----- Attempting Submission 1 (Fri Sep 17 22:26:52 GMT 2010) ----- The Bullet Cluster Reloaded? An in-depth study of two post-collision cluster mergers Detailed X-ray/lensing studies of the massive X-ray selected galaxy clusters 1E0657-56 (the "Bullet Cluster") and MACSJ0025.4-1222 have exploited the segregation of collisional and collisionless matter in cluster mergers to obtain first constraints on the self-interaction cross section of dark matter. We here propose a similar study of two newly discovered very X-ray luminous galaxy clusters from the MACS sample that have been confirmed as linear, post-collision mergers of similar or higher mass. The proposed ACIS-I and HST/ACS observations will allow an accurate mapping of collisional (X-ray gas) and collisionless matter (galaxies and, presumably, dark matter) and thus yield improved constraints on the properties of dark matter.
The depth of the proposed observations is dictated by the requirements of an accurate measurement of the gravitational shear field. Resolved colour information from non-overlapping passbands is critical for the secure identification of faint, multiply imaged background objects and thus an unambiguously determined lens model, as well as for the elimination of unlensed galaxies in the cluster and the foreground, which cause a dilution of the measured shear. We note that several strong-lensing features have already been tentatively identified in the existing HST snapshot. The weak-lensing analysis of the large-scale mass distribution will be conducted in the F814W passband which provides the greatest depth. Since this requires high-precision measurements of the ellipticity of faint background galaxies across the full field of view, we request LOW SKY mode. Earth glow would drastically reduce the sensitivity of these observations by raising the limiting magnitude, and thus lower the number of background galaxies for which accurate shape measurements can be obtained. A single pointing with ACS will suffice since the mass-sheet degeneracy of the weak-lensing shear field can be broken with strong-lensing constraints. However, coverage of the full ACS/WFC field of view is critical to probe the gravitational shear field to a radius of at least 400 kpc. We request SAME ORIENTATION for all observations, as random orientations of the ACS field of view for observations in different passbands would severely compromise the availability of colour information (important to mitigate dilution of the weak-shear signal from unlensed foreground and cluster galaxies) in the outer cluster regions. 12 false 0.0 0.0 false false false false false false SUPPORTED CLUSTER OF GALAXIES GRAVITATIONAL LENS RICH CLUSTER SHOCK FRONT J2000 05 53 24.0000 -33 42 30.00 J2000 ACS/WFC Imaging No No F435W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W POS-TARG DITHER DITHER 3 3.011 6-position dither ACS-WFC-DITHER-LINE true true 0.138 2 85.28 POS-TARG ACS-WFC-DITHER-LINE 32.99 1 true POS-TARG DITHER DITHER 2 3.011 4-position dither ACS-WFC-DITHER-LINE true true 0.138 2 85.28 POS-TARG ACS-WFC-DITHER-LINE 32.99 2 true /Users/ebeling/Desktop/OP-cache/12362-v01.xml The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12362 :visit-id "01" :proposal-version -1 :trans-version "20100505" :date "17-Sep-2010 12:22:22.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 5 :saa-model "27" :si-bright-earth-limb-angle 40.0 :data-volume 4547584 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12362 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12362 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2762 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "Y" :pcs-visibility 3191 :pcs-gap 425 ) (Tic-Orbit :proposal-id 12362 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 2680 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "Y" :pcs-visibility 3191 :pcs-gap 425 ) (Tic-Orbit :proposal-id 12362 :visit-id "01" :orbit-number 3 :pcs-time 222 :si-vis-utilization 2762 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "Y" :pcs-visibility 3191 :pcs-gap 425 ) (Tic-Orbit :proposal-id 12362 :visit-id "01" :orbit-number 4 :pcs-time 222 :si-vis-utilization 2740 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "Y" :pcs-visibility 3191 :pcs-gap 425 ) (Tic-Orbit :proposal-id 12362 :visit-id "01" :orbit-number 5 :pcs-time 222 :si-vis-utilization 2763 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "Y" :pcs-visibility 3191 :pcs-gap 425 ) true 5 0