We propose WFC3/IR imaging and grism spectroscopy of two newly discovered galaxy clusters at z~2, both of which are independently identified using Spitzer/IRAC colors and accurate photometric redshifts from medium-band near-infrared filters. Only with WFC/IR can we obtain the ~hundreds of spectra needed to separate both active and quiescent cluster galaxies from the field. We will combine the grism spectroscopy with our multi-wavelength data to measure redshifts for ~30 massive members and push down the luminosity function to L~0.3L* with an accuracy of sigma(z)/(1+z)=0.0034. The spectra also provide D4000 to measure stellar ages and oxygen emission to track star formation. The WFC3/IR imaging provides resolution of ~2 kpc to measure the physical properties of individual members and classify fainter members and their companions to H(160)~25.5. We will combine these WFC3/IR observations with public CANDELS and 3D-HST datasets of field galaxies to: (1) Compare the morphologies, sizes, stellar masses, luminosities, color maps, and star formation histories of cluster galaxies at z~2 to well-studied relations at lower redshift to determine how cluster galaxies evolve; (2) Compare to the field at z~2 to test for accelerated evolution in the clusters; (3) Measure the minor/major merger fraction to determine if cluster galaxies grow by accretion or in-situ star formation; and (4) Determine why cluster galaxies stop forming stars, i.e. via "mass-quenching" or "environment-quenching". We will effectively double our cluster sample at this pivotal epoch when members are transitioning from intensely star-forming systems to the passive galaxies that dominate local clusters. Obtain WFC3/IR imaging and grism spectroscopy of two galaxy clusters at z~2 For IRC0222A (z~1.6), we require 10 orbits with the G102 grism and for IRC0222B (z~2), we require 8 orbits with the G141 grism. Each set of orbits will be divided into two visits separated by ORIENT of ~10 degrees to minimize overlap of spectra. To maximize the scheduling window, we have selected the primary roll angle to be ~60 degrees and the second roll angle to be ~70 degrees. Each galaxy cluster will also be imaged with WFC3/IR for 3 orbits: F105W (1 orbit), F125W (2/3rd orbits), and F160W (4/3rd orbits). UNRESOLVED STELLAR POPULATIONS AND GALAXY STRUCTURE 12 false 0.0 0.0 false false false false SUPPORTED CLUSTER OF GALAXIES HIGH REDSHIFT CLUSTER J2000 J2000 redshift=1.6 CLUSTER OF GALAXIES HIGH REDSHIFT CLUSTER Potentially bright star ~1.3 arcmin to Southwest J2000 J2000 redshift=1.95 WFC3/IR Imaging No No F125W WFC3/IR Imaging No No F160W WFC3/IR Imaging No No F105W WFC3/IR Spectroscopic No No G102 WFC3/IR Spectroscopic No No G102 WFC3/IR Imaging No No F125W WFC3/IR Imaging No No F160W WFC3/IR Imaging No No F105W WFC3/IR Spectroscopic No No G141 WFC3/IR Spectroscopic No No G141 The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. Direct imaging of high redshift galaxy cluster using F105W, then multiple exposures using Grism 102W. Second visit of 2 where each visit contains 5 orbits. The visits are separated by ORIENT of ~10 degrees to minimize overlapping spectra. 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CASM was run in Phase II mode. Direct imaging of high redshift galaxy cluster using F105W, then multiple exposures using Grism 102W. Second visit of 2 where each visit contains 5 orbits. The visits are separated by ORIENT of ~10 degrees to minimize overlapping spectra. 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Imaging of high redshift cluster in 3 orbits split between F105W (1 orbit), F125W (2/3 orbit), and F160W (4/3 orbit) (Tic-Visit :proposal-id 12896 :visit-id "04" :proposal-version -1 :trans-version "20120604" :date "10-Jul-2012 15:49:23.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 3272912 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12896 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12896 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2630 :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 3015 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12896 :visit-id "04" :orbit-number 2 :pcs-time 222 :si-vis-utilization 2773 :al-overhead-at-end 0 :internal-dur-at-end 728 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3015 :pcs-gap 0 ) false 2 0 The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. Direct imaging of high redshift galaxy cluster using F140W, then multiple exposures using Grism 141. First visit of 2 where each visit contains 4 orbits. The visits are separated by ORIENT of ~10 degrees to minimize overlapping spectra. 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CASM was run in Phase II mode. Imaging of high redshift cluster in 3 orbits split between F105W (1 orbit), F125W (2/3 orbit), and F160W (4/3 orbit) (Tic-Visit :proposal-id 12896 :visit-id "11" :proposal-version -1 :trans-version "20120604" :date "10-Jul-2012 15:51:59.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 1775728 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12896 :visit-id "11" :obset-id "11" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12896 :visit-id "11" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2680 :al-overhead-at-end 0 :internal-dur-at-end 557 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3015 :pcs-gap 0 ) false 1 0 The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. 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