We propose to study the physical nature of dark matter by using massive, merging clusters of galaxies. As shown with the Bullet Cluster (1E0657-56), such massive well-measured systems are critical for our understanding of dark matter. By more than doubling the number of clusters in the sample and obtaining systems at different observation angles, impact parameters, geometrical arrangements, and merger velocities, the systematic uncertainties in the dark matter cross section calculations can be improved substantially, allowing us to move from rough order of magnitude estimates to measurements with quantifiable uncertainties that can be compared usefully with the predictions from numerical simulations, and the constraints on alternate gravity models become unambiguous. Our proposed targets are three extraordinary, merging galaxy clusters with X-ray and optical offsets that are placed at ideal redshifts for such a study; A520, A1758N, and A2163. To pin down the position of the dark matter component we require high resolution, absolutely calibrated mass maps. High resolution gravitational lensing data is needed to attain this goal, which can only be achieved with the excellent resolving power of the HST.

The goal of this program is to obtain a weak lensing mass reconstruction of three merging cluster systems and measure if the centroid of the lensing signal is offset from the position of the X-ray plasma, which contains the bulk of the visible mass in clusters. Required Area: Because we are needing to calculate the centroid of the weak lensing signal from measurements of the gravitational shear, which is a non-local statistic, we need to measure shapes of background galaxies in a wide region around the possible locations of the centroids. We therefore are requesting the minimum number of pointings per cluster which provide us with a ∼ 1.5' buffer between the image edge and the expected centroid locations. Using this criteria we need 2 pointings in A1758N, 4 in A520, and 2 in A2163. Required Filters: In addition to measuring the shapes of faint galaxies, we also need to be able to determine which galaxies are behind the clusters and therefore being lensed, and which are foreground or cluster dwarf galaxies, which are not being lensed. Without the ability to remove the non-lensed dwarf galaxies from the background galaxy sample, the signal-to-noise of the mass reconstruction is greatly diminished (Clowe & Schneider 2001) and the error in the centroid of the weak lensing peaks greatly increases. We found with the Bullet Cluster observations that a 3 passband set of F814W, F606W, and F435W allowed us to use cuts in color-color space to remove the dwarf galaxies while still having a sufficiently large number density of background galaxies to measure the weak lensing centroid with high significance. Required Depth: Our target sensitivity for this program is defined by the need to get shape measurements for ∼ 90 background galaxies/arcmin2 to measure the weak lensing peak centroids to the desired accuracy. To do this we need to obtain photometry at a S/N = 10σ at IF 814W = 27.5 for a galaxy, which we can get (calculated from both the ACS ETC and our experience with the 1E0657-558 dataset) in 2 orbits with the F814W filter. For color selection, a single orbit in F435W and F606W will provide us the necessary photometric S/N. Dithering and Mosaicing Pattern: Our observing plan is to take 4 exposures per orbit, with a chip gap spanning offset between each exposure. Due to the 5 orbit limit per visit, we will schedule each mosaic pointing as a block of 4 orbits to take all the passbands for that pointing in the same visit. The other pointings for the same cluster will need to be constrained to occur at a similar rotation angle as the first (or 180 degrees to it). We will keep a small ∼ 10′ overlap region between the pointings to detect any systematic errors between the shape measurements of the different pointings. COSMOLOGY 12 false 0.0 0.0 false false false false false false SUPPORTED CLUSTER OF GALAXIES RICH CLUSTER J2000 13 32 44.5568 +50 32 44.79 J2000 CLUSTER OF GALAXIES RICH CLUSTER J2000 13 32 44.5568 +50 32 44.79 J2000 CLUSTER OF GALAXIES RICH CLUSTER J2000 04 54 10.6356 +02 56 27.21 J2000 CLUSTER OF GALAXIES RICH CLUSTER J2000 04 54 10.6356 +02 56 27.21 J2000 CLUSTER OF GALAXIES RICH CLUSTER J2000 04 54 10.6356 +02 56 27.21 J2000 CLUSTER OF GALAXIES RICH CLUSTER J2000 04 54 10.6356 +02 56 27.21 J2000 CLUSTER OF GALAXIES RICH CLUSTER J2000 16 15 47.5510 -06 08 51.63 J2000 CLUSTER OF GALAXIES RICH CLUSTER J2000 16 15 47.5510 -06 08 51.63 J2000 ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F435W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F435W 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 85.389987 1 true The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. A1758N pointing 1 (Tic-Visit :proposal-id 12253 :visit-id "01" :proposal-version -1 :trans-version "20100505" :date "25-Jun-2010 01:57:28.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 4547584 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12253 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12253 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3110 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3445 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3220 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3445 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "01" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3223 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3445 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "01" :orbit-number 4 :pcs-time 222 :si-vis-utilization 3223 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3445 :pcs-gap 0 ) true 4 0 The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. A1758N pointing 2 (Tic-Visit :proposal-id 12253 :visit-id "02" :proposal-version -1 :trans-version "20100505" :date "25-Jun-2010 01:56:46.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 4547584 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12253 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12253 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3110 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3445 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "02" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3220 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3445 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "02" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3223 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3445 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "02" :orbit-number 4 :pcs-time 222 :si-vis-utilization 3223 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3445 :pcs-gap 0 ) true 4 0 Same orient constraint links visits (03 04 05 06) The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. A520 pointing 1 (Tic-Visit :proposal-id 12253 :visit-id "03" :proposal-version -1 :trans-version "20100505" :date "25-Jun-2010 01:48:28.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 4547584 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12253 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12253 :visit-id "03" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2898 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "03" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3012 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "03" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3011 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "03" :orbit-number 4 :pcs-time 222 :si-vis-utilization 3011 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) true 4 0 Same orient constraint links visits (03 04 05 06) The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. A520 pointing 2 (Tic-Visit :proposal-id 12253 :visit-id "04" :proposal-version -1 :trans-version "20100505" :date "25-Jun-2010 01:48:34.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 4547584 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12253 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12253 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2898 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "04" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3012 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "04" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3011 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "04" :orbit-number 4 :pcs-time 222 :si-vis-utilization 3011 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) false 4 0 Same orient constraint links visits (03 04 05 06) The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. A520 pointing 3 (Tic-Visit :proposal-id 12253 :visit-id "05" :proposal-version -1 :trans-version "20100505" :date "25-Jun-2010 01:48:41.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 4547584 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12253 :visit-id "05" :obset-id "05" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12253 :visit-id "05" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2898 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "05" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3012 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "05" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3011 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "05" :orbit-number 4 :pcs-time 222 :si-vis-utilization 3011 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) true 4 0 Same orient constraint links visits (03 04 05 06) The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. A520 pointing 4 (Tic-Visit :proposal-id 12253 :visit-id "06" :proposal-version -1 :trans-version "20100505" :date "25-Jun-2010 01:48:47.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 4547584 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12253 :visit-id "06" :obset-id "06" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12253 :visit-id "06" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2898 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "06" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3012 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "06" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3011 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "06" :orbit-number 4 :pcs-time 222 :si-vis-utilization 3011 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3234 :pcs-gap 0 ) true 4 0 The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. A2163 pointing 1 (Tic-Visit :proposal-id 12253 :visit-id "07" :proposal-version -1 :trans-version "20100505" :date "01-Jul-2010 16:55:41.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 4547584 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12253 :visit-id "07" :obset-id "07" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12253 :visit-id "07" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2906 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3242 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "07" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3020 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3242 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "07" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3019 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3242 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "07" :orbit-number 4 :pcs-time 222 :si-vis-utilization 3019 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3242 :pcs-gap 0 ) true 4 0 The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. A2163 pointing 2 (Tic-Visit :proposal-id 12253 :visit-id "08" :proposal-version -1 :trans-version "20100505" :date "25-Jun-2010 01:50:04.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 4547584 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12253 :visit-id "08" :obset-id "08" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12253 :visit-id "08" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2906 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3242 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "08" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3020 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3242 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "08" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3019 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3242 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12253 :visit-id "08" :orbit-number 4 :pcs-time 222 :si-vis-utilization 3019 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3242 :pcs-gap 0 ) false 4 0