Assigned ID: 913 ----- Attempting Submission 1 (Fri Feb 24 23:32:01 GMT 2012) ----- ----- Attempting Submission 1 (Wed Jul 11 15:23:41 GMT 2012) ----- Pattern is centred correctly with POS_TARG and mosaic shows correct field. 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. 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 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). COSMOLOGY 12 false 0.0 0.0 false false false false SUPPORTED CLUSTER OF GALAXIES GRAVITATIONAL LENS J2000 flux of brightest cluster galaxy ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F435W POS-TARG DITHER DITHER 2 6.10486 LINE false true 3.085553 2 91.16385 POS-TARG ACS-WFC-DITHER-LINE 85.38997 1 true 1341951860840 /home/hrr/Home/PROPOSALS/A2146_HST/OP-cache/12871_phase2mosaic2targetsv3-v01.xml The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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