The physical factors that determine the linear sizes of massive star clusters are not well understood. Their scale sizes were long thought to be governed by the tidal field of the parent galaxy, but major questions are now emerging. Globular clusters, for example, have mean sizes nearly independent of location in the halo. Paradoxically, the recently discovered "anomalous extended clusters" in M31 and elsewhere have scale sizes that fit much better with tidal theory, but they are puzzlingly rare. Lastly, the persistent size difference between metal-poor and metal-rich clusters still lacks a quantitative explanation. Many aspects of these observations call for better modelling of dynamical evolution in the outskirts of clusters, and also their conditions of formation including the early rapid mass loss phase of protoclusters. A new set of accurate measurements of scale sizes and structural parameters, for a large and homogeneous set of globular clusters, would represent a major advance in this subject. We propose to carry out a (WFC3+ACS) imaging survey of the globular clusters in the supergiant Virgo elliptical M87 to cover the complete run of the halo. M87 is an optimum target system because of its huge numbers of clusters and HST's ability to resolve the cluster profiles accurately. We will derive cluster effective radii, central concentrations, luminosities, and colors for more than 4000 clusters using PSF-convolved King-model profile fitting. In parallel, we are developing theoretical tools to model the expected distribution of cluster sizes versus galactocentric distance as functions of cluster mass, concentration, and orbital anisotropy.

A sequence of 2-orbit visits will be executed for four target fields in the halo of M87, plus one additional target field in the outskirts of the globular cluster M3 for PSF calibration. On each visit, one orbit will be spent with the F475W filter and one orbit with the F814W filter. The scientific goal is to measure the linear scale sizes (effective radii) of the M87 globular clusters and to trace the dependence of r_h versus galactocentric distance. The M3-SOUTH target field is to be used for precise definition of the PSF shape as a function of location on the WFC3 and ACS/WFC cameras, through identical filters as will be used for the M87 fields. Accurate measurement of the globular cluster radii will require convolving the measured PSF with King-type models for the cluster profiles. RESOLVED STELLAR POPULATIONS 12 false 0.0 0.0 false false false false SUPPORTED GALAXY HALO J2000 J2000 V magnitude is brightest globular cluster GALAXY HALO J2000 J2000 GALAXY HALO J2000 J2000 GALAXY HALO J2000 J2000 GALAXY HALO J2000 J2000 WFC3/UVIS Imaging No No F475W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F475W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F475W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F475W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F475W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F475W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F475W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F475W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F475W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F475W ACS/WFC Imaging No No F814W POS-TARG DITHER 0.265 LINE 0.187 false false 69.05 4 20.67 ACS-WFC-DITHER-BOX 1 false POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 2 false POS-TARG DITHER 0.145 LINE false false 2 46.84 WFC3-UVIS-DITHER-LINE 3 false POS-TARG MOSAIC DITHER 4 2.414 WFC3-UVIS-DITHER-BOX false 81.785 true 0.112 0.173 2 85.759 POS-TARG WFC3-UVIS-GAP-LINE 23.884 4 false POS-TARG DITHER 0.135 LINE false false 3 46.84 WFC3-UVIS-DITHER-LINE-3PT 5 false The scheduling data was generated by CASM 19.3. CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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CASM was run in Phase II mode. (Tic-Visit :proposal-id 12532 :visit-id "04" :proposal-version -1 :trans-version "20110509" :date "29-Jul-2011 10:29:14.0" :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 3437840 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12532 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12532 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2912 :al-overhead-at-end 93 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3245 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12532 :visit-id "04" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3023 :al-overhead-at-end 116 :internal-dur-at-end 1019 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3245 :pcs-gap 0 ) true 2 0 The scheduling data was generated by CASM 19.3. 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