Beyond MACS: A Snapshot Survey of the Most Massive Clusters of Galaxies at z>0.5Truly massive galaxy clusters play a pivotal role for a wealth of extragalactic and cosmological research topics, and SNAPshot observations of these systems are ideally suited to identify the most promising cluster targets for further, in-depth study. The power of this approach was demonstrated by ACS/WFC3 SNAPshots of X-ray selected MACS and eMACS clusters at z>0.3 obtained by us in previous Cycles (44 of them in all of F606W, F814W, F110W, and F140W). Based on these data, the CLASH MCT program selected 16 out of 25 of their targets to be MACS clusters. Similarly, all but one of the six most powerful cluster lenses selected for in-depth study by the HST Frontier Fields initiative are MACS detections, and so are 16 of the 29 z>0.3 clusters targeted by the RELICS legacy program.
We propose to extend our spectacularly successful SNAPshot survey of the most X-ray luminous distant clusters to a redshift-mass regime that is poorly sampled by any other project. Targeting only extremely massive clusters at z>0.5 from the X-ray selected eMACS sample (median velocity dispersion: 1180 km/s), the proposed program will (a) identify the most powerful gravitational telescopes at yet higher redshift for the next generation of in-depth studies of the distant Universe with HST and JWST, (b) provide constraints on the mass distribution within these extreme systems, (c) help improve our understanding of the physical nature of galaxy-galaxy and galaxy-gas interactions in cluster cores, and (d) unveil Balmer Break Galaxies at z~2 and Lyman-break galaxies at z>6 as F814W dropouts.
Acknowledging the broad community interest in our sample we waive our data rights for these observations.We propose ACS and WFC3 SNAPshot observations of all eMACS clusters at z>0.5 not yet observed with HST in our program's four filters. Our targets are uniformly distributed over the high-Galactic latitude sky (|b|>20 deg), dec > -30 deg, making ours an ideal SNAPshot sample. Since all of our science goals require or greatly benefit from color information, we aim for three independent visits per cluster target (one each for ACS/F606W and F814W imaging, and a third one for SNAPshots with WFC3/F110W/F140W). Counting each visit as a separate target (as indicated in Section 3.3.3 of the CfP) but discounting visits already performed for some of our targets in previous SNAP programs, we are left with 174 distinct "targets". Some of these will be used as backup (we suspect the respective clusters to be less massive than suggested by the RASS X-ray luminosity). From the remainder, 166 visits will be selected to match the number of orbits awarded.Cosmology0falseSUPPORTEDCLUSTER OF GALAXIESHIGH REDSHIFT CLUSTERJ2000ACS/WFCImagingNoNoF606WACS/WFCImagingNoNoF814WWFC3/IRImagingNoNoF110WWFC3/IRImagingNoNoF140WPOS-TARGDITHER3.011LINEfalsetrue385.28ACS-WFC-DITHER-LINE1falsePOS-TARGDITHER0.636LINEfalsefalse241.788WFC3-IR-DITHER-LINE2falseThe scheduling data was generated by CASM 25.2.CASM was run in Phase II mode./private/var/folders/56/c_jk44c970v8fj73rtx427cm0006v6/T/OPVisit2811958167907656357.xml(Tic-Visit
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