'Missing' Evolutionary Link for Compact Galaxies Found

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The evolution of stars and galaxies in the Universe takes a long time. A really, REALLY long time. It takes lots of data from many astronomical objects sampled of different ages, plus simulations and a bit of patience to figure out the evolutionary history of something astronomical — as is the case of the giant ellipticals that were recently exposed with the help of many ground-based and space-based telescopes.

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My first astronomy professor put it this way: Imagine that you are part of an alien race, come down to Earth to study humanity. But, you’ve only got a few minutes to fly around the world and collect your data. How might you try and understand the life cycle of a human being? You’d have lots of pictures of small people, some so small to be almost helpless. Then there are lots of big people, too. People come in all shapes, sizes, and colors. But with that snapshot of humanity, you have to piece together the life stages of a human.

It is like this with galaxies in the Universe. We see big galaxies and small galaxies, spirals and ellipticals and irregular galaxies, too. We’ve had to put together the history of these galaxies as best we can without actually having the millions and billions of years to watch them grow up. Sometimes we get it dead wrong, as in the early 20th century it was thought that elliptical galaxies can eventually flatten out to become disks and spirals. We now know that larger galaxies are built in the mergers of smaller galaxies, but even in that paradigm there is room for diversity.

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This method of galaxy building takes quite a bit of time, so you only expect to see larger galaxies nearby but not far away due to lookback time. However, some galaxies just won’t fit that mold. Some seem to defy a simple explanation.

Size comparison of the modern Milky Way to a compact, giant ellitpical.
NASA, ESA, S. Toft, A. Feild

Take the case of the giant, compact elliptical galaxies seen at a redshift of 2. This means they are being seen as they were when the Universe was just over three billion years old. These galaxies are already massive, having almost the mass of the present Milky Way but compacted into a much smaller space. They also appear “dead.” That is, they have very little star formation and almost no gas left with which to make new stars. Just 3 billion years after the Big Bang, these galaxies have reached the end of their life cycle. Their stars will simply age and die until only the reddest, smallest, longest-lasting ones are left.

In order to have these in existence, there must have been an incredibly powerful galaxy-building epoch even earlier, at redshifts 3 to 6, or 1 to 2 billion years after the Big Bang. Such galaxies would be full of dust and gas that would obscure their optical light, as well as being redshifted to longer wavelengths by their distance.

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Such a population of galaxies has finally been uncovered and categorized by a group of astronomers led by Sune Toft, of the DARK Cosmology Center at the Niels Bohr Institute, University of Copenhagen. In a paper that contains data from many telescopes around the world and in space — including Hubble, Spitzer, Herschel, Keck, the Very Large Array, the Sub-Millimeter Array, and many more — they characterize the sample of submillimeter-emitting gas-rich galaxies that exist in the early Universe and find that it has just the right characteristics to merge and make the giant ellipticals that are “red and dead” 10 billion years ago, and even more so today.

For the first time, this evolutionary link has been established, solving the mystery of these giant but quiet behemoths of the Universe. The dusty, gassy, merging galaxies of the distant past live a chaotic but short youth before settling down for a long, long, quiet middle age.

This work has been accepted for publication by the Astrophysical Journal, and a preprint is available on arXiv.org. 

Credit for the human analogy goes to Dr. Erickson at Lycoming College! At least, that is who I learned it from. 

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