Massive galaxy cluster contains a super-mom of star birth

The Phoenix Cluster (SPT-CLJ2344-4243) is a galaxy cluster located in the constellation Phoenix. It measures about 7.3 million light years wide, making it one of the most massive galaxy cluster.

It was possibly dormant for billions of years, but has recently become active in star formation. The rate of star formation occurring is the highest ever recorded in the middle of a galaxy cluster. Observations by NASA’s Chandra X-ray Observatory show the recent creation of 740 solar masses (stars) per year. This is considerably higher than the Perseus Cluster, where stars are formed around 20 times slower.

Phoenix Cluster is also producing more X-rays than any other known massive cluster. Data from observations suggest that hot gas is cooling in the central regions at the highest rate ever recorded. The central galaxy in the cluster contains vast amounts of hot gas. More normal matter is present there, than the total of all the other galaxies in the cluster.

In the center of the system, there is a supermassive black hole growing very rapidly. It is expanding approximately at a rate of 60 times the mass of our Sun each year. Its current size is about 20 billion times the mass of the Sun.

The Phoenix Cluster was initially detected by Rashid Sunyaev and others using the Sunyaev-Zeldovich effect by the South Pole Telescope.

Image: Phoenix Cluster

Most massive ever?
The Phoenix Cluster is extraordinarily massive: about 2,000 times the apparent mass of the Milky Way, or 2.5 quadrillion times the mass of the sun.

It’s in a dead heat for the most massive galaxy cluster. The record holder,  El gordo slightly more massive, but the uncertainty in this estimate is high — it could turn out that with more careful measurements, Phoenix is more massive.”

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Indeed, the Phoenix Cluster is the most  X-ray luminous galaxy cluster found yet, about 35 percent brighter than the previous record holder. All the X-rays it sheds suggest the Phoenix Cluster is also the fastest-cooling cluster yet known, since cooling of hot gas in the cluster is what is thought to produce the X-rays.

Star formation
Researchers say the central galaxy in the Phoenix cluster is apparently forming a whopping 740 stars per year, based on images acquired from 10 different telescopes in space and on the ground around the world that observed the cluster at a variety of wavelengths. The previous record holder was Abell 1835, a galaxy that forms about 100 stars per year.

“If you look at normal galaxy clusters, the central, most massive galaxy is typically forming stars at a rate of one new star every few years,” McDonald said. “It’s a huge difference.”

The central massive galaxies are essentially the first to form within such clusters.  Being billions of years old, they are expected to have been dormant a very long time. That makes this “starburst” activity seen in the Phoenix cluster’s central galaxy even more extraordinary.

“Central galaxies have typically been referred to as ‘red and dead’ — just a bunch of old stars orbiting a massive black hole, and there’s nothing new happening,” McDonald said. “But the central galaxy in this cluster has somehow come to life and is giving birth to prodigious numbers of new stars.”

This colossal rate of star formation may shed new light on a decades-old conundrum regarding the evolution of galaxy clusters. Gas at the core of a cluster streaming from nearby galaxies and supernova explosions should naturally cool over time, forming a flow cold enough to condense and form new stars. However, scientists have long been at a loss to find any galaxy cluster that actually cools at the rates predicted.

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The cooling problem
One explanation for this “cooling flow problem” may be that a cluster’s natural cooling somehow gets interrupted. For instance, the super massive black hole suspected to lurk at the centers of many of these clusters might emit jets of particles that reheat the core, preventing it from cooling entirely.

“What’s interesting about the Phoenix Cluster is that we see such a large fraction of the cooling that was predicted,”  “It could be that this is earlier in the evolution where there’s nothing stopping it, so it cools and becomes a starburst.”

In the future, “ideally we’d like to find more systems like this, “It’s really difficult to draw meaningful conclusions based on a single system.

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