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Scores of galaxies found packed in clusters

A mind-boggling new image of space packed with thousands of tiny dots of color, each one a distant galaxy, is showing astronomers that the brightest galaxies tend to be in busiest parts of the universe.
Image: Thousands of galaxies
Thousands of galaxies crowd into this Herschel image of the distant universe. Each dot is an entire galaxy containing billions of stars. This image image was taken in a region of space called the Lockman hole, which allows a clear line of sight out into the distant Universe.
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A mind-boggling new image of space packed with thousands of tiny dots of color, each one a distant galaxy, is showing astronomers that the brightest galaxies tend to be in busiest parts of the universe.

For over a decade, astronomers have been puzzled by some strange, bright galaxies in the distant universe which appear to be forming stars at phenomenal rates, making them very hard to explain with conventional theories of galaxy formation. The crucial new image from the European Space Agency's Herschel Space Observatory can help astronomers firm up their theories of galaxy formation.

Among the important questions: What fueled the formation of these bizarre galaxies, and what kind of environment they are located in? The answer to part of that question hinges on how close the galaxies are to each other. That's where Herschel came in. With its ability for very sensitive mapping over wide areas, the orbiting telescope managed to see thousands of these galaxies and identify their location, showing for the first time that they are packed closely together in the center of large galaxy clusters.

Herschel sees material that cannot be seen at visible wavelengths, namely cold gas and dust between the stars that shine in infrared light.

The SPIRE instrument on Herschel has been surveying large areas of the sky, currently totaling 15 square degrees — around 60 times the apparent size of the full moon. The two regions mapped so far are in the constellations of Ursa Major and Draco.

Galaxies which appear brightest in the far-infrared wavelengths that Herschel sees are typically observed as they were around 10 billion years ago — the length of time it took their light to reach Earth.

In Herschel's new image of one of these regions — a false-color view of a small portion of the sky — the telescope found thousands of tiny colored points of light, each of which is an entire galaxy that in turn contains billions of stars.

The colors represent the far-infrared wavelengths measured by Herschel, with redder galaxies either being further away or containing colder dust, while brighter galaxies are forming stars more vigorously.

The image shows that the galaxies are not scattered randomly — there are regions which have more galaxies, and regions that have fewer. This clustering of galaxies through space provides information about the way they have interacted over the history of the universe.

These esults show that the bright galaxies detected with the SPIRE instrument preferentially occupy regions of the universe that contain more dark matter, the invisible, mysterious stuff that makes up three quarters of the matter in the universe. This preference seems to be especially true about 10 billion years ago, when these galaxies were forming stars at a much higher rate than most galaxies are today.

The closeness of galaxies in the early universe means they were more likely to collide with one another. It is these collisions that stir up the gas and dust in the galaxies and cause the rapid bouts of star formation.

"All indications are that these galaxies are busy. They are crashing, merging, and possibly settling down at centers of large dark matter halos," said Asantha Cooray, of the University of California and one of the members of the team that made the new observations.

The work, which was presented last week at the Herschel First Results Symposium in the Netherlands, will also be detailed in an upcoming special issue of journal Astronomy & Astrophysics dedicated to the first science results from Herschel.