The 8 O'Clock Arc: A Serendipitous Discovery of a Strongly Lensed Lyman Break Galaxy in the SDSS DR4 Imaging Data

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Abstract
We report on the serendipitous discovery of the brightest Lyman break galaxy (LBG) currently known, a galaxy at z = 2.73 that is being strongly lensed by the z = 0.38 luminous red galaxy (LRG) SDSS J002240.91+143110.4. The arc of this gravitational lens system, which we have dubbed the "8 o'clock arc" due to its time of discovery, was initially identified in the imaging data of the Sloan Digital Sky Survey Data Release 4; followup observations on the Astrophysical Research Consortium (ARC) 3.5 m telescope at Apache Point Observatory confirmed the lensing nature of this system and led to the identification of the arc's spectrum as that of an LBG. The arc has a spectrum and a redshift remarkably similar to those of the previous record-holder for brightest LBG (MS 1512-cB58, also known as cB58), but, with an estimated total magnitude of (g, r, i) = (20.0, 19.2, 19.0) and surface brightness of (μg, μr, μi) = (23.3, 22.5, 22.3) mag arcsec-2, the 8 o'clock arc is thrice as bright. The 8 o'clock arc, which consists of three lensed images of the LBG, is 162° (9.6'') long and has a length-to-width ratio of 6 : 1. A fourth image of the LBG—a counterimage—can also be identified in the ARC 3.5 m g-band images. A simple lens model for the system assuming a singular isothermal ellipsoid yields an Einstein radius of θEin = 3.32'' ± 0.16'', a total mass for the lensing LRG (within the 12.1 ± 0.6 h-1 kpc enclosed by the Einstein radius) of 1.35 × 1012 h-1M☉, and a magnification factor for the LBG of 12.3. The LBG itself is intrinsically quite luminous (≈11L*) and shows indications of massive recent star formation, perhaps as high as 160 h-1 M☉ yr-1.
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