12871( 2) - 11-Jul-2012 15:39:17 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 12871 Version: 2 Check-in Time: 11-Jul-2012 15:39:17 STScI Edit Number: 0 Title When Giants Collide: Mapping the Mass in the Cluster Merger Abell 2146 ------------------------------------------------------------------------------------ Type Cycle GO 20 ------------------------------------------------------------------------------------ Investigators Contact? PI: Prof. Lindsay J. King University of Texas at Dallas Y CoI: Dr. Helen R Russell University of Waterloo N CoI: Ms. Rebecca E. Canning University of Cambridge N CoI: Prof. Douglas Clowe Ohio University N CoI: Mr. Brett Ragozzine Ohio University N CoI: Mr. Christopher Parish University of Texas at Dallas N CoI: Dr. Adrienne Leonard CEA/DSM/DAPNIA/Service d'Astrophysique N CoI: Dr. Baojiu Li University of Durham N CoI: Dr. Stefi A. Baum Rochester Institute of Technology N CoI: Dr. Alastair C. Edge University of Durham N CoI: Prof. Andrew C. Fabian University of Cambridge N CoI: Dr. Poshak Gandhi Institute of Space & Astronautical Science N CoI: Dr. Brian R. McNamara University of Waterloo N CoI: Dr. Christopher P. O'Dea Rochester Institute of Technology N CoI: Dr. Jeremy S Sanders University of Cambridge N ------------------------------------------------------------------------------------ Abstract Galaxy cluster mergers are critical for our understanding of dark matter, as shown with the Bullet Cluster (1E0657-56). These violent events result in a separation of the dark matter in these systems from the hot X-ray emitting plasma, the dominant baryonic component. Our proposed target, Abell 2146, is an extraordinary post-merger system, discovered using Chandra X-ray observations. It is a unique system, presenting two unambiguous shock fronts that allow us to estimate that the collision between the two clusters occured ~0.1-0.2 Gyr ago. We propose ACS/WFC observations of this system, from which we will obtain a high-resolution map of the total mass distribution, using gravitational lensing analysis of distant background galaxies. By comparing this mass map with our X-ray images, and with the locations of cluster galaxy populations, we will study dark matter and test gravity on cluster scales. We will map the detailed mass substructure in the clusters, essential to our understanding of their formation, facilitated by the exquisite resolution of HST. We will use the window that HST provides in conjunction with gravitational lensing, to reveal the dark matter haloes that harbour cluster members in this extreme environment, studying their characteristic mass and size, and the physical processes at play. ------------------------------------------------------------------------------------ Observations Description ------------------------ The primary goal of our programme is to map the mass in the system Abell 2146 using gravitational lensing, and measure if the centroids of the mass are offset from the peaks in our X-ray image, containing most of the luminous mass. We will also map the mass substructure in this system using gravitational 12871( 2) - 11-Jul-2012 15:39:17 - [ 2] flexion, which is especially sensitive to gradients in the mass distribution. Required pointings: Gravitational shear is a non-local statistic and as such we require observations in a wide region about the possible locations of the centroid of the weak lensing signal. We therefore request two pointings, one centred on each of the centroids of the galaxy distributions. A section of the shear field between the subcluster and primary cluster locations will be observed with both pointings, therefore further increasing the signal-to-noise in the inner regions, where we see elongated background galaxies consistent with the expected direction of the shear field. Required filters: In order to accurately determine the lensed background galaxy population and to remove both foreground objects and the cluster member galaxies, we require imaging in 3 filters. The signal-to-noise of the mass reconstruction is greatly reduced (Clowe & Schneider 2001) if non-lensed dwarf galaxies cannot be removed from the population. We propose to observe Abell 2146 with the F814W, F606W and F435W filters. These have been successfully used to remove dwarf galaxies while maintaining a high number density of background galaxies (~90 arcmin^2) in observations of the Bullet Cluster (Clowe et al. 2006), which is close in redshift to Abell 2146. Required depth: Our target sensitivity for this programme is defined by the need to get shape measurements for ~90 background galaxies arcmin^2 to measure the weak lensing peak centroids accurately. To do this we need to obtain photometry at a S/N= 10 at IF814W = 27.5 for a galaxy, which we can get in 2 orbits with the F814Wfilter (calculated from both the ACS ETC, and from Clowe’s experience with the Bullet Cluster dataset). For colour selection, a single orbit in F435W and F606W will provide us with the necessary photometric S/N. Dithering pattern: Our observing plan is to take 4 exposures per orbit, with a small offset between the first two images, a chip gap spanning offset before the 3rd image, and another small offset for the fourth image. We will schedule each pointing as a block of 4 orbits to take all the filters for that pointing in the same visit. The other pointing for the cluster will need to be constrained to occur at a similar rotation angle as the first (or 90 degrees to it). Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 12871( 2) - 11-Jul-2012 15:39:17 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 ABELL2146 CLUSTER OF GALAXIES, RA=15H56M3.2415S +/- 0.5", J2000 V = 16 flux of brightest cluster galaxy GRAVITATIONAL LENS DEC=+66D22'10.22" +/- 0.5" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 12871( 2) - 11-Jul-2012 15:39:17 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-LINE LINE Pattern_Purpose DITHER DITHER Number_Of_Points 2 2 Point_Spacing 6.10486 3.085553 Line_Spacing Coordinate_Frame POS-TARG POS-TARG Pattern_Orient 91.16385 85.38997 Angle_Between_Sides Center_Pattern YES NO ------------------------------------------------------------------------------------------------------------------------------------ 12871( 2) - 11-Jul-2012 15:39:17 - [ 5] Visit: 01 Visit Requirements: ORIENT 310D TO 330 D; ORIENT 130D TO 150 D 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 ABELL2146 ACS/WFC ACCUM WFC F814W 1 500 S POS TARG 0.0, see below -93.82200482232652; PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 ABELL2146 ACS/WFC ACCUM WFC F814W 1 500 S POS TARG 0.0, see below -93.82200482232652; PATTERN 1 2 ------------------------------------------------------------------------------------------------------------------------------------ 3 ABELL2146 ACS/WFC ACCUM WFC F606W 1 500 S POS TARG 0.0, see below -93.82200482232652; PATTERN 1 3 ------------------------------------------------------------------------------------------------------------------------------------ 4 ABELL2146 ACS/WFC ACCUM WFC F435W 1 500 S POS TARG 0.0, see below -93.82200482232652; PATTERN 1 4 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ ABELL2146 1 ACS/WFC ACCUM WFC F814W 1 1 none none 1 635.0 S ABELL2146 1 ACS/WFC ACCUM WFC F814W 1 2 none none 1 635.0 S ABELL2146 1 ACS/WFC ACCUM WFC F814W 2 1 none none 1 635.0 S ABELL2146 1 ACS/WFC ACCUM WFC F814W 2 2 none none 1 635.0 S ABELL2146 2 ACS/WFC ACCUM WFC F814W 1 1 none none 2 684.0 S ABELL2146 2 ACS/WFC ACCUM WFC F814W 1 2 none none 2 684.0 S ABELL2146 2 ACS/WFC ACCUM WFC F814W 2 1 none none 2 684.0 S ABELL2146 2 ACS/WFC ACCUM WFC F814W 2 2 none none 2 684.0 S ABELL2146 3 ACS/WFC ACCUM WFC F606W 1 1 none none 3 670.0 S ABELL2146 3 ACS/WFC ACCUM WFC F606W 1 2 none none 3 670.0 S ABELL2146 3 ACS/WFC ACCUM WFC F606W 2 1 none none 3 670.0 S ABELL2146 3 ACS/WFC ACCUM WFC F606W 2 2 none none 3 670.0 S ABELL2146 4 ACS/WFC ACCUM WFC F435W 1 1 none none 4 668.0 S ABELL2146 4 ACS/WFC ACCUM WFC F435W 1 2 none none 4 668.0 S ABELL2146 4 ACS/WFC ACCUM WFC F435W 2 1 none none 4 668.0 S ABELL2146 4 ACS/WFC ACCUM WFC F435W 2 2 none none 4 668.0 S 12871( 2) - 11-Jul-2012 15:39:17 - [ 6] Visit: 02 Visit Requirements: SAME ORIENT AS 01 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 ABELL2146 ACS/WFC ACCUM WFC F814W 1 500 S POS TARG 0.0, see below 93.82200482232652; PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 ABELL2146 ACS/WFC ACCUM WFC F814W 1 500 S POS TARG 0.0, see below 93.82200482232652; PATTERN 1 2 ------------------------------------------------------------------------------------------------------------------------------------ 3 ABELL2146 ACS/WFC ACCUM WFC F606W 1 500 S POS TARG 0.0, see below 93.82200482232652; PATTERN 1 3 ------------------------------------------------------------------------------------------------------------------------------------ 4 ABELL2146 ACS/WFC ACCUM WFC F435W 1 500 S POS TARG 0.0, see below 93.82200482232652; PATTERN 1 4 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ ABELL2146 1 ACS/WFC ACCUM WFC F814W 1 1 none none 1 635.0 S ABELL2146 1 ACS/WFC ACCUM WFC F814W 1 2 none none 1 635.0 S ABELL2146 1 ACS/WFC ACCUM WFC F814W 2 1 none none 1 635.0 S ABELL2146 1 ACS/WFC ACCUM WFC F814W 2 2 none none 1 635.0 S ABELL2146 2 ACS/WFC ACCUM WFC F814W 1 1 none none 2 684.0 S ABELL2146 2 ACS/WFC ACCUM WFC F814W 1 2 none none 2 684.0 S ABELL2146 2 ACS/WFC ACCUM WFC F814W 2 1 none none 2 684.0 S ABELL2146 2 ACS/WFC ACCUM WFC F814W 2 2 none none 2 684.0 S ABELL2146 3 ACS/WFC ACCUM WFC F606W 1 1 none none 3 670.0 S ABELL2146 3 ACS/WFC ACCUM WFC F606W 1 2 none none 3 670.0 S ABELL2146 3 ACS/WFC ACCUM WFC F606W 2 1 none none 3 670.0 S ABELL2146 3 ACS/WFC ACCUM WFC F606W 2 2 none none 3 670.0 S ABELL2146 4 ACS/WFC ACCUM WFC F435W 1 1 none none 4 668.0 S ABELL2146 4 ACS/WFC ACCUM WFC F435W 1 2 none none 4 668.0 S ABELL2146 4 ACS/WFC ACCUM WFC F435W 2 1 none none 4 668.0 S ABELL2146 4 ACS/WFC ACCUM WFC F435W 2 2 none none 4 668.0 S 12871( 2) - 11-Jul-2012 15:39:17 - [ 7] Summary Form for Proposal 12871 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 130D TO 150 D, ORIENT 310D TO 330 D, PATTERN 1 1, PATTERN 1 2, PATTERN 1 3, PATTERN 1 4, POS TARG 0.0, -93.82200482232652, POS TARG 0.0, 93.82200482232652, SAME ORIENT AS 01 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F435W, F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names ABELL2146 ------------------------------------------------------------------------------------------------------------------------------------