Unusual Broad-Line MgII Emitters Among Luminous Galaxies in BOSS
  Many classes of active galactic nuclei (AGN) have been observed and recorded
since the discovery of Seyfert galaxies. In this paper, we examine the sample
of luminous galaxies in the Baryon Oscillation Spectroscopic Survey (BOSS). We
find a potentially new observational class of AGN, one with strong and broad
MgII $2799$\AA\ line emission, but very weak emission in other normal
indicators of AGN activity, such as the broad line H$\alpha$, H$\beta$, and the
near-ultraviolet AGN continuum, leading to an extreme ratio of broad
H$\alpha$/MgII flux relative to normal quasars. Meanwhile, these objects'
narrow-line flux ratios reveal AGN narrow-line regions with levels of activity
consistent with the MgII fluxes and in agreement with that of normal quasars.
These AGN may represent an extreme case of the Baldwin effect, with very low
continuum and high equivalent width relative to typical quasars, but their
ratio of broad MgII to broad Balmer emission remains very unusual. They may
also be representative of a class of AGN where the central engine is observed
indirectly with scattered light. These galaxies represent a small fraction of
the total population of luminous galaxies ($\simeq 0.1$\%), but are more likely
(about 3.5 times) to have AGN-like nuclear line emission properties than other
luminous galaxiess. Because MgII is usually inaccessible for the population of
nearby galaxies, there may exist a related population of broad-line MgII
emitters in the local universe which are currently classified as narrow-line
emitters (Seyfert 2s) or LINERs.
