Gary J. Hill

We present a spectroscopic database and analysis of satellite streaks in Public Data Release 1 (PDR1) from the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX), covering 6778 observations and ∼90 deg ² of sky from 2017 January to 2024 July. We detect 527 streaks across 492 observations, a contamination fraction of 7.3% that rises to monthly values above 20% by 2023–2024, although the streaks mask only 0.09% of the survey area. Of the 527 streaks, 475 (90.1%) are identified with a cataloged object using archival two-line elements: 258 (54%) lie in low or medium Earth orbit, and 217 (46%) are geostationary or highly elliptical. Only 35% were launched since 2010; the rest are spent rocket bodies, debris, and older spacecraft. This reflects HETDEX’s sensitivity to faint objects and its 2017–2024 observing baseline. The Hobby–Eberly Telescope’s large 10 m aperture and HETDEX’s ∼6 minute exposures result in similar observed magnitudes for low Earth orbit (LEO) and geostationary Earth orbit (GEO) objects (median g _AB = 16.2 ± 1.1 for LEO, 16.7 ± 1.1 for GEO). However, once transit time is corrected for, instantaneous magnitudes differ widely: g _inst = 7.4 for LEO versus g _inst = 13.0 for GEO objects, a difference of 5.6 mag, while the exposure-averaged magnitudes differ by only 0.5 mag. Normalized to 550 km, the 34 Starlink streaks give g (550 km) = 5.95, consistent with the mean V (550 km) = 5.92 ± 0.04 reported for VisorSat-design spacecraft. The spectra are consistent with reflected sunlight, with modest, signal-to-noise-driven differences in slope. The catalog, spectra, and code are archived on Zenodo (doi: 10.5281/zenodo.22962479 ).

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.

Context . Ly α emitters (LAEs) are galaxies with strong Ly α emission, tracing early star formation and ionizing radiation. Their connection to active galactic nuclei (AGNs) is key to understanding the mechanisms behind (extended) Ly α emission. Aims . For this study, we measured the fraction of LAEs identified as radio-emitting AGNs ( f _AGN,radio ) and the fraction of radio sources that exhibit Ly α emission ( f _{Ly α} ) to investigate the connection of radio AGN activity to Ly α emission at 1.88 < z < 3.52. Methods . We identified 928 sources that were detected in both the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) and the LOw Frequency ARray (LOFAR) surveys. These matches were drawn from 55 109 spectroscopically confirmed LAEs and 27 625 radio sources. Results . After applying completeness corrections, we obtain f _AGN,radio = 1.77 ± 0.04% and f _{Ly α} = 18.15 ± 0.14% (for Ly α fluxes above 3×10 ⁻¹⁷ erg s ⁻¹ cm ⁻² and radio flux densities above 0.1 mJy at 144 MHz). The fraction f _AGN,radio increases from 0.4 ± 0.1% at log ₁₀ L _{Lyα,ergs ⁻¹} ≈ 42.6 to 9.7 ± 1.3% at log ₁₀ L _{Lyα,ergs ⁻¹} ≈ 44.5. The fraction f _Lyα rises from 0.7 ± 0.1% at log ₁₀ L _{150,WHz ⁻¹} ≈ 24.6 to 55.8 ± 14.5% for log ₁₀ L _{150,WHz ⁻¹} ≈ 28.3. ‘LAEs with radio AGNs’ and ‘optical AGNs with Ly α emission’ exhibit similar radio luminosity distributions above the AGN threshold ( L _150MHz > 10 ^23.9 W Hz ⁻¹ ), though optical AGNs show systematically higher Ly α luminosities. We find no statistically significant correlations of Ly α luminosity with both radio luminosity and a low-frequency spectral index, suggesting that Ly α emission is regulated by gas properties rather than jet power alone. We also find a positive correlation between the full width at half maximum (FWHM) and luminosity of the Ly α line, suggesting that more luminous sources have broader lines, potentially due to enhanced outflows or velocity dispersions. However, we find no correlation between FWHM and radio size, indicating minimal direct jet-gas interaction in these systems. Conclusions . Our results show that most Ly α emission at 1.88 < z < 3.52 is powered by star formation, with radio AGN activity confined to a small luminous subset (1.77 ± 0.04%). While luminous systems preferentially host radio AGNs, the lack of correlation between Ly α and radio luminosities in individual sources indicates that Ly α emission is governed primarily by host galaxy gas properties rather than direct AGN jet coupling.

