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 1640, and the C iv 1548, 1550 and O vi 1032, 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-maxim 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 foation or high velocity shocks. Additionally, S82-20 is not detected at wavelengths longer than 2 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 foation and accretion on to a supeassive black hole coexist.