A Lyα-only active galactic nucleus from the Sloan Digital Sky Survey

The Sloan Digital Sky Survey has discovered a z = 2.4917 radio-loud active galactic nucleus (AGN) with a luminous, variable, low-polarization UV continuum, H I two-photon emission, and a moderately broad Lyα line (FWHM ≃ 1430 km s^-1) but without obvious metal-line emission. SDSS J113658.36+024220.1 does have associated metal-line absorption in three distinct, narrow systems spanning a velocity range of 2710 km s^-1. Despite certain spectral similarities, SDSS J1136+0242 is not a Lyman break galaxy. Instead, the Lyα and two-photon emission can be attributed to an extended, low-metallicity narrow-line region. The unpolarized continuum argues that we see SDSS 11136+0242 very close to the axis of any ionization cone present. We can conceive of two plausible explanations for why we see a strong UV continuum but no broad-line emission in this "face-on radio galaxy" model for SDSS J1136+0242: the continuum could be relativistically beamed synchrotron emission that swamps the broad-line emission, or more likely, SDSS J1136+0242 could be similar to PG1407+265, a quasar in which for some unknown reason the high-ionization emission lines are very broad, very weak, and highly blueshifted.