Bright galaxies like to stick together

For more than a decade, astronomers have been puzzled by some strange bright galaxies in the distant universe that appear to be forming stars at phenomenal rates, making them very hard to explain with conventional theories of galaxy formation. One important question has been the environments in which they are located, such as their closeness. The Herschel Space Observatory, with its ability for sensitive mapping over wide areas, has been able to see thousands of these galaxies and identify their locations, showing for the first time that they are packed closely together in the center of large galaxy clusters.

A project using the Spectral and Photometric Imaging Receiver (SPIRE) instrument aboard 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, well away from the confusion of our own galaxy. Galaxies that are brightest at Herschel’s far-infrared wavelengths are typically seen as they were around 10 billion years ago, the light having traveled toward us since then.

The image shows a false-color view of a small portion of the sky observed by Herschel. Almost every point of light is an entire galaxy, each containing billions of stars. The colors represent the far-infrared wavelengths measured by Herschel, with redder galaxies either being farther away or containing colder dust, while brighter galaxies are forming stars more vigorously. While at a first glance the galaxies look to be scattered randomly over the image, they are not. A closer look reveals there are regions that 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 amazing new results from Herschel are just a taste of things to come as Herschel continues to unlock the secrets of the early stages of star birth and galaxy formation in our universe,” said David Parker, director of Space Science and Exploration at the United Kingdom Space Agency. “We’re immensely proud of our United Kingdom scientists who are playing key roles in this highly successful space mission.”

Herschel sees material that cannot be seen at visible wavelengths, namely cold gas and dust between the stars. This is well-illustrated by looking at much closer galaxies that can be seen in more detail. The Antennae Galaxies, lying a mere 50 million light-years away, are actually two galaxies that are in the process of colliding, and were observed as part of a different observing program. Herschel does not see the light from stars, but the clouds of dust within which new stars are forming. The collision of these galaxies has caused a surge in star formation, but such collisions are relatively rare in the universe today. Billions of years ago, however, when galaxies were more tightly packed, such events were more common.

Despite the new window to the universe afforded by the far-infrared light, Herschel is still not seeing the full picture. Three-quarters of the matter in our universe is made up of mysterious “dark matter” that does not shine at all. Because we cannot see dark matter, we do not know what it is made of, but we can measure its effect on the matter around it. Although it does not emit or absorb light, dark matter does interact with the rest of the universe through gravity, gradually pulling groups of galaxies together into huge clusters over the course of billions of years. While many computer simulations exist of how this occurs, the ability to measure this at different times through the history of the universe allows astronomers to compare the simulations with real measurements.

These latest results from Herschel, part of the “HerMES” key program, show that the bright galaxies detected with the SPIRE instrument preferentially occupy regions of the universe that contain more dark matter. This seems to be especially true about 10 billion years ago when these galaxies were forming stars at a higher rate than most galaxies are today.

Our Milky Way Galaxy resides in the suburbs of a large supercluster centered about 60 million light-years away. The neighboring supercluster of galaxies to us is around 300 million light-years away. By comparison, 10 billion years ago galaxies were only 20 to 30 million light-years apart on average. Their proximity means that many of the galaxies will eventually 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. “Thanks to the superb resolution and sensitivity of the SPIRE instrument on Herschel, we managed to map in detail the spatial distribution of massively star-forming galaxies in the early universe,” said Asantha Cooray of the University of California, Irvine. “All indications are that these galaxies are busy. They are crashing, merging, and possibly settling down at centers of large dark matter halos.”

It has required the sensitivity and resolution of Herschel to be able to identify the brightest galaxies and establish the way in which they are clustering. “We have known for a long time that environment plays an important role in shaping galaxies’ evolution,” said Lingyu Wang of the University of Sussex in England. “With Herschel, we are able to pierce through huge amounts of dust and study the impact of the environment right from the birth of these massive galaxies forming stars at colossal rates. This is allowing us to witness the active past of today’s dead elliptical galaxies at times when they were in rich environments.”

“This result from Asantha’s team is fantastic; it is just the kind of thing we were hoping for from Herschel and was only possible because we can see so many thousands of galaxies,” said Seb Oliver from the University of Sussex. It will certainly give the theoreticians something to chew over.”