Luminous Mid-IR Selected Obscured Quasars at Cosmic Noon in SDSS Stripe82 II: Spectroscopic Diversity and Broad H$α$ Emissions

Ben Wang, Yuzo Ishikawa, Joseph F. Hennawi, Zheng Cai, Gordon T. Richards, Nadia L. Zakamska, Daming Yang, Jan-Torge Schindler

First listed 2025-08-04 | Last updated 2025-08-04

Abstract

We present a multiwavelength spectroscopic survey of 23 luminous mid-infrared-selected Type-2 quasars at redshifts z = 0.88 to 3.49. The targets were selected in the SDSS Stripe 82 field based on their bright WISE W4 detections (flux > 5 mJy) and extremely faint or red optical counterparts (e.g., r > 23 or r - W4 > 8.4), designed to identify heavily obscured quasars. Deep near-infrared (Gemini/GNIRS) and optical (Keck/LRIS and KCWI) spectroscopy confirm 23 out of 24 candidates as Type-2 quasars in this redshift range, including 12 objects at z > 2. The spectra exhibit strong rest-frame UV and optical emission lines (Ly-alpha, C IV, [O III], H-alpha) with a wide range of line widths, indicating significant spectral diversity. Approximately one-third of the sample (8 of 23) shows broad H-alpha emission (FWHM > 2000 km/s) despite their Type-2 classification, while the rest have only narrow lines (FWHM < 2000 km/s) characteristic of classical obscured quasars. Notably, these broad-line Type-2 quasars share similar spectral energy distributions with the JWST-discovered "little red dot" (LRD) AGNs, suggesting that our sample could be lower-redshift analogues of the heavily obscured broad-line AGNs uncovered by JWST. We also find that the [O III] 5007 angstrom emission is relatively weak for their high bolometric luminosities, deviating from trends seen in lower-redshift Type-2 QSOs. A new composite spectrum for Type-2 QSOs is built using our sample. Overall, our results demonstrate that mid-IR selection efficiently uncovers a diverse population of obscured quasars and that spectroscopic follow-up is crucial for revealing their true nature. This study provides new insights into heavily obscured SMBH growth at cosmic noon and bridges the gap to the obscured AGN populations now being revealed by JWST.

Short digest

Survey of 24 WISE W4–bright, optically faint/red SDSS Stripe 82 candidates confirms 23 luminous Type‑2 quasars at z = 0.88–3.49 using Gemini/GNIRS plus Keck/LRIS and KCWI spectra. About one‑third (8/23) unexpectedly show broad Hα (FWHM > 2000 km/s) while sharing SEDs with JWST “little red dots,” marking them as lower‑z analogues of heavily obscured broad‑line AGN. The sample also exhibits relatively weak [O III] λ5007 given their high bolometric luminosities, and the authors construct a new Type‑2 QSO composite spectrum. Results demonstrate that simple mid‑IR plus extreme r−W4 color selection efficiently recovers a spectroscopically diverse obscured quasar population at cosmic noon, informing SMBH growth demographics that connect to the LRD population.

Key figures to inspect

• Target selection plane: r − W4 versus W4 (or r magnitude), showing the Stripe 82 DOG cut (e.g., r > 23 or r − W4 > 8.4) and the >5 mJy W4 threshold; verify where the confirmed Type‑2s and broad‑Hα objects sit relative to these cuts.
• GNIRS rest‑optical spectra around Hα for the 8 broad‑line sources; inspect line decompositions, measured FWHM (>2000 km/s), and any accompanying narrow components or [N II]/[S II] constraints.
• LRIS/KCWI rest‑UV spectra displaying Lyα, C IV, and other lines across the sample; compare profile diversity and widths between pure narrow‑line objects and the broad‑Hα subset.
• [O III] λ5007 luminosity versus Lbol (or mid‑IR luminosity) comparison plot; look for the systematic [O III] deficit relative to low‑z Type‑2 trends and outliers that drive the deviation.
• Type‑2 QSO composite spectrum built from this sample; check contributing objects, normalization, and key line ratios that encode the weak‑[O III] behavior and broad‑line fraction.