A huge supercluster of galaxies in the early Universe

A false-color image of the far-infrared emission from a huge protocluster of galaxies (in the circle) dating to the epoch about 1.4 billion years after the Big Bang. Astronomers have made deep optical and infrared observations of the complex and have concluded that the star-forming processes at work, while exceptionally active, generally appear to follow the same processes as in our galaxy. Credit: NASA/ESA/Herschel; Miller et al.

The structure of the universe is often described as a cosmic web of filaments, knots and voids, the knots being clusters of galaxies, the largest gravitationally bound objects known. These nodes are thought to have been seeded by small-amplitude density fluctuations, such as those observed in the cosmic microwave background (CMB) that grew until they collapsed into the structures we see today. While the CMB is well understood and details of current galaxy clusters well described, the intermediate stages of evolution lack sufficient observations to limit the models. Traditional searches for galaxy clusters assume that these objects have had enough time to equilibrate so that the intergalactic gas has warmed up enough to be detected in X-rays. To detect the more distant galaxies and protoclusters that are too faint to detect in the X-rays, astronomers instead use their bright infrared or submillimeter emission.

The supercluster SPT2349-56, discovered in the submillimeter band by the South Pole telescope, is so distant that its light has been traveling for more than 12 billion years. It is home to more than thirty submillimeter bright galaxies and dozens of other luminescent and/or spectroscopically confirmed star-forming galaxies. It is one of the most active star-forming complexes known and produces more than ten thousand stars a year. One of the bright sources appears to be the amalgamation of more than twenty galaxies. However, the system’s stellar mass was unknown, making it impossible, for example, to know whether the massive outburst of stars was the result of extraordinary efficiency or simply because the system was so extremely large.

CfA astronomer Matthew Ashby was part of a team that has now made very deep observations in optical and infrared wavelengths to obtain the stellar masses through spectral energy distribution (SED) analyses. They used the Gemini and Hubble space telescopes to obtain optical/near infrared flux measurements and Spitzer’s IRAC camera for the infrared flux. To model the SEDs, the many detected point sources at all wavelengths must be matched. This is a complex undertaking, and the scientists describe the processes for doing it, while also addressing the severe mixing that can occur due to insufficient spatial resolution in the infrared.

According to their results published in Monthly Notices from the Royal Astronomical Societythe astronomers find that the stellar mass in this primordial cluster compared to its star formation rate is close to the value measured in nearby (“normal”) galaxies, a conclusion that suggests that the star-forming processes at work are similar to those in the local universe. However, the cluster is deficient in molecular gas, suggesting that activity is nearing the end of this tumultuous phase as the gaseous raw material for stars is drained.

A huge protocluster of merging galaxies in the early Universe

More information:
Ryley Hill et al, Rapid build-up of the stellar contents in the protocluster nucleus SPT2349-56 at z = 4.3, Monthly Notices from the Royal Astronomical Society (2021). DOI: 10.1093/mnras/stab3539

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