The structures and star populations of massive galaxies appear to change as they age, but much about how these galaxies formed and evolved remains mysterious.
Many of the oldest and most massive galaxies reside in clusters, enormous structures where numerous galaxies are found concentrated together. Galaxy clusters in the early universe are thought to be key to understanding the life cycles of old galaxies, but to date astronomers have located only a handful of these rare, distant structures.
Galaxies are normally found in groups or larger collections called clusters. The Local Group consists of our own galaxy, the larger spiral galaxy Andromeda (M31) and several smaller satellites, including the Large and Small Magellenic Clouds.
New research from a team led by Carnegie’s Andrew Newman has confirmed the presence of an unusually distant galaxy cluster, JKCS 041.
“Our observations make this galaxy cluster one of the best-studied structures from the early universe,” Newman said.
Although the team began studying JKCS 041 in 2006, it has taken years of observing with many of the world’s most powerful telescopes to finally confirm its distance. The team used the Hubble Space Telescope to capture sharp images of the distant cluster and split the starlight from the galaxies into its constituent colors, a technique known as spectroscopy. They found 19 galaxies at precisely the same great distance of 9.9 billion light-years, the telltale sign of an early galaxy cluster.
A previous study using the Chandra X-ray Observatory discovered X-ray emissions in the location of JKCS 041.
“These X-rays likely originate from hot gas in JKCS 041, which has been heated to a temperature of about 80 million degrees by the gravity of the massive cluster,” said team member Stefano Andreon of the Osservatorio Astronomico di Brera, who led a companion paper published by Astronomy & Astrophysics.
Today the largest and oldest galaxies are found in clusters, but there is a mystery about when and why these giant galaxies stopped forming new stars and became dormant, or inactive. Peering back to a time when the galaxies in JKCS 041 were only 1 billion years old — or 10 percent of their present age — the team found that most had already entered their dormant phase.
“Because JKCS 041 is the most-distant known cluster of its size, it gives us a unique opportunity to study these old galaxies in detail and better understand their origins,” Newman said.
Once massive galaxies enter their dormant phase, they continue to expand in overall size. This is thought to occur as galaxies collide with one another and evolve into a new, larger galaxy. Early clusters are suspected to be prime locations for these collisions, but to the team’s surprise they found that the galaxies in JKCS 041 were growing at nearly the same rate as non-cluster galaxies.