Massive galaxies may grow from cold molecular gas that condenses as stars rather than forming in violent mergers, according to an international team of astronomers that studied a protocluster of still-forming galaxies.
B.H.C. Emonts et al found that the Spiderweb, a massive galaxy in the center of a distant protocluster, is forming from a large reservoir of molecular gas. In this artist’s conception of the protocluster, the protogalaxies are shown in white and pink, and the blue indicates the location of the carbon monoxide gas. Image credit: M. Kornmesser / ESO.
“The formation of the largest galaxies in the Universe is thought to be a two-stage process. For the past 10 billion years, these giant galaxies have grown mostly by cannibalizing smaller galaxies,” the astronomers said.
“However, computer simulations predict that in an earlier phase, lasting a few billion years, their stars condensed directly out of large reservoirs of accreted gas.”
To investigate this, the team studied the Spiderweb galaxy (MRC 1138-262), an object nearly 10 billion light-years away.
“The Spiderweb is not a single galaxy, but rather an aggregation of protocluster galaxies. They are embedded in a giant halo of atomic (neutral and ionized) hydrogen gas,” the authors explained.
“The central protocluster galaxy has a supermassive black hole at its core, which emits jets of relativistic particles visible in radio observations.”
“Observations suggest that the protocluster galaxies will eventually merge and evolve into a single giant elliptical galaxy in the center of the cluster.”
The astronomers used the Australia Telescope Compact Array (ATCA) and NSF’s Karl G. Jansky Very Large Array (VLA) to detect carbon monoxide gas in the Spiderweb. The presence of this gas indicates a larger quantity of molecular hydrogen, which is much more difficult to detect.
They estimated that the molecular gas totals more than 100 billion times the mass of the Sun.
Not only is this quantity of gas surprising, but the gas also must be unexpectedly cold, about minus 200 degrees Celsius (minus 328 degrees Fahrenheit). Such cold molecular gas is the raw material for new stars.
“It is surprising how cold this gas is, at some 200 degrees below zero Celsius,” said team member Dr. Matthew Lehnert, from the Astrophysics Institute of Paris, France.
“We would have expected a lot of collapsing galaxies, which would have heated the gas, and for that reason we thought that the carbon monoxide would be much more difficult to detect.”
Carbon monoxide in this gas indicates that it has been enriched by the supernova explosions of earlier generations of stars.
The ATCA observations revealed the total extent of the gas, and the VLA observations, much more narrowly focused, provided another surprise. Most of the cold gas was found, not within the protogalaxies, but instead between them.
“This is a huge system, with this molecular gas spanning three times the size of our own Milky Way Galaxy,” said team member Dr. Preshanth Jagannathan, of the National Radio Astronomy Observatory.
“The Spiderweb is an astonishing laboratory, which lets us witness the birth of supergalaxies in the interiors of clusters, which are the ‘cosmic cities’ of the Universe,” added team member Dr. George Miley, from the University of Leiden.
“We are beginning to understand how these giant objects formed from the ocean of gas which surrounds them.”
The findings were published in the Dec. 2, 2016 issue of the journal Science.
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B.H.C. Emonts et al. 2016. Molecular gas in the halo fuels the growth of a massive cluster galaxy at high redshift. Science 354 (6316): 1128-1130; doi: 10.1126/science.aag0512
