Black Holes are Bad for Star Formation

   

Black holes, regions of space so dense that even light can't escape, lie at the center of virtually every galaxy including our own Milky Way. New research suggests that black holes are at least partially responsible for the size of their host galaxy as well; it seems galaxies with more active black holes produce fewer stars. It's the first piece of concrete evidence astronomers have about the relationship between a black hole and its host galaxy.

Young galaxies are without concrete shape, made of the raw material that eventually becomes stars, planets and everything else that will reside inside them. At this early stage, black holes are called galactic nuclei and they are giant, luminous and much more energetic than their adult versions.

Similarly, star formation is much livelier in galactic youth. As a galaxy matures, two things happen: the young central black hole consumes more material and more stars form. The processes run side-by-side, at least for a little while. Astronomers think inflows of gas fuel new stars and supermassive black holes. Any gas that falls onto the black holes is heated and accelerated around it, releasing energy. If a black hole takes in too much material, it will start spewing radiation into the galaxy. This heats up and disperses the reservoirs of cool gases necessary for star formation.

One of the big questions in galactic evolution is how much the black hole’s process of heating gases has slowed star formation, particularly in large elliptical galaxies that typically have little cold gas and few young stars. Previous studies have only provided "snapshots" of this process that hint at the relationship between active galactic nuclei and star formation. The relationship over cosmic history has never been clear.

Now, new data from the Herschel Observatory -- a European Space Agency cornerstone mission with important participation by NASA -- is shedding some light on the mystery. "To understand how active galactic nuclei affect star formation over the history of the universe, we investigated a time when star formation was most vigorous, between eight and 12 billion years ago," said James Bock, senior research scientist at NASA's Jet Propulsion Laboratory and co-coordinator of the Herschel Multi-tiered Extragalactic Survey (HerMES). "At that epoch, galaxies were forming stars 10 times more rapidly than they are today on average. Many of these galaxies are incredibly luminous, more than 1,000 times brighter than our Milky Way."

The team of researchers used Herschel data that probed 65 galaxies in the far-infrared spectrum. These wavelengths reveal the rate of star formation; the energy released by developing stars heats the surrounding dust which re-radiates starlight in far-infrared wavelengths. They compare these readings with X-rays streaming from the galaxies’ active central black holes as measured by NASA’s Chandra X-ray Observatory.

They found that the galaxies with most powerful black holes at their cores, heat the raw material and spew more radiation, produce fewer stars than galaxies with less active black holes. It’s the first evidence that black holes began suppressing galactic star formation early, when the universe was less than half its current age. It’s an interesting relationship, and it’s certain to bring up more questions about the intertwined lives of galaxies and black holes.