The 8 o'clock arc: A serendipitous discovery of a strongly lensed lyman break galaxy in the SDSS DR4 imaging data

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 (mu(g), mu(r), mu(i)) = (23.3, 22.5, 22.3) mag arcsec-2, the 8 o'clcok arc is thrice as bright. The 8 o'clock arc, which consists of three lensed images of the LBG, is 162 degrees (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 theta(Ein) = 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 x 10(12) h(-1)M(circle dot), and a magnification factor for the LBG of 12.3(-3.6)(+15.0). The LBG itself is intrinsically quite luminous (approximate to 11L(*)) and shows indications of massive recent star formation, perhaps as high as 160 h(-1) M-circle dot yr(-1).