Most distant dense galaxy cluster found

One of the “big questions” in modern astronomy is how do galaxies form and evolve and how is galaxy evolution effected by environment?  Observationally we know that young galaxies contain a lot of gas and hence are efficient at forming stars.  The light from these young galaxies is dominated by short-lived giant O and B type stars and hence the galaxies are blue. As galaxies age, they uses up their supply of gas and star formation is exhausted. The O and B stars are extinguished and the galaxy becomes “red and dead”.  When we look at high redshift galaxies, this is what we find – galaxies are generally blue.

But not all galaxies are not found in isolation. They are found in larger gravitationally-bound groups and clusters. In the local Universe, galaxy clusters typically comprise a few hundred to a few thousand galaxies. If we look at high redshift clusters beyond about z~1.5, we find rather diffuse proto-clusters containing young blue star-forming galaxies.

Swinburne astronomers Lee Spitler, Karl Glazebrook, Greg Poole and Glenn Kacprzak are part of the international FourStar Galaxy Evolution Survey (Z-FOURGE) team who this week have announced the most distant discovered dense galaxy cluster which might contain the “missing link” between cluster that contain young blue galaxies and old “red and dead” galaxies. The cluster contains about 30 galaxies at a redshift of z=2.2, or about 10.5 billion light years away, in the Hubble Space Telescope’s COSMOS field.  While finding the most distant galaxy cluster is pretty exciting, even more amazing is to find it in such a well studied region as the COSMOS field.

Z-FOURGE cluster

Galaxy number density map showing the z=2.2 galaxy cluster and surrounding regions. (Credit: Spitler et al. 2012)

The COSMOS field has been observed for thousands of hours by astronomers, but the new cluster was not visible at optical wavelengths. Using the FourStar infrared camera on the Magellan 6.5-m telescope in Chile, the team have applied a new observational technique whereby they conduct deep, near-infrared imaging with a set of medium bandwidth filters (compared to traditional broadband filters) between 1 and 1.8 μm.  This allows them to trace the old stellar populations in galaxies within the redshift range of z = 1.5-3.5 via the Balmer/4000Å break (as the 4000Å lines gets redshifted to about 1 μm). The Balmer break at 4000Å  results from absorption by metal lines in the atmospheres of old stars.

It is hard to study these high redshift galaxies spectroscopically because they are very faint, and traditional broadband photometric techniques cannot accurately determine their redshift.  Using this medium broadband filters, Spitler and collaborators are able to accurately determine the redshifts of 313 galaxies in the COSMOS field in a redshift slice from 2.1 to 2.3.  They found an over-density of 29 galaxies which they claim is a galaxy cluster.  The cluster is broken into three sub-clumps, two of which contain a high fraction of red galaxies.  The team suggest that they might have caught three sub-clusters in the process of merging, and that the cluster might represent a transition from high redshift blue “proto-clusters” to low redshift, red dominated clusters.  The results will be published in The Astrophysical Journal Letters.  For more information, see

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