Galaxies come of age in cosmic blobs

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 25 June, 2009

Growing supermassive black holes and bursts of star formation have been discovered in cosmic blobs – immense reservoirs of hydrogen gas located in the early Universe – representing the coming of age of galaxies.

Astronomers have known about the half-million-light-year-wide blobs for around a decade, but how and why they glowed brightly in optical light had remained a mystery. Now, thanks to new observations using telescopes such as the Chandra X-ray Observatory, Spitzer Space Telescope and Hubble, astronomers have found that the source of power is coming from growing supermassive black holes, with a significant contribution from newly forming stars. The discovery settles some important arguments about what role the blobs played in the original construction of galaxies and black holes.

A composite image of a blob observed in the study with a growing black hole in the centre of the galaxy. Image: X-ray (NASA/CXC/Durham Univ./D.Alexander et al.); Optical (NASA/ESA/STScI/IoA/S.Chapman et al.); Lyman-alpha Optical (NAOJ/Subaru/Tohoku Univ./T.Hayashino et al.); Infrared (NASA/JPL-Caltech/Durham Univ./J.Geach et al.)

“We used Chandra to stare at a patch of sky that astronomers call SSA-22,” describes Jim Geach of Durham University. SSA-22 contains the largest number of blobs that astronomers know about and are seen when the Universe is only about two billion years old. “Our most exciting result is that we have discovered that a significant fraction of blobs contain galaxies with actively growing black holes, and we believe that the radiation associated with the black holes is responsible for the formation of blobs. So these strange objects could be giving us a glimpse into the evolution of galaxies, the signature of galaxies coming of age.”

Galaxies are believed to form when gas flows inwards under the pull of gravity and cools by emitting radiation, a process that should stop when the gas is heated by radiation and outflows from galaxies and their black holes. But Geach and colleagues show that it is the heating of gas by growing supermassive black holes and bursts of star formation, rather than cooling of gas, that most likely powers the blobs. That is, the blobs represent the evolutionary stage when the galaxies and black holes are just starting to switch off their rapid growth because of these heating processes, a stage known as feedback and one that astronomers have long been trying to understand.

“Witnessing feedback in action is extremely exciting to astronomers who have come up with models of galaxy formation because this is the phase where the galaxy has nearly finished growing, and then tries to prevent further growth by stopping fresh gas from cooling,” says Geach. “This is basically a form of self-regulation. These new Chandra observations are currently the best evidence that these blobs are powered by heating rather than cooling.”

Co-author Brett Lehmer adds: “Massive galaxies must go through a stage like this or they would form too many stars and so end up ridiculously large by the present day.”

Artist impression of what a galaxy might look like inside a blob. A two-sided outflow is powered by the supermassive black hole buried inside the middle of the galaxy. Image: NASA/CXC/M.Weiss.

Five of the blobs displayed evidence that the black holes were rapidly growing, and further observations provided evidence that the galaxies embedded within these blobs are forming stars at rates of hundreds of times that of the Milky Way. “The process of star formation is very energetic, it is almost certain that it is contributing to the elimination of these blobs,” says Lehmer. “We think that the blobs themselves are likely to be leftovers of the gas that surrounds the ancestors of some of the most massive galaxies and their black holes in the present-day Universe.”

Besides explaining the power source of the blobs, these results also help explain their future. Under the heating scenario, the gas in the blobs will not cool down to form stars but will add to the hot gas found between galaxies. SSA22 itself could evolve into a massive galaxy cluster.

“In the beginning the blobs would have fed their galaxies, but what we see now are more like leftovers,” says Geach. “This means we’ll have to look even further back in time to catch galaxies and black holes in the act of forming from blobs.”

While an important step in unravelling the evolutionary pathway of galaxies, the team caution that there is still a lot to learn. “In the future we need to conduct wide-area hunts for these blobs to improve our statistical understanding of this population,” concludes Lehmer.