Astronomers vindicated by frenetic Phoenix Cluster star formation

Posted by TG Daily Staff

Evidence for what astronomers suspect happens at the cores of the largest galaxy clusters is somewhat scarce.

Indeed, the prevailing theory hypothesizes that cooling flows of gas should sink toward a cluster's center, thereby sparking extreme star formation.

However, scientists lacked actual proof for their hypothesis - until the recent analysis of the unique Phoenix Cluster.

To be sure, the situation changed dramatically when a large international team of over 80 astronomers, led by Massachusetts Institute of Technology's Hubble Fellow Michael McDonald, studied the cluster and found evidence of a starburst, or extreme star formation.

"It is indeed reassuring to see this process in action. Further study of this system may shed some light on why other clusters aren't forming stars at these high rates, as they should be," explained McDonald. 

"Our first observations of this cluster with the Gemini South telescope in Chile really helped to ignite this work. They were the first hints that the central galaxy in this cluster was such a beast!"

Matthew Bayliss and Jonathan Ruel of Harvard used the Gemini data to subsequently determine the cluster's distance, while simultaneously corroborating its huge mass with estimates from X-ray data obtained with the Chandra X-ray Observatory. 

Astronomers now believe they have finally seen, at least in this one large cluster of galaxies, what they expected to find all along - a massive burst of star formation, presumably fueled by an extensive flow of cooling gas streaming inward toward the cluster's central core galaxy. 

The sinking gas is likely sparking star formation and a lively, dynamic environment - similar a cold front triggering thunderstorms on a hot summer's day. This stands in stark contrast to most other large galaxy clusters where central galaxies appear to have stopped forming new stars billions of years ago - a controversial discrepancy known as the "cooling-flow problem."

Astronomers vindicated by frenetic Phoenix Cluster star formation

According to theory, the hot plasma that fills the spaces between galaxy cluster members should glow in X-rays as it cools, in much the same way that hot coals glow red. As the galaxy cluster forms, this plasma initially heats up due to the gravitational energy released from the infall of smaller galaxies.

As the gas cools, it should condense and sink inward, a process known as a "cooling flow." 

In the cluster's center, this cooling flow can lead to very dense cores of gas, termed "cool cores," which should fuel bursts of star formation in all clusters that go through this process. Most of these predictions had been confirmed with observations - the X-ray glow, the lower temperatures at the cluster centers - but starbursts accompanying this cooling remain rare.

SPT-CLJ2344-4243, nicknamed the "Phoenix Cluster," lies in the direction of the southern constellation Phoenix, which McDonald feels is only fitting.

"The mythology of the Phoenix – a bird rising from the dead – is a great way to describe this revived object," said McDonald. "While galaxies at the center of most clusters may have been dormant for billions of years, the central galaxy in this cluster seems to have come back to life with a new burst of star formation."