OQ 208 is a nearby, partially obscured quasar ( z = 0.077) that is a young, bright, parsec-scale radio source. We assemble archival and new high-frequency Very Long Baseline Array (VLBA) and Very Large Array (VLA) observations and optical spectra to form a dataset spanning 39 yr. Radio light curves covering 58 yr were also compiled. We utilize new spectrophotometry to calibrate previous spectroscopy using forbidden narrow lines that are expected to be stable on much longer timescales. VLBA and VLA observations of a light-year-scale bright nuclear flare at 15.4 and 22 GHz reveal a rise (fade) beginning in mid-1996 (early 2000). Quasi-contemporaneously, from 1997 February 7 to 2000 June 3, the H α broad-line equivalent widths (EWs) and fluxes dropped dramatically. In the context of the tendency of radio-loud quasars to have a depressed extreme-ultraviolet (EUV) continuum (the main source of ionizing flux for H α ) relative to radio-quiet quasars at matched UV luminosity (the EUV deficit of radio-loud quasars), this may not be a coincidence. Analytic models previously developed to explain the relationship between jet power and the EUV deficit are consistent with (but not direct observational proof of) the small EWs being a consequence of transient magnetically arrested accretion states from ∼1997 to 2001. The 22 GHz VLBA nucleus gradually fades; in 2023 the flux density was <5% of its value in 2000. The environs of the nucleus also fade at 22 GHz, but in a time-delayed fashion.

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 ).

We present the first statistical census of emission-line variable active galactic nuclei (EVA) at cosmic noon by combining untargeted and deep Hobby–Eberly Telescope Dark Energy Experiment (HETDEX) spectroscopy with multiepoch spectra from the Sloan Digital Sky Survey, DESI, and LAMOST. Anchoring all candidates to a HETDEX spectroscopic epoch and requiring an active galactic nucleus (AGN) classification in either the HETDEX or external epoch(s), we identify a homogeneous sample of 100 EVA at z ∼ 1.5, including 98 that are newly identified. Emission-line variability is selected primarily through statistically significant line-flux changes, supplemented by extensive visual inspections using contemporaneous photometric light curves. The resulting incidence fraction is f _EVA ∼ 0.9%. The rest-frame intervals between spectroscopic epochs span ∼1–10 yr, with brightening and dimming events exhibiting statistically indistinguishable characteristic timescales (Δ T ∼ 2.2 and ∼2.6 yr, respectively). A key result is the characterization of the Baldwin effect in the time domain: while many EVA follow the ensemble Baldwin effect (eBeff) between two epochs, a substantial fraction exhibit apparent anti-eBeff responses. Time-resolved spectroscopy of an individual source reveals that the intrinsic equivalent width–luminosity relation is nonstationary, with the line-to-continuum responsivity systematically evolving from stronger to weaker across successive variability cycles; sparse two-epoch sampling of this evolving intrinsic Baldwin evolution naturally produces both eBeff-like and anti-eBeff behaviors. Finally, EVA show no strong preference for extreme Eddington ratios but exhibit a mild tendency toward lower λ _Edd values relative to matched control samples, driven primarily by sources observed in their dim states. Together, these results establish a coherent framework for interpreting emission-line variability in AGN at the peak epoch of cosmic black hole growth.

The Hobby–Eberly Dark Energy Experiment (HETDEX) is an untargeted ∼540 deg ² spectroscopic survey of Ly α emission in the 1.9 < z < 3.5 Universe. In surface brightness, this survey reaches 1 σ Ly α sensitivities of ∼2–5 × 10 ⁻¹⁸ erg s ⁻¹ cm − 2 arcsec − 2 , allowing large samples of extended Ly α nebulae (LAN) to be studied. We selected a sample of 70,691 Ly α -emitting galaxies (LAEs) with an emission-line signal-to-noise ratio greater than 6 and modeled the Ly α emission as a point-source component with an optional exponential envelope. Half (∼47.5%) of the LAE sample (33,612 objects) exhibits significant extended emission and is best fit by the two-component model. The fraction of resolved sources increases with Ly α flux and luminosity. Their isophotal areas range from 10 to 130 arcsec ² (median 15 arcsec ² ), with integrated Ly α fluxes from 6 to 2000 × 10 ⁻¹⁷ erg s ⁻¹ cm ⁻² (median 20 × 10 ⁻¹⁷ erg s ⁻¹ cm ⁻² ). Comparison between point-spread function-weighted and isophotal flux measurements shows that the HETDEX pipeline underestimates the total Ly α flux by ∼30% on average, reflecting the substantial halo contribution in extended sources. Approximately 420 LANs are found per deg ² over 79.5 deg ² of noncontiguous sky. About 12% of resolved sources show active galactic nuclei signatures and are bright in Ly α and continuum. The remaining 88% span a wide range of morphologies and often lack continuum counterparts. Exponential scale lengths show no strong correlation with Ly α flux or luminosity (median 11.6 ± 1.9 kpc). Only 2.9% of the full S/N > 6 LAE population with ancillary data have radio counterparts, but 64% of those are found to be extended, with the radio fraction increasing with Ly α size. We present a catalog of all modeled sources, with their positions, redshifts, luminosities, and structural parameters for over 70,000 LAEs consisting of 33,000 spatially extended LAN. The catalog can be found at https://hetdex.org/data-results/ and in the online version of this paper.

We present a measurement of the Ly α intensity mapping power spectrum from the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). We measure the cross-power spectrum of the Ly α intensity and Ly α -emitting galaxies (LAEs) in a redshift range of 1.9 ≤ z ≤ 3.5. We calculate the intensity from HETDEX spectra that do not contain any detected LAEs above a signal-to-noise ratio of 5.5. To produce a power spectrum model and its covariance matrix, we simulate the data using lognormal mocks for the LAE catalog and Ly α intensity in redshift space. The simulations include the HETDEX sensitivity, selection function, and mask. The measurements yield the product of the LAE bias, the intensity bias, the mean intensity of undetected sources, and the ratio of the actual and fiducial redshift-space distortion parameters, b g b I 〈 I 〉 F ¯ RSD / F ¯ RSD fid = (6.7 ± 3.1), (11.7 ± 1.4), and (8.3 ± 1.5) × 10 ⁻²² erg s ⁻¹ cm ⁻² arcsec ⁻² Å ⁻¹ in three redshift bins centered at z ¯ = 2.1 , 2.6, and 3.2, respectively. The results are reasonably consistent with cosmological hydrodynamical simulations that include Ly α radiative transfer. They are, however, significantly smaller than previous results from cross-correlations of quasars with Ly α intensity. These results demonstrate the statistical power of HETDEX for Ly α intensity mapping and pave the way for a more comprehensive analysis. They will also be useful for constraining models of Ly α emission from galaxies used in modern cosmological simulations of galaxy formation and evolution.

The Hobby–Eberly Dark Energy Experiment (HETDEX) is an untargeted spectroscopic galaxy survey that uses Ly α- emitting galaxies (LAEs) as tracers of 1.9 < z < 3.5 large-scale structure. Most detections consist of a single emission line, whose identity is inferred via a Bayesian analysis of ancillary data. To determine the accuracy of these line identifications, HETDEX detections were observed with the Dark Energy Spectroscopic Instrument (DESI). In two DESI pointings, high-confidence spectroscopic redshifts are obtained for 1157 sources, including 982 LAEs. The DESI spectra are used to evaluate the accuracy of the HETDEX object classifications and tune the methodology to achieve the HETDEX science requirement of ≲2% contamination of the LAE sample by low-redshift emission-line galaxies, while still assigning 96% of the true Ly α emission sample with the correct spectroscopic redshift. We compare emission-line measurements between the two experiments assuming a simple Gaussian line fitting model. Fitted values for the central wavelength of the emission line, the measured line flux, and line widths are consistent between the surveys within uncertainties. Derived spectroscopic redshifts, from the two classification pipelines, when both agree as an LAE classification, are consistent to within 〈Δ z /(1 + z )〉 = 6.9 × 10 ⁻⁵ with an rms scatter of 3.3 × 10 ⁻⁴ . Data are available at https://data.desi.lbl.gov/desi/public/dr1/vac/dr1/hetdex .

We explore the neutral hydrogen (H i) gas around 1.9 < z < 3.5 Lyman alpha emitters (LAEs) from the Hobby–Eberly Telescope Dark Energy Experiment using faint Lyα absorption. This absorption is the result of H i in the halo of the LAE scattering Lyα photons from the integrated light of background galaxies along the line of sight. We stack millions of spectra from regions around ∼88,000 LAEs, in order to focus on the physics of the gas at large radii. The extensive number of fiber spectra contributing to the stacks ensures a sufficient signal-to-noise ratio to detect the faint Lyα absorption, which would otherwise be buried within the noise. We detect absorption out to a projected ∼350 kpc around an average LAE at z ∼ 2.5. We use these results to create an empirical radial W _λ (Lyα) profile around LAEs. Comparison with numerical simulations reveals a profile similar to the empirical one within this region. Compared to previous studies, the profile is similar but modestly higher. We also outline a simple physical picture motivated by the observed trends in the data. We plan to quantify this radial profile as a function of redshift, local density, and Lyα luminosity, to explore the relationship between LAE environments and H i distribution.