10486( 3) - 05/12/05 06:36 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 10486 Version: 3 Check-in Date: 12-May-2005 10:35:20 1.Proposal Title: A Cosmic String Lens Candidate ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle 4. Parallel Pointing Tolerance GO 14 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Prof. Eric Agol University of Washington CoI: Prof. Craig Hogan University of Washington N CoI: Mr. Richard M. Plotkin University of Washington N ------------------------------------------------------------------------------------ 6. Abstract We propose two-band imaging observations with ACS of a cosmic string lens candidate, CSL-1, to look for a feature predicted by the cosmic string model: a low-surface brightness discontinuity in between the two galaxy images. ------------------------------------------------------------------------------------ 10486( 3) - 05/12/05 06:36 - [ 2] Observations Description ------------------------ We were awarded 3 orbits to observe the gravitational lens candidate CSL-1. It is necessary that we either 1) detect the discontinuity in the light profile that would be produced in the scenario where a cosmic string is present, or 2) we recover the light profile expected from 2 elliptical galaxies (z=0.46) separated by 1.9 arcsec in the scenario where there is no cosmic string. We must expose long enough to image the second scenario, otherwise a non-detection of a discontinuity would not be evidence of the absence of a string. We used the properties of CSL-1 listed in Sazhin et al. 2003 and calculated the lowest surface brightness in the area between the two galaxies to be around 22 mag arcsec^-2 in the Johnson R band. We therefore desire exposures at least 4200 s and 2000 s long in filters F625W and F775W respectively (calculated using the Exposure Time Calculator on the HST website) using WFC on ACS. These times are calculated to result in a signal to noise slightly better than 10 per 2x2 pixel region, and they were chosen to minimize the exposure time and provide color information. We defined a single visit and placed CSL-1 to fall in a region of the WFC chip that uses a single amplifier (using the POS-TARG offset command.) We obtained the coordinates of CSL-1 from Alcala et al. (2004, A&A, 428, 339). We packed our orbits using the orbit planner in APT. We decided to split F625W observations into 4 exposures taken over 2 orbits, dithering a box pattern between exposures to reject cosmic rays and hot pixels. The F775W observations were packed with 3 exposures over 1 orbit, dithering with a line pattern. The orbit planner shows a total of 5062 and 2409 seconds exposing our target in F625W and F775W respectively, easily providing the requisite signal to noise. The visit planner in APT shows CSL-1 being visible for only 2-3 weeks during cycle 14. Our program has no special time requirements. We can therefore split our observations into multiple visits if necessary due to possible scheduling constraints imposed by the limited source visibility. The data reduction will not require any special processing. We will utilize the cosmic-ray rejection and image combining from the pipeline, carrying out a deconvolution of the images using Tiny Tim, and fit the deconvolved images with (a) a gravitational lens model (using the publicly available code of Chuck Keeton); (b) a cosmic string lens model; and (c) two elliptical galaxy models. We will quantify the statistical significance of each fit with a maximum likelihood analysis, and determine at what confidence we can reject the gravitational lens and two- elliptical galaxy models (assuming predicted cosmic-string edge shows up in the data). In the two-elliptical galaxy fit, we will extract parameters such as central brightness, ellipse axis sizes and orientations. The string model predicts that the galaxies should be clones, (i.e. the same in both images, with the same parity.) If the parameter fits rule out the images being the same within errors, we can rule out the string. On the other hand, if the images are consistent with being the same to interesting precision, it is evidence for a string even if the discontinuity is not seen. That would be grounds for a followup proposal to find the edge. Real Time Justification ----------------------- Calibration Justification ------------------------- 10486( 3) - 05/12/05 06:36 - [ 3] Additional Comments ------------------- ------------------------------------------------------------------------------------ 10486( 3) - 05/12/05 06:36 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 CSL-1 GALAXY, ELLIPTICAL, RA=12H23M30.6040S +/- 0.3", J2000 Z = V = 20.71+/-0.03 GRAVITATIONAL LENS, DEC=-12D38'56.89" +/- 0.3" 0.463 INTERACTING GALAXY Coordinate Source: Other_Source: Alcala et al. 2004, AA, 428, 339 10486( 3) - 05/12/05 06:36 - [ 5] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.265 Line_Spacing 0.187 Coordinate_Frame POS-TARG Pattern_Orient 20.67 Angle_Between_Sides 69.05 Center_Pattern YES ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 3 Point_Spacing 3.011 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 85.28 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 10486( 3) - 05/12/05 06:36 - [ 6] Visit: 01 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 CSL-1 ACS/WFC ACCUM WFC2 F625W CR-SPLIT=NO 1 1100 S POS TARG 40, 0; PATTERN 1 see below 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 CSL-1 ACS/WFC ACCUM WFC2 F775W CR-SPLIT=NO 1 750 S POS TARG 40, 0; PATTERN 2 see below 2 ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ CSL-1 1 ACS/WFC ACCUM WFC2 F625W 1 none none none 1 1245.0 S CSL-1 1 ACS/WFC ACCUM WFC2 F625W 2 none none none 1 1245.0 S CSL-1 1 ACS/WFC ACCUM WFC2 F625W 3 none none none 2 1286.0 S CSL-1 1 ACS/WFC ACCUM WFC2 F625W 4 none none none 2 1286.0 S CSL-1 2 ACS/WFC ACCUM WFC2 F775W 1 none none none 3 803.0 S CSL-1 2 ACS/WFC ACCUM WFC2 F775W 2 none none none 3 803.0 S CSL-1 2 ACS/WFC ACCUM WFC2 F775W 3 none none none 3 803.0 S 10486( 3) - 05/12/05 06:36 - [ 7] Summary Form for Proposal 10486 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements POS TARG 40, 0; PATTERN 1 1 POS TARG 40, 0; PATTERN 2 2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F625W F775W ------------------------------------------------------------------------------------------------------------------------------------ Target Names CSL-1 ------------------------------------------------------------------------------------------------------------------------------------