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1. The Active Galactic Nuclei in the Hobby–Eberly Telescope Dark Energy Experiment Survey (HETDEX). I. Sample Selection

   https://doi.org/10.3847/1538-4365/ac6ba6  

   Liu, Chenxu; Gebhardt, Karl; Cooper, Erin Mentuch; Davis, Dustin; Schneider, Donald P.; Ciardullo, Robin; Farrow, Daniel J.; Finkelstein, Steven L.; Gronwall, Caryl; Guo, Yuchen; et al (July 2022, The Astrophysical Journal Supplement Series)

   Abstract We present the first active galactic nuclei (AGN) catalog of the Hobby–Eberly Telescope Dark Energy Experiment Survey (HETDEX) observed between 2017 January and 2020 June. HETDEX is an ongoing spectroscopic survey (3500–5500 Å) with no target preselection based on magnitudes, colors or morphologies, enabling us to select AGN based solely on their spectral features. Both luminous quasars and low-luminosity Seyferts are found in our catalog. AGN candidates are selected with at least two significant AGN emission lines, such as the Lyαand Civλ1549 line pair, or with a single broad emission line with FWHM > 1000 km s⁻¹. Each source is further confirmed by visual inspections. This catalog contains 5322 AGN, covering an effective sky coverage of 30.61 deg². A total of 3733 of these AGN have secure redshifts, and we provide redshift estimates for the remaining 1589 single broad-line AGN with no crossmatched spectral redshifts from the Sloan Digital Sky Survey Data Release 14 of QSOs. The redshift range of the AGN catalog is 0.25 <z< 4.32, with a median ofz= 2.1. The bolometric luminosity range is 10⁹–10¹⁴L_☉with a median of 10¹²L_☉. The medianr-band magnitude of our AGN catalog is 21.6 mag, with 34% havingr> 22.5, and 2.6% reaching the detection limit atr∼ 26 mag of the deepest imaging surveys we searched. We also provide a composite spectrum of the AGN sample covering 700–4400 Å. 

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2. The Active Galactic Nuclei in the Hobby–Eberly Telescope Dark Energy Experiment Survey (HETDEX). II. Luminosity Function

   https://doi.org/10.3847/1538-4357/ac8054  

   Liu, Chenxu; Gebhardt, Karl; Cooper, Erin Mentuch; Zhang, Yechi; Schneider, Donald P.; Ciardullo, Robin; Davis, Dustin; Farrow, Daniel J.; Finkelstein, Steven L.; Gronwall, Caryl; et al (August 2022, The Astrophysical Journal)

   Abstract We present the Lyαemission line luminosity function (LF) of the active galactic nuclei (AGN) in the first release of the Hobby–Eberly Telescope Dark Energy Experiment Survey (HETDEX) AGN catalog. The AGN are selected either by emission line pairs characteristic of AGN or by a single broad emission line, free of any photometric preselections (magnitude/color/morphology). The sample consists of 2346 AGN spanning 1.88 <z< 3.53, covering an effective area of 30.61 deg². Approximately 2.6% of the HETDEX AGN are not detected at >5σconfidence atr∼ 26 in the deepestr-band images we have searched. The Lyαline luminosity ranges from ∼10^42.3to 10^45.9erg s⁻¹. Our LyαLF shows a turnover luminosity with opposite slopes on the bright end and the faint end: The space density is highest at $L_{\text{Ly}\text{α}}^{\ast }={10}^{43.4}$erg s⁻¹. We explore the evolution of the AGN LF over a broader redshift range (0.8 <z< 3); constructing the rest-frame ultraviolet (UV) LF with the 1450 Å monochromatic luminosity of the power-law component of the continuum (M₁₄₅₀) fromM₁₄₅₀∼ −18 to −27.5. We divide the sample into three redshift bins (z∼ 1.5, 2.1, and 2.6). In all three redshift bins, our UV LFs indicate that the space density of AGN is highest at the turnover luminosity $M_{1450}^{*}$with opposite slopes on the bright end and the faint end. TheM₁₄₅₀LFs in the three redshift bins can be well fit with a luminosity evolution and density evolution model: the turnover luminosity ( $M_{1450}^{*}$) increases, and the turnover density (Φ*) decreases with increasing redshift. 

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3. HETDEX Public Source Catalog 1—Stacking 50,000 Lyman Alpha Emitters ^∗

   https://doi.org/10.3847/1538-4357/ace4c2  

   Davis, Dustin; Gebhardt, Karl; Cooper, Erin Mentuch; Bowman, William P.; Garcia Castanheira, Barbara; Chisholm, John; Ciardullo, Robin; Fabricius, Maximilian; Farrow, Daniel J.; Finkelstein, Steven L.; et al (September 2023, The Astrophysical Journal)

   Abstract We describe the ensemble properties of the 1.9 <z< 3.5 Lyman alpha emitters (LAEs) found in the HETDEX survey’s first public data release, HETDEX Public Source Catalog 1. Stacking the low-resolution (R∼ 800) spectra greatly increases the signal-to-noise ratio (S/N), revealing spectral features otherwise hidden by noise, and we show that the stacked spectrum is representative of an average member of the set. The flux-limited, LyαS/N restricted stack of 50,000 HETDEX LAEs shows the ensemble biweightaveragez∼ 2.6 LAE to be a blue (UV continuum slope ∼ −2.4 andE(B – V)< 0.1), moderately bright (M_UV∼ −19.7) star-forming galaxy with strong Lyαemission (logL_Lyα∼ 42.8 andW_λ(Lyα) ∼ 114 Å), and potentially significant leakage of ionizing radiation. The rest-frame UV light is dominated by a young, metal-poor stellar population with an average age of 5–15 Myr and metallicity of 0.2–0.3Z_⊙. 

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4. Discovery of a Damped Lyα Absorber Originating in a Spectacular Interacting Dwarf Galaxy Pair at z = 0.026

   https://doi.org/10.3847/2041-8213/ac5250  

   Boettcher, Erin; Gupta, Neeraj; Chen, Hsiao-Wen; Chen, Mandy C.; Józsa, Gyula I. G.; Rudie, Gwen C.; Cantalupo, Sebastiano; Johnson, Sean D.; Balashev, S. A.; Combes, Françoise; et al (February 2022, The Astrophysical Journal Letters)

   Abstract We present the discovery of neutral gas detected in both damped Lyαabsorption (DLA) and Hi21 cm emission outside of the stellar body of a galaxy, the first such detection in the literature. A joint analysis between the Cosmic Ultraviolet Baryon Survey and the MeerKAT Absorption Line Survey reveals an Hibridge connecting two interacting dwarf galaxies (log (M_star/M_⊙) = 8.5 ± 0.2) that host az= 0.026 DLA with log[N(Hi)/cm⁻²] = 20.60 ± 0.05 toward the QSO J2339−5523 (z_QSO= 1.35). At impact parameters ofd= 6 and 33 kpc, the dwarf galaxies have no companions more luminous than ≈0.05L_*within at least Δv= ±300 km s⁻¹andd≈ 350 kpc. The Hi21 cm emission is spatially coincident with the DLA at the 2σ–3σlevel per spectral channel over several adjacent beams. However, Hi21 cm absorption is not detected against the radio-bright QSO; if the background UV and radio sources are spatially aligned, the gas is either warm or clumpy (with a spin temperature to covering factor ratioT_s/f_c> 1880 K). Observations with VLT-MUSE demonstrate that theα-element abundance of the ionized interstellar medium (ISM) is consistent with the DLA (≈10% solar), suggesting that the neutral gas envelope is perturbed ISM gas. This study showcases the impact of dwarf–dwarf interactions on the physical and chemical state of neutral gas outside of star-forming regions. In the SKA era, joint UV and Hi21 cm analyses will be critical for connecting the cosmic neutral gas content to galaxy environments. 

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5. AGN STORM 2. II. Ultraviolet Observations of Mrk 817 with the Cosmic Origins Spectrograph on the Hubble Space Telescope*

   https://doi.org/10.3847/1538-4357/acc45a  

   Homayouni, Y.; De Rosa, Gisella; Plesha, Rachel; Kriss, Gerard A.; Barth, Aaron J.; Cackett, Edward M.; Horne, Keith; Kara, Erin A.; Landt, Hermine; Arav, Nahum; et al (May 2023, The Astrophysical Journal)

   Abstract We present reverberation mapping measurements for the prominent ultraviolet broad emission lines of the active galactic nucleus Mrk 817 using 165 spectra obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. Our ultraviolet observations are accompanied by X-ray, optical, and near-infrared observations as part of the AGN Space Telescope and Optical Reverberation Mapping Program 2 (AGN STORM 2). Using the cross-correlation lag analysis method, we find significant correlated variations in the continuum and emission-line light curves. We measure rest-frame delayed responses between the far-ultraviolet continuum at 1180 Å and Ly α λ 1215 Å ( 10.4 − 1.4 + 1.6 days), N v λ 1240 Å ( 15.5 − 4.8 + 1.0 days), Si iv + ]O iv λ 1397 Å ( 8.2 − 1.4 + 1.4 days), C iv λ 1549 Å ( 11.8 − 2.8 + 3.0 days), and He ii λ 1640 Å ( 9.0 − 1.9 + 4.5 days) using segments of the emission-line profile that are unaffected by absorption and blending, which results in sampling different velocity ranges for each line. However, we find that the emission-line responses to continuum variations are more complex than a simple smoothed, shifted, and scaled version of the continuum light curve. We also measure velocity-resolved lags for the Ly α and C iv emission lines. The lag profile in the blue wing of Ly α is consistent with virial motion, with longer lags dominating at lower velocities, and shorter lags at higher velocities. The C iv lag profile shows the signature of a thick rotating disk, with the shortest lags in the wings, local peaks at ±1500 km s −1 , and a local minimum at the line center. The other emission lines are dominated by broad absorption lines and blending with adjacent emission lines. These require detailed models, and will be presented in future work. 

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