Gemini Frontier Fields: Wide-field Adaptive Optics K s -band Imaging of the Galaxy Cluster MACS J0416.1-2403 1 M. Schirmer 1 , R. E. Carrasco 1 , P. Pessev 1 , V. Garrel 1 , C. Winge 1 , B. Neichel 2,1 , F. Vidal 3,1 1 Gemini Observatory, Casilla 603, La Serena, Chile 2 Aix Marseille Universit´ e, CNRS, Laboratoire d’Astrophysique de Marseille, UMR 7326, 13388, Marseille, France 3 Observatoire De Paris, Place Jules Janssen, 92190 Meudon, France mschirme@gemini.edu ABSTRACT The Hubble Space Telescope (HST) Frontier Fields Campaign targets six massive clusters of galaxies, exploiting the strong gravitational lensing effect to study the distant Universe. At Gemini South we ob- serve the three southern-most clusters in K s -band, overcoming HST/WFC3’s sensitivity cut-off redwards of 1.7μm. We use the Gemini Multi-Conjugate Adaptive Optics System (GeMS) and the Gemini South Adaptive Optics Imager (GSAOI), delivering near diffraction-limited images on arcminute scales. In this paper we describe our public release of 100 ′′ × 110 ′′ wide images of the first target, MACS J0416.1-2403. With a 5σ depth of K s ∼ 23.8 mag for extended sources our images are shallower than the HST/WFC3 data. Yet we have achieved an angular resolution of 0. ′′ 07−0. ′′ 10, twice as high as HST/WFC3, using a single natural guide star only. The data were distortion corrected and registered with sub-pixel ac- curacy despite only a few low-S/N extended sources are visible in the individual exposures. This is a demonstration that even for fields at high galactic latitude, where natural guide stars are scarce, current multi-conjugated adaptive optics technology at 8m-telescopes has opened a new window on the distant Universe. We provide fully calibrated, co-added images matching the native GSAOI pixel scale as well as the larger plate scales and the WCS projections of the HST release. Two more Frontier Fields, Abell 2744 and Abell S1063, will be observed with GeMS/GSAOI, adding to the legacy value of this community effort. Subject headings: Techniques: image processing, Instrumentation: adaptive optics, Galaxies: Clusters: individual (MACS J0146.1-2403) 1. Introduction The HST Frontier Fields campaign combines deep observations of six massive galaxy clusters in the optical and the infrared, using the Advanced Camera for Surveys (ACS) and WFC3, respectively. 1 Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Minist´ erio da Ciˆ encia, Tecnologia e Inovac ¸˜ ao (Brazil) and Ministerio de Ciencia, Tecnolog´ ıa e Innovaci´ on Productiva (Argentina). The magnification effect from strong gravitational lensing in these clusters provides a unique oppor- tunity for studies of the galaxy population in the young Universe. The wavelength coverage with HST/ACS and HST/WFC3 ranges from 0.43μm to 1.6μm. Complementary observations have been ob- tained with Chandra (PI: Murray, S.), Spitzer (PIs: Soifer, T., Capak, P.), Subaru/SuprimeCam (Postman et al. 2011), VLT/HAWK-I (Owers et al. 2011) and AAT/AAOmega (Ebeling et al. 2014). HST/WFC3 is insensitive for wavelengths longer than 1.7μm 2 . This design was chosen to allow for 2 http://www.stsci.edu/hst/nicmos/documents/handbooks/ current NEW/c04 imaging.6.8.html 1 arXiv:1409.1820v1 [astro-ph.GA] 5 Sep 2014