Daniel J. Farrow

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.

Small-scale interactions such as minor mergers, flybys and harassment are considered common driving mechanisms that can both initiate and quench star-forming processes within post-starburst (PSB) galaxies. PSBs have been linked to disturbed morphologies and faint tidal features in numerous works. We aim to independently study these features in E+A galaxies found in the Galaxy and Mass Assembly Fourth Data Release (GAMA DR4). Using image stacking techniques, we derive the averaged radial surface brightness profiles of E+A image stacks alongside the Sérsic parameters that best fit these profiles. These E+A stacks consist of 57 images extracted from the Sloan Digital Sky Survey (SDSS) in g, r & i bands. The E+A stacks are shown to have brighter extended regions than similar non-PSB galaxies, which is indicative of faint tidal features that could be caused by small-scale interactions. Additionally, we make use of Dark Energy Spectroscopic Instrument (DESI) Legacy Survey imaging, which is deeper than SDSS, to further support our conclusions and find that the Legacy Survey images independently strengthen the evidence for excess light at extended radii relative to the comparison samples. We discuss these findings in the context of future surveys such as the Vera C. Rubin Legacy Survey of Space and Time (LSST) survey.

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 present a deep learning framework to enhance the identification of Ly α emitters (LAEs) in the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX), an untargeted spectroscopic survey of LAEs at 1.9 < z < 3.5 without imaging preselection. We primarily address the low signal-to-noise ratio (S/N) regime (4.8 ≤ S/N ≤ 5.5), where LAE candidates suffer from substantial noise contamination. To distinguish LAE candidates from artifacts and sky residuals, we employ a convolutional neural network (CNN) trained on two-dimensional spectral images of single emission lines. The training sample is constructed from the HETDEX COSMOS catalog, with external validation from ancillary observations and our participatory science project, Dark Energy Explorers. For small-format, low-resolution spectroscopic data, the model achieves a balanced accuracy, precision, and recall of 94.1%, 97.5%, and 97.5%, respectively, in the high-S/N regime (S/N > 5.5), and 85.1%, 78.2%, and 84.4% in the low-S/N regime. Using HETDEX LAEs independently identified by Dark Energy Spectroscopic Instrument (DESI) spectroscopy, the model recovers 99% and 93% of the high- and low-S/N LAEs, respectively. Visual attribution indicates that the CNN attends to smooth, spatially extended central emission in true positives and to irregular or noisy features in true negatives. Applied to the full HETDEX catalog, the CNN enables an S/N threshold down to 4.8 by suppressing spurious spikes across z ∼ 1.9–2.5 in the redshift distribution. Our approach facilitates HETDEX cosmological analyses by mitigating false positives in galaxy clustering and highlights the value of domain-specific deep learning for refining low-S/N spectroscopic identification in untargeted surveys.

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.

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.