 . ″ 8 image quality, enabling an untargeted census of emission-line galaxies across 540 deg ² . We present HETDEX Public Data Release 1 (PDR1), comprising 431,713 IFU observations covering 86.67 deg ² of noncontiguous sky in the Spring (13 ^h , +51°) and Fall (1 . h 5, 0°) fields, along with legacy regions (Cosmic Evolution Survey, Great Observations Origins Deep Survey North, North Ecliptic Pole, SA22). PDR1 includes the HETDEX Public Source Catalog 2 (HPSC2), an expanded and reprocessed version of E. Mentuch Cooper et al. (2023) incorporating four additional years of data, improved quality control, and new machine learning classifiers. HPSC2 contains 426,654 LAEs, 491,411 [O II ] emitters, 19,457 low- z galaxies, 18,303 active galactic nuclei, and 150,608 stars, providing coordinates, redshifts or stellar velocities, and 1D spectra for each source. Because the data cubes use local sky subtraction optimized for faint emission-line detection, they are not suited for absolute surface-brightness measurements or very extended nearby galaxies. Appendix materials include the full detection catalog, the 1.6 million–candidate LAE sample, and raw detection databases. All products are publicly accessible through the HETDEX data portal ( https://hetdex.org/data-results/ ), including access to a public JupyterLab . HPSC2 is also publicly available via Zenodo (doi: 10.5281/zenodo.19581262 ).

Gravitational waves (GWs) from compact binary coalescences are standard sirens that provide a direct measure of the source’s luminosity distance, enabling an independent measurement of the Hubble constant ( H ₀ ). While a bright siren —a GW event with an identified electromagnetic (EM) counterpart—provided the first such constraint, most detections, currently dominated by black hole mergers, lack EM signatures. A measurement of H ₀ is still possible with these dark sirens by statistically associating GW events with galaxies in existing catalogs based on the sky localization. In this work, we explore the potential of two subsets of dark sirens categorized by their localization precision: ’golden’ dark sirens, defined by a sky area localization ≤0.1deg ² , and ’silver’ dark sirens, which are more common but less precisely localized (≤1deg ² ). Using the fifth internal data release of the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX), we assess the suitability of the Visible Integral-field Replicable Unit Spectrograph (VIRUS) for spectroscopic follow-up of dark sirens. VIRUS exposures of the standard HETDEX depth provide precise redshifts and exquisite completeness within z = 0.2. After a single year of observations with the upgraded LIGO-A ^# network, the combined sample of golden and silver dark sirens with z ≤ 0.2 at H ₀ = 70 km s ⁻¹ Mpc ⁻¹ and follow-up VIRUS observations can potentially yield a few-percent constraint on H ₀ . Our predictions suggest that spectroscopic redshift surveys such as HETDEX can play a key role in realizing high-precision cosmology with dark sirens in the near future.

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.

We investigate extended Ly α emission using integral-field unit (IFU) data from the Hobby–Eberly Telescope Dark Energy Experiment (or HETDEX). We apply the modified Shepard method to image 106,853 Ly α emitters (LAEs) and median-stack them as functions of rest-frame Ly α equivalent width (EW), Ly α line flux, and redshift to obtain accurate flux measurements for the faintest Ly α detections. After careful consideration of point-spread function, centroiding, IFU correlation, and signal-to-noise ratio errors, we calculate that the one-dimensional exponential scale lengths of our LAEs range from ∼3.0 to 7.1 kpc. The scale lengths show a slight negative correlation with the Ly α EW, with lower-EW sources having higher scale lengths. We also detect positive correlations between the scale length of the sample and its Ly α flux and luminosity within flux- and luminosity-binned subsamples. Overall, we find evidence for a decrease in Ly α halo size with increasing redshift. We discuss the mechanisms that could be powering these halos and driving their complex evolution.