First Results from Z-FOURGE: Discovery of a Candidate Cluster at z = 2.2 in COSMOS

Spitler et al. 2012, ApJL in press
[English press release][Dutch press release] [astro-ph] [journal] [ADS] [webpage]

We have discovered the most-distant example of a dense galaxy cluster
in a well-studied region called the COSMOS field.  We used new imaging
to identify very distant galaxies.  The cluster literally 'popped out' in the
FourStar images, as shown below.

Galaxy number density map showing the z=2.2 galaxy cluster and surrounding regions.

This Figure highlights regions on a piece of a sky where there are
dense concentrations of galaxies.  Dark regions have few galaxies,
while the red regions have many galaxies packed into a small region.
The individual galaxies are the small dots, while the big circles
highlight the location of the main galaxy cluster structure.  The bar
at the bottom corresponds to a length of 1.6 million light years.
Compare to other survey data here.

Animation blinking between images in the optical wavelength (from the Hubble Space Telescope) and the near-infrared (from FourStar/Magellan).

This figure shows how our new filters are able to sort out very
distant galaxies from other objects like stars in our galaxy the Milky
Way and nearby galaxies.  With images taken at optical wavelengths, the
clusters do not stand out at all.  When we observed with the new
filters on the FourStar imager on the Magellan telescope at
near-infrared wavelengths, distant galaxies and the galaxy cluster
immediately jumped out at us.  The red objects are very distant
galaxies.

Z-FOURGE near-infrared images (J medium bands) showing subclusters A, B and C.  Here red objects are galaxies in the cluster.