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 thez=0.38 Luminous Red Galaxy (LRG) SDSS J002240.91+143110.4. The arc of thisgravitational lens system, which we have dubbed the "8 o'clock arc" due to itstime of discovery, was initially identified in the imaging data of the SloanDigital Sky Survey Data Release 4 (SDSS DR4); followup observations on theAstrophysical Research Consortium (ARC) 3.5m telescope at Apache PointObservatory confirmed the lensing nature of this system and led to theidentification of the arc's spectrum as that of an LBG. The arc has a spectrumand a redshift remarkably similar to those of the previous record-holder forbrightest LBG (MS 1512-cB58, a.k.a "cB58"), but, with an estimated totalmagnitude 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'clock arc is thriceas bright. The 8 o'clock arc, which consists of three lensed images of the LBG,is 162deg (9.6arcsec) long and has a length-to-width ratio of 6:1. A fourthimage of the LBG -- a counter-image -- can also be identified in the ARC 3.5mg-band images. A simple lens model for the system assuming a singularisothermal ellipsoid potential yields an Einstein radius of 2.91+/-0.14 arcsec,a total mass for the lensing LRG (within the (10.6+/-0.5)/h kpc enclosed by thelensed images) of 1.04x10^12/h Msun, and a magnification factor for the LBG of12.3(+15/-3.6). The LBG itself is intrinsically quite luminous (approximately6L*) and shows indications of massive recent star formation, perhaps as high as160/h Msun/year.Comment: 4 pages 5 figures, submitted to ApJ Letter