How massive, huge galaxies were able to form shortly after the Big Bang is one of the great mysteries of astronomy. Observations in an early galaxy cluster are now providing the first clues. Images from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile reveal gas clumps and tidal structures in these protoclusters, indicating a turbulent formation history. Accordingly, the galaxies in the cluster were formed by the collapse of a precursor structure. This created turbulence and tidal forces that triggered a pileup in this protocluster. A few hundred million years after the observed state, this cluster could merge into a massive elliptical galaxy, as astronomers report. These multiple mergers could explain how early galaxies were able to grow so large so early.
It is one of the great mysteries of astronomy: In recent decades, astronomers have repeatedly discovered early galaxies that are more massive and more developed than expected. Actually, these collections of stars haven’t had enough time since the Big Bang and the formation of the first stars to grow so far – at least according to the current models of galaxy evolution. Among the surprising discoveries are, for example, a massive Milky Way twin that was already fully mature a billion years after the Big Bang: The galaxy has spiral arms, a large disk of stars and a central bulge with old stars. Even older are early galaxies, which existed 500 to 700 million years after the Big Bang and also contained ten to 100 billion solar masses of stars. Astronomers have also discovered early forms of galaxy clusters in the early cosmos. How all of these structures were able to mature so quickly so early remains unclear.
Protoclusters, tidal streams and a “string of pearls” of gas clumps
Now observations of an early proto-cluster of galaxies with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile provide new clues about the formation of such early giants. Astronomers led by Nikolaus Sulzenauer from the Max Planck Institute for Radio Astronomy in Bonn analyzed the radio signatures of cold gas and dust in the center of SPT2349-56, a protocluster in the southern constellation Phoenix. This early cluster of galaxies existed around 1.4 billion years after the Big Bang and is characterized by a high rate of star formation and an enormous mass of gas between the clusters of stars. “At the center of SPT2349-56 we found three closely interacting galaxies that produce a star every 40 minutes,” reports co-author Ryley Hill from the University of British Columbia in Canada. For comparison: The Milky Way produces an average of three to four stars in a whole year.
Using the high-resolution radio data, the astronomers discovered a surprisingly complex structure in the vicinity of the central three galaxies. “They are connected by huge, coherent streams of ionized carbon,” they report. These spiral tidal currents extend over an area larger than the Milky Way. The ionized carbon is excited by shock waves in the gas from these streams and therefore shines brightly in the radio wave spectrum. Thanks to this bright emission, the team was able to precisely measure the movement of the gas. This revealed dense clumps of gas strung like a string of pearls along these tidal arms. “To our surprise, the clumps are connected to 20 other galaxies that are located in the outer regions of the structure. This suggests a common origin,” says Sulzenauer. The speed and direction of movement of the tidal streams and gas clumps suggests that they all originate from the vicinity of the central three galaxies of the protocluster.
First collapse, then multiple mergers
But what caused this strong turbulence in this proto-galaxy cluster? Using model simulations, the researchers developed a plausible scenario for this. Accordingly, the observed structures of SPT2349-56 must be due to a “monolithic collapse” of a dense, massive gas cloud. Such a collapse triggers an explosion of star formation, creating several massive galaxies in a short period of time. “The dynamic friction leads to a separation of the masses and the three central galaxies migrate to the center of the halo with a loss of energy,” explains the team. This event leaves behind massive gas flows, local gas clumps and numerous smaller, gas-rich galaxies in the nearby area. However, as evolution continues, these smaller galaxies will not be preserved: “Most of the 40 gas-rich galaxies in this core will be destroyed and eventually form a single, giant elliptical galaxy in less than 300 million years,” reports Sulzenauer. “For the first time we are observing the beginning of a cascading merger.”
(Video: Max Planck Society)
Such a multiple merger of smaller galaxies in early proto-galaxy clusters could therefore explain how massive large galaxies were able to form in a short time in the early cosmos: First, a huge halo of gas and dust collapses into gas clumps and galaxies. Then these smaller galaxies merge into one large one in a pileup. “Instead of slowly accumulating mass, a massive elliptical galaxy could form in just a few hundred million years,” says Sulzenauer. The new observations thus provide valuable information about the processes at the beginning of the universe. “It may be too early to claim that we fully understand the early childhood of giant elliptical galaxies,” says co-author Scott Chapman of Dalhousie University in Canada. “However, we have come a long way in connecting the tidal arms in protoclusters and the formation process of massive galaxies in today’s galaxy clusters.”
Source: Nikolaus Sulzenauer (Max Planck Institute for Radio Astronomy, Bonn) et al., The Astrophysical Journal, doi: 10.3847/1538-4357/ae2ff0