# Presentation: 116024

## Application of Gravitational Lensing Models to the Brightest Strongly Lensed Lyman Break Galaxy – the 8 o’clock arc

***Application of Gravitational Lensing Models to the Brightest Strongly Lensed Lyman Break Galaxy – the 8 o’clock arc***
***E. Buckley-Geer******1******, S. Allam******1,2******, H. Lin******1******, H.T. Diehl******1******, D.Tucker******1******, J. Annis******1******, J. Frieman******3***
**1 Fermi National Accelerator Laboratory, 2 University of Wyoming, 3 University of Chicago**

**ABSTRACT**
**We report on modeling of the recently discovered brightest strongly lensed Lyman Break Galaxy (LBG), the so called "8 o'clock arc". This galaxy is at redshift 2.73 and is lensed by a luminous red galaxy (LRG, z=0.38) in the Sloan Digital Sky Survey (SDSS) DR4 imaging data. We observe four images, three in the arc plus a counterimage. The three lensed images in the arc subtend an angle of 160 degrees around the LRG and extend over more than 9.6 arcsec in length. For the modeling we are using information from all four images extracted from SPICAM images taken on the 3.5m telescope at APO. Using the GRAVLENS package with an Singular Isothermal Ellipse (SIE) mass profile we obtain a velocity dispersion of 392 km/sec for the LRG and a total magnification of about 12.**

***Follow-up observations***
-    **Follow-up observations were carried out using the 3.5m at Apache Point Observatory on 24 August 2006. Three 300 sec exposures were taken using the SPIcam CCD imager in each of the SDSS gri filters under seeing conditions of 1.0-1.2” (FWHM). **

-   **Spectroscopy was taken using the DISIII spectrograph. The resulting spectrum indicated that the arc was a Lyman Break Galaxy at a redshift of 2.73. (See Poster #256.03 S. Allam et al. and astro-ph/0611138)**

***SPIcam g-band co-added image***

-  ***Discovery***
-   **The 8 o’clock arc was discovered while performing a visual scan of an interacting/merging galaxy pair catalog extracted from the SDSS DR4. The central Luminous Red Galaxy (LRG) has an SDSS spectrum which indicates that it is at a redshift of 0.38. The associated arc is a very blue extended object which does not have an SDSS spectrum. There are 3 images in the arc and a 4****th**** counter image below the LRG which is not visible in the SDSS image.**

***SDSS DR5 image***

***Modeling the LRG***
-   **We used the GALFIT program (Peng et al. 2002, AJ, 124: 266 ) to model the LRG and subtract it from the image prior to modeling the lens. We chose to work with the g-band co-added image as the counter image A4 is clearly visible. A PSF image was generated from one of the stars in the SPIcam image using IRAF’s mkobject. We chose to mask out the pixels in the arc and counter image to avoid biasing the galaxy modeling.**

**A3**

**A2**

**A1**

**A4**

**LRG**

***Input image***

***PSF Image***

***Pixel mask***

**We generated a galaxy model based on a de Vaucouleurs profile****.**

_***Fitted parameters***_
**Magnitude = 20.41 ****± ****0.03**
**r****e**** = 3.66 ****± 0.18"**
**Ellipticity = 0.37 ± 0.02**
**PA = 14.72 ± 2.78 deg**
**χ****2****/dof = 1.33**

**n = 4 *****k***** = 7.67 **
**Σ****(r) = surface brightness at radius r**
**Σ****e**** = surface brightness at radius r****e**

***Galaxy Subtracted image***

***Galaxy model***

**A3**

**A2**

**A1**

**A4**

***Modeling the Lens***
**   ****We used the gravlens/lensmodel software (C. Keeton astro-ph/0102340) to model the lens. We used the four image positions obtained by running SExtractor (Bertin & Arnout, 1996 A&AS, 117, 393) on the g-band galaxy subtracted image. The LRG position was taken from the GALFIT subtraction. We assumed a singular isothermal ellipse (SIE) for the mass profile of the lens. We assume a flat cosmology, ****Ω****m****=0.3, ****Ω****λ****=0.7 H****0****=100*****h***** km s****-1 ****Mpc****-1****.**

**κ**** = surface mass density**
**ξ**** = x****2**** + y****2****/q****2 ****where q = ellipse axis ratio**

**b****'**** is related to the Einstein radius b****SIE**** such that**

***The image plane***

***The source plane***

_***Fitted parameters***_
**b****' = 2.94 ± 0.14"**
**Ellipticity = 0.53 ± 0.05**
**PA = 12.76      deg**
**χ****2****/dof = 1.04 **

**The velocity dispersion is given by**

**D****s**** = 1141 *****h*****-1**** Mpc angular diameter distance to source**
**D****L**** = 752 *****h*****-1**** Mpc** **angular diameter distance to lens**
**D****LS**** = 863 *****h*****-1**** Mpc** **angular diameter distance between lens**
**          ****and source**

_***Calculated parameters***_
**b****SIE**** = 3.35 ± 0.16"**
**σ**** = 392 ± 9 km s****-1**
**R****EIN**** = 12.2 ****± 0.6 *****h*****-1 ****kpc**
**M****EIN**** = 1.37 ± 0.13 x10****12**** *****h*****-1**** M******
**Mass-to light ratio = 17.7 *****h***** M********/ L******** **

_***Image magnifications***_
**A1 = 4.01     A2 = -4.44 **
**A3 = 3.79     A4 = -0.52 ****± 0.04**
**Total magnification = 12.76 **

**   ****A S****é****rsic profile yields a very similar fit with a slightly smaller r****e**** of 2.88 ****± 0.2"**

**+11.6**
** ****- 3.4**

**+ 1.74**
** ****- 1.56**

**+4.42**
**-1.11**

**+ 5.59**
** ****- 1.59**

**+1.17**
**-0.69**

***D. Tucker, E. Buckley-Geer, H.T. Diehl, S. Allam, H. Lin***