Luminous mid-IR-selected type 2 quasars at cosmic noon in SDSS Stripe 82 – I. Selection, composite photometry, and spectral energy distributions

ABSTRACT We analyse 23 spectroscopically confirmed type 2 quasars (QSOs) selected from Wide-field Infrared Survey Explorer 22 $\rm{\mu m}$ band in the Sloan Digital Sky Survey Stripe 82 region, focusing on their multiband photometry and spectral energy distributions (SEDs). The 24 candidates were selected to be infrared (IR) luminous (${\rm flux}_{W4} > 5 \, {\rm mJy}$), optically faint ($r > 23$), or with red colour ($r - W4 >8.38$). Gemini/Gemini Near-InfraRed Spectrograph and Keck/Low-Resolution Imaging Spectrometer observations confirm 23 to be type 2 QSOs at $z = 0.88 \!-\! 3.49$. Multiband photometry is used for SED fitting, covering 0.1–10 $\mu$m in the rest frame. The IR emission is dominated by the dust torus, with an average luminosity of $L_{\rm torus} = 10^{46.84} \, \rm erg\, s^{-1}$. We present three possibilities for the origin of the rest-ultraviolet/optical: scattered light, stellar emission, and the reddened accretion disc. Assuming an obscured:unobscured ratio of 1:1, the targets have bolometric luminosities of $L_{\rm bol} = 10^{46.28} \!-\! 10^{48.08} \, \rm erg\, s^{-1}$ and supermassive black hole masses of $10^{8.18} \!-\! 10^{9.98} \, {\rm M}_{\odot }$, averaging $L_{\rm bol} = 10^{47.04} \, {\rm erg\,s^{-1}}$ and $M_{\rm BH} = 10^{8.94} \, {\rm M}_{\odot }$, assuming the Eddington limit. Compared to previous type 2 active galactic nuclei SEDs, our targets have a brighter dust torus and redder optical–IR colour. By comparing the SED to JWST ‘little red dots’ (LRDs), we find that these IR-selected type 2 QSOs have similar SED shapes to the LRDs. This survey demonstrates mid-IR selection as an efficient method to find luminous type 2 QSOs and the composite photometry generated by this sample provides a guide for finding more type 2 QSOs at higher redshift in the future.