The Beginnings of Galaxy Clusters: Proto-Clusters Finally Seen!

The deeper we peer through the cosmos, the more we are looking into the distant past.  Light from other galaxies takes millions of years to reach us, and so when that light has finally arrived at Earth, it is millions of years old, a snapshot in time of the distant galaxy.  The furthest we can see is so far back in the history of the universe, that galaxies haven’t even formed yet.  As we look at the large-scale structure of the Universe, we see it filled with a cosmic web of galaxy clusters, containing tens of thousands of galaxies each.  How these immense structures formed so quickly during the early epochs of the universe is an unanswered question in astrophysics.  A new result published by a team of astronomers using the Planck and Hershel telecopes have found the first images of ‘proto-clusters,’ massive clumps containing vast pockets of star formation that will eventually become the massive clusters we see today.

The Planck all-sky map at submillimeter wavelengths (545 GHz). The band running through the middle corresponds to dust in our Milky Way galaxy. The black dots indicate the location of the proto-cluster candidates identified by Planck and subsequently observed by Herschel. The inset images showcase some of the observations made by Herschel’s SPIRE instrument; the contours represent the density of galaxies. Credit: ESA and the Planck Collaboration/ H. Dole, D. Guéry & G. Hurier, IAS/University Paris-Sud/CNRS/CNES

The data was gathered with the Planck all sky survey, whose goal is to map the entire sky in nine different wavelengths in order to produce the highest resolution map of the cosmic microwave background radiation, the leftover radiation from the Big Bang.  By imaging in so many wavelengths, it is possible for Planck to eliminate emission from all foreground sources, such as the Milky Way and other Galaxies in the Universe.  But in one of the Planck maps, scientists found 234 sources with characteristics expected of objects in the very early universe.  Further data from the Hershel Space Telescope, in far infrared to submillimeter wavelengths, allowed astronomers to study the objects in much higher detail and angular resolution.  Using the data, astronomers concluded that the objects are massive concentrations of galaxies in the early universe that are rapidly forming new stars, at 1500 times the rate of the Milky Way.

“Because we are looking so far back in time, and because the the universe is assumed to be homogenous in all directions, we think it’s very similar to looking at the equivalent of what a baby cluster might look like,” said Brenda L. Frye, an assistant astronomer at the University of Arizona’s Steward Observatory who was involved in the research.  “In contrast to previous observations, for which the odd one or two baby clusters was found which one would put in a zoo, we now have found a real sample of 200 baby clusters.”

Before this study, it was not known how galaxy clusters formed.  One of the leading theories was that they formed gradually, and came together as stars slowly formed in their component galaxies.  But with these new observations, the stars are forming like fireworks, rapidly populating the galaxies in the cluster.

“Hints of these kinds of objects had been found earlier in data from Herschel and other telescopes, but the all-sky capability of Planck revealed many more candidates for us to study,” said Hervé Dole of the Institut d’Astrophysique Spatiale in Orsay, France, lead scientist of the analysis published in the journal Astronomy & Astrophysics. “We still have a lot to learn about this new population, requiring further follow-up studies with other observatories. But we believe that they are a missing piece of cosmological structure formation.”

It shows how two telescopes with differing capabilities can work together to narrow down and study objects in detail, in an impressive display of synergy.  This is a truly beautiful part of scientific discovery.  The goal is to work together to further our understanding of the cosmos, instead of eliminating competition.

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