In the news this month... a record-breaking distant cluster
X-ray and optical composite of JKCS041 CREDIT: X-ray: NASA/CXC/INAF/S.Andreon et al Optical: DSS; ESO/VLT
Look deep enough with a sensitive telescope and a seemingly empty patch of sky is full of galaxies. Look closely and you'll see that they are often gathered together in clusters. These massive collections of galaxies are the largest gravitationally bound objects in the universe, but it is uncertain how long ago these clusters formed. Now, using a variety of instruments, a team led by Stefano Andreon of the National Institute for Astrophysics in Milan, Italy, has discovered the most distant galaxy cluster ever found.
The cluster, known as JKCS041, is located in the constellation of Cetus and lies about 10.2 billion light years away, beating the previous record holder by almost one billion light years. It is so far away that the light now arriving at Earth was produced by the cluster when the universe was only about a quarter of its current age.
The astronomers first discovered the galaxy in infra-red observations made with the UK Infra Red Telescope, UKIRT, in 2006. The optical light from galaxies this far away is shifted into the infra-red part of the spectrum due to the expansion of the universe, so old galaxies like these are often detected by infra-red telescopes. Further observations with both optical and infra-red telescopes confirmed the distance to the object, but could not rule out the possibility that, rather than being a genuine gravitationally bound cluster, the object could just be a chance alignment of galaxies along our line of sight. To test this, the team examined X-ray observations from the Chandra space telescope.
Nearby galaxy clusters have extended X-ray emission, caused by hot gas in the space between the galaxies. This gas, known as the hot intra-cluster medium, is only observed in genuine gravitationally bound clusters of galaxies and so is a good test of whether a group of galaxies just lie along the same line of sight by chance, or are physically associated. When the astronomers examined the Chandra observations of JKCS041, they found a significant amount of extended X-ray emission within the cluster coming from hot gas of the intra-cluster medium, showing that it is a physically connected group of galaxies.
This is an important discovery because this is close to the distance limit expected for a galaxy cluster based on how long it should take for them to assemble following the big bang, and studying its characteristics can reveal more about how the universe evolved.
Andreon, S., Maughan, B., Trinchieri, G., & Kurk, J. (2009). JKCS041: a colour-detected galaxy cluster at zphot~1.9 with deep potential well as confirmed by X-ray data Astronomy and Astrophysics DOI: 10.1051/0004-6361/200912299