Luminous Mid-IR Selected Type-2 Quasars at Cosmic Noon in SDSS Stripe82 I: Selection, Composite Photometry, and Spectral Energy Distributions

Ben Wang, Joseph F. Hennawi, Zheng Cai, Gordon T. Richards, Jan-Torge Schindler, Nadia L. Zakamska, Yuzo Ishikawa, Hollis B. Akins, Zechang Sun

First listed 2025-01-23 | Last updated 2025-05-22

Abstract

We analyze 23 spectroscopically confirmed Type-2 quasars (QSOs) selected from the WISE 22$\rm μ$m band in the SDSS Stripe 82 region, focusing on their multi-band photometry and spectral energy distributions (SEDs). These objects were selected to be IR-luminous ($\rm flux_{W4} > 5mJy$, i.e., $12.62 < W4 < 14.62 \rm\ AB \, magnitude$), optically faint ($r > 23$) or with red color ($r - W4 >8.38$). Gemini/GNIRS observations were conducted for all 24 candidates, and 18/24 were also observed with Keck/LRIS. The observations confirm 23 to be real Type-2 QSOs in the redshift range $0.88 - 2.99$ (12 are at $z>2$). We collect multi-band photometry and conduct SED fitting. The composite photometry probes the wavelength from 0.1$\rm μ$m to 10$\rm μ$m at the rest frame. The IR emission is dominated by dust torus implying an average torus luminosity for the sample of $L_{\rm torus} 10^{46.84} \rm erg/s$. The origin of the rest-UV/optical light is not definitive, but we present three possible scenarios: scattered light, stellar emission, and the reddened accretion disk. Assuming an obscured:unobscured ratio of approximately 1:1, our targets have $L_{\rm bol} = 10^{46.28} \rm erg \,s^{-1} - 10^{47.49} \rm erg \,s^{-1}$ and around SMBH masses $\rm 10^{8.18} M_{\odot} - 10^{9.39} M_{\odot}$, assuming they accreate at the Eddington limit. Compared to previous Type-2 AGN SEDs, our targets have a brighter dust torus and redder optical-IR color. By comparing the SED to the results from JWST `little red dots' (LRDs), we find that these IR-selected Type-2 QSOs have similar SED shapes to the LRDs. This pilot Type-2 QSO survey demonstrates that mid-IR selection is an efficient way to find luminous Type-2 QSOs at $z>2$. Finally, the composite photometry and Type-2 QSOs SED model generated by this sample provide a guide for finding more Type-2 QSOs at higher redshift.

Short digest

WISE W4>5 mJy selection in SDSS Stripe 82 yielded 24 candidates, with GNIRS (all) plus LRIS (18/24) confirming 23 luminous Type‑2 QSOs at 0.88<z<2.99, including 12 at z>2. Multi‑band photometry (DESI Legacy, PS1, UKIDSS, Spitzer, WISE) enables rest‑frame 0.1–10 μm composite photometry and AGNfitter SEDs. The IR is torus‑dominated with average L_torus ≈10^46.84 erg s−1; inferred L_bol spans 10^46.28–10^47.49 erg s−1 and Eddington masses ~10^8.18–10^9.39 M⊙, and the sample is redder with brighter tori than earlier Type‑2 SEDs. Their SED shapes closely resemble JWST “little red dots,” highlighting mid‑IR selection as an efficient path to luminous z>2 Type‑2s; a key open point is the origin of the rest‑UV/optical light (scattered, stellar, or a reddened disk).

Key figures to inspect

• Selection plane: r versus r−W4 (and W4 flux>5 mJy) showing how the Stripe 82 cut isolates IR‑luminous, optically faint/red sources; check where confirmed Type‑2s lie relative to contaminants.
• Spectroscopic confirmation panels: GNIRS+LRIS examples illustrating narrow‑line spectra and redshift measurements; compare success rate (96%) and the distribution across 0.88<z<2.99, emphasizing the z>2 subset.
• Composite SED and AGNfitter decomposition: rest‑frame 0.1–10 μm photometry with model components, demonstrating torus dominance and the uncertain UV/optical component; read off average L_torus and L_bol range.
• Redshift and photometry summary: histograms or tables of z, W4 fluxes, and optical magnitudes (r>23 or r−W4>8.38) to visualize sample definition and luminosity regime.
• Comparison to JWST LRDs: overlaid median SEDs showing the similarity in optical‑IR slope/shape between these mid‑IR Type‑2 QSOs and LRDs, motivating selection strategies at higher z.