A peculiar Type II QSO identified via broad-band detection of extreme nebular line emission

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 × 10⁴⁴ 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.