Why some black holes eat more

An Irish researcher at the American Museum of Natural History has a theory on why some black holes are gobbling up huge amounts of matter while others don't consume at all, writes DICK AHLSTROM

A GIANT cloud of interstellar gas is on a direct collision course with our Milky Way galaxy. There is no escape and when it hits there will be chaos, with the cloud collapsing to form new stars and the ever-present risk of an exploding supernova.

Happily the so-called Smith Cloud won’t get here for about 50 million years, so we don’t have to worry about it just yet. Astronomers don’t know exactly what part of the Milky Way the cloud will hit, but it is likely to be at some distance from our corner of the galaxy, says Prof Barry McKernan of the Department of Astrophysics at the American Museum of Natural History in New York.

Originally from Kilmacud in Dublin, McKernan is now attached to the museum and to the City University of New York.

Last month he and collaborators Ariyeh Maller and Saavik Ford had their research into giant hydrogen gas clouds like the Smith Cloud published online in the Astrophysical Journal Letters.

McKernan usually conducts research into what are known as “active galactic nuclei”, galaxies whose ferociously active centres harbour massive black holes that gobble up any matter that comes their way. Active galactic nuclei are not very common however.

Most of the hundreds of billions of galaxies across the universe have quiescent galactic centres. This happens when there is little or no matter available for the black hole to consume, and represents the current status of the Milky Way.

Then there are galaxies where the black hole seems to be “grazing”, feeding at a low rate but at a level sufficient to light up the galactic nucleus. The glow is produced because energy is released when matter disappears into the black hole, says McKernan.

In a small number of galaxies the central black hole is feeding voraciously, causing the nucleus to glow intensely. They can give off more light than all the rest of the stars combined.

These active galactic nuclei are the focus of McKernan’s research. “I am interested in what is going on in the centre of the galaxy, not in what is happening in the distant galactic halo,” he says.

And yet the two unexpectedly became linked in Prof McKernan’s research.

“For astronomers, the big mystery is why most black holes are quiet while some are feeding lightly and a few are feeding vigorously,” he says.

There are a number of theories but they didn’t match up well with astronomical observations. The paper’s authors then considered the possibility that perