A Peculiar Type~II QSO Identified via Broad-band Detection of Extreme
  Nebular Line Emission
  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 $\lambda$1640,
and the CIV $\lambda\lambda$1548, 1550 and OVI $\lambda\lambda$1032, 1038
doublets. The high Ly$\alpha$ luminosity ($3.5\times 10^{44}$ erg s$^{-1}$),
the high emission line equivalent widths ($>200$\r{A} for Ly$\alpha$), 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$\mu$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.
