A Peculiar Type~II QSO Identified via Broad-band Detection of Extreme Nebular Line Emission

Abstract

We present S82-20, an unusual redshift $\approx$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 HeII $\hat{I}»$1640, and the CIV $\hat{I}»\hat{I}»$1548, 1550 and OVI $\hat{I}»\hat{I}»$1032, 1038 doublets. The high Ly$\hat{I}\pm$ luminosity ($3.5\times 10^{44}$ erg s${}^{-1}$), the high emission line equivalent widths ($>200$Ã for Ly$\hat{I}\pm$), the FWHM of the emission lines ($<800$km s${}^{-1}$), and the high ionization OVI line strongly support the interpretation that S82-20 is a Type~II QSO. However, photoionization models using Type~II QSO do not fully explain the measured CIV/HeII 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$\hat{I}¼$m, in tension with the expectation of isotropically IR 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 onto a supermassive black hole coexist.

13 pages, 8 figures