A Lyman-alpha-only AGN from the Sloan Digital Sky Survey

Abstract

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 Lyman-alpha line (FWHM = 1430 km/s) 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. Despite certain spectral similarities, SDSS J1136+0242 is not a Lyman-break galaxy. Instead, the Ly-alpha and two-photon emission can be attributed to an extended, low-metallicity narrow-line region. The unpolarized continuum argues that we see SDSS J1136+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 which swamps the broad-line emission; or, more likely, SDSS J1136+0242 could be similar to PG 1407+265, a quasar in which for some unknown reason the high-ionization emission lines are very broad, very weak, and highly blueshifted.

arXiv commentAJ, in press, 10 pages emulateapj format

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• A Ly-alpha Emitter with an Extremely Large Rest-frame Equivalent Width of ~900A at z=6.5: A Candidate of Population III-dominated Galaxy?
• Broad Absorption Line Quasars with Redshifted Troughs: High-Velocity Infall or Rotationally Dominated Outflows?
• Intranight Optical Variability of Radio-Quiet Weak Emission Line Quasars-III
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• X-rays from the radio-quiet quasar PG 1407+265: relativistic jet or accretion disc emission?
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• Rest-frame optical and far-infrared observations of extremely bright Lyman-break galaxy candidates at z ~ 2.5
• Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). XIV. A Candidate Type-II Quasar at z=6.1292