Assigned ID: 989 ----- Attempting Submission 1 (Sat Feb 27 00:53:35 GMT 2010) ----- This is a resubmission of the Phase for our Cycle 18 program, which has not executed yet, but is on the calendar for later in 2011. POS TARGs were changed on all exposures, based on recommendation of the WFC3 team. ----- Attempting Submission 1 (Fri Jul 09 13:19:59 GMT 2010) ----- ----- Attempting Submission 2 (Thu Jul 21 14:22:55 GMT 2011) ----- ----- Attempting Submission 3 (Mon Aug 22 19:09:10 GMT 2011) ----- Intermediate Mass Black Holes (IMBHs) are objects of considerable astrophysical significance, and the centers of globular clusters may be our best chance of detecting them. Tentative evidence has been presented for, e.g., G1, Omega Cen, and M54. These are all suspected to be the remnant nuclei of disrupted dwarf galaxies, with M54 residing at the center of the Sagittarius dwarf galaxy. IMBHs in these clusters could indicate an important connection between globular cluster black holes and the super-massive black holes known to exist in galactic nuclei. However, the line-of-sight velocity studies from which the IMBH evidence was obtained are not clear-cut. We have demonstrated that HST proper motion studies can provide much better constraints. We recently published 170,000 proper motions for Omega Cen, and our modeling of the results significantly weakened the case for an IMBH. Here we propose a similar study for M54. Ground-based spectroscopy by one of us found a peak in velocity dispersion which could be fit by either a 10^4 solar mass IMBH or an anisotropic velocity distribution; it also found a surprising offset between the kinematic and star count centers. The proposed proper motions will validate and refine our knowledge of the dispersion peak; they will directly measure the anisotropy through the ratio of radial and tangential proper motions; and they will allow an improved determination of the kinematic center from the proper motion dispersion field. We will derive a high quality proper motion catalog of thousands of stars to put strong new constraints on the M54 cluster dynamics, structure, and possible presence of an IMBH.

We will observe M54 with the WFC3/UVIS, with the F438W filter, using the subarray mode, for 2 orbits. Our observation strategy will be focused on the core of the cluster, for two reasons. First, the core is where an IMBH would reside and leave its mark. Second, the velocity dispersion in M54 drops with radius. Outside the core the proper motion velocity dispersion would be hard to measure, because it becomes comparable to our expected random errors. The choice of a blue filter is motivated by the fact that it reduces the prominence of the brightest red giant stars in the images. The choice of WFC3/UVIS over ACS/WFC is motivated by three considerations: WFC3/UVIS has a smaller pixel size, better blue sensitivity, and higher cosmetic quality (very little charge transfer efficiency degradation). The choice of subarray mode reduces overheads associated with buffer dumps, therefore allowing for more exposure time and exposures per orbit. We are planning 10 2048x2048 exposures, per orbit. The resulting field of view (FOV; 80x 80 arcsec) is more than sufficient to cover the M54 core (core radius equals 7 arcsec, half-mass radius equals 30 arcsec). The FOV will overlap 100% with the epoch 1 ACS/WFC data from GO-10775, independent of the telescope orientation. The 20 total exposures are split over 10 positions. At each position we will obtain one short (30s) and one long (200-300s) exposure. The 10 positions were chosen by Jay Anderson based on the output of custom scripts. N/A N/A N/A RESOLVED STELLAR POPULATIONS 12 false 0.0 0.0 false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER NUCLEUS Distance is 26.8 kpc. J2000 J2000 WFC3/UVIS Imaging No No F438W 1314040054439 /private/var/folders/Us/UsFtG5GQEiuyiHNsecbW+E++4bQ/-Tmp-/OPVisit3091126569544636001.xml The scheduling data was generated by CASM 19.3. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12274 :visit-id "01" :proposal-version -1 :trans-version "20110509" :date "22-Aug-2011 15:07:32.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1407792 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12274 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12274 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2939 :al-overhead-at-end 167 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3275 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12274 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3049 :al-overhead-at-end 167 :internal-dur-at-end 654 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3275 :pcs-gap 0 ) true 2 0