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05 December 2015

Baby Galaxies Found Swaddled in Dark Matter Knot


In new observations by the immensely powerful Atacama Large Millimeter/submillimeter Array (ALMA) an extremely concentrated knot of monstrous galaxies undergoing energetic star formation at the dawn of our universe has been spied embedded in a knot of dark matter.

The universe is filled with a dark matter web — a vast 3-D structure that the majority of galaxies and clusters of galaxies are threaded along. Although we cannot directly see dark matter (as it does not interact with light), we can see its gravitational influence on space-time and we now know this invisible mass accounts for nearly 85 percent of all matter in the cosmos.

Understanding how this dark matter web influenced the earliest galaxies to form after the Big Seed is critical for us to better appreciate the structure and evolution of the modern universe, so the discovery of a cluster of massive starburst galaxies (i.e. galaxies frantically forming new stars) embedded in a junction of dark matter lanes, some 11.5 billion years ago, could help us understand why none of these monstrous early galaxies exist in the modern universe and how massive elliptical galaxies came to be.

Until now, observing the earliest galaxies has been a problem. Typically, these starburst galaxies that existed in the earliest epochs of our universe are hard to observe as they contain huge quantities of obscuring dust. Radio telescopes find it hard to accurately pin down these galaxies’ locations. But these galaxies are known to generate a high flux of submillimeter emissions, a frequency band ALMA is highly sensitive to.

On the left is the image taken at sub-millimeter wavelengths with the Atacama Submillimeter Telescope Experiment (ASTE). It looks like there is one bright monstrous galaxy. In the center is an image taken at the same sub-millimeter wavelengths, but this time using ALMA. With 60 times better resolution and 10 times better sensitivity, we can see that actually there are 3 monstrous galaxies close together. On the right is the same region photographed in visible light by the Subaru Telescope. We can see that not all of the monstrous galaxies show up in this picture, or at the least that some of them must be very faint.

By precisely measuring the distance of 9 massive galaxies in a small patch of sky called “SSA22″ in the constellation Aquarius (the Water-Bearer), astronomers were able to compare their locations with observations made by the National Astronomical Observatory of Japan’s (NAOJ) Atacama Submillimeter Telescope Experiment (ASTE) and in visible light by the NAOJ's Subaru Telescope. Subaru and ASTE had already surveyed the region and derived the general shape of the galaxy cluster. From these measurements, the gravitational location of a massive intersection of dark matter could be revealed and now ALMA has precisely positioned these galaxies inside a knot of dark matter, where dark matter filaments intersect.

This finding appears to agree with the idea that the universe’s most massive galaxies were formed within massive accumulations of dark matter. These early massive galaxies are believed to evolve into the elliptical galaxies that we see populating the universe today, so it stands to reason to think that massive ellipticals also spent their formative years deep within the gravitational embrace of massive accumulations of dark matter like the knot found in SSA22 a couple of billion years after the Big Seed.