First results from Z-FOURGE: Discovery of a candidate cluster at z = 2.2 in COSMOS

Lee R. Spitler, Ivo Labbé, Karl Glazebrook, S. Eric Persson, Andy Monson, Casey Papovich, Kim Vy H. Tran, Gregory B. Poole, Ryan Quadri, Pieter Van Dokkum, Daniel D. Kelson, Glenn G. Kacprzak, Patrickj McCarthy, David Murphy, Caroline M.S. Straatman, Vithal Tilvi

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We report the first results from the Z -FOURGE survey: the discovery of a candidate galaxy cluster at z = 2.2 consisting of two compact overdensities with red galaxies detected at ≳ 20σ above the mean surface density. The discovery was made possible by a new deep (Ks ≲ 24.8 AB 5σ) Magellan/FOURSTAR near-IR imaging survey with five custom medium-bandwidth filters. The filters pinpoint the location of the Balmer/4000 break in evolved stellar populations at 1.5 < z < 3.5, yielding significantly more accurate photometric redshifts than possible with broadband imaging alone. The overdensities are within 1′ of each other in the COSMOS field and appear to be embedded in a larger structure that contains at least one additional overdensity (10σ). Considering the global properties of the overdensities, the z = 2.2 system appears to be the most distant example of a galaxy cluster with a population of red galaxies. A comparison to a large ΛCDM simulation suggests that the system may consist of merging subclusters, with properties in between those of z > 2 protoclusters with more diffuse distributions of blue galaxies and the lower-redshift galaxy clusters with prominent red sequences. The structure is completely absent in public optical catalogs in COSMOS and only weakly visible in a shallower near-IR survey. The discovery showcases the potential of deep near-IR surveys with medium-band filters to advance the understanding of environment and galaxy evolution at z > 1.5.

Original languageEnglish (US)
Article numberL21
JournalAstrophysical Journal Letters
Issue number2
StatePublished - Apr 1 2012

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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