A peculiar Type II QSO identified via broad-band detection of extreme nebular line emission. Issue 1 (18th October 2021)

• Record Type: Journal Article
• Authors: Lin, Yu-Heng; Scarlata, Claudia; Hayes, Matthew; Feltre, Anna; Charlot, Stephane; Bongiorno, Angela; Väisänen, Petri; Mogotsi, Moses
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 509:Issue 1(2022)
• Page Start: 489
• Page End: 500
• Publication Date: 2021-10-18
• Subjects: galaxies:active -- galaxies: high-redshift -- quasars: emission lines; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stab2978
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: We present S82-20, an unusual redshift ≈3 object identified in SDSS-Stripe 82 broad-band images. The rest-frame ultraviolet spectrum of S82-20 shows emission lines from highly ionized species, including He ii λ1640, and the C iv λλ1548, 1550 and O vi λλ1032, 1038 doublets. The high Ly α luminosity (3.5 × 10 44 erg s −1 ), the high emission line equivalent widths (>200 Å for Ly α), the full width at half-maximum of the emission lines (<800 km s −1 ), and the high ionization O vi line strongly support the interpretation that S82-20 is a Type II Quasi-Stellar Objects (QSOs). However, photoionization models using Type II QSO do not fully explain the measured C iv /He ii line ratio, which requires either some contribution from star formation or high velocity shocks. Additionally, S82-20 is not detected at wavelengths longer than 2 μm, in tension with the expectation of isotropically infrared emission of a luminous QSO. We consider the possibility that S82-20 is a rare example of a changing-look QSO, observed in a temporarily low state, where the broad line region has faded, while the narrow line region still emits emission line. Otherwise, it may be a rare case of the short phase of the life of a massive galaxy, in which active star formation and accretion on to a supermassive black hole coexist.