Luminous mid-IR selected obscured quasars at cosmic noon in SDSS Stripe82 II: spectroscopic diversity and broad H α emissions

ABSTRACT We present a multiwavelength spectroscopic survey of 23 luminous mid-infrared-selected Type-2 quasars at the redshifts of $z=0.88$–3.49. The targets were selected in the SDSS Stripe 82 field based on their bright WISE 22 $\mu$m detections and extremely faint or red optical counterparts. Near-infrared (Gemini/Gemini Near-InfraRed Spectrograph) and optical (Keck/Low Resolution Imaging Spectrometer and Keck Cosmic Web Imager Integral Field Spectrograph) spectroscopy confirm 23 out of 24 candidates as Type-2 quasars, 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 (full width at half-maximum, FWHM$>2000$ km s$^{-1}$) despite their Type-2 classification, while the rest have only narrow lines (FWHM $< 2000$ km s$^{-1}$) characteristic of classical obscured quasars. Notably, these broad-line Type-2 quasars share similar spectral energy distributions with the James Webb Space Telescope (JWST)-discovered ‘little red dot’, 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] $\lambda$5007 emission is relatively weak for their high-bolometric luminosities, deviating from trends seen in lower-z Type-2 quasi-stellar objects (QSOs). A new composite spectrum for Type-2 QSOs is built using our sample. Overall, our results demonstrate that mid-infrared selection efficiently uncovers diverse populations of obscured quasars and that spectroscopic follow-up is crucial for revealing their true nature. This study provides new insights into heavily obscured supermassive black hole growth at cosmic noon and bridges the gap to the obscured active galactic nuclei populations being revealed by JWST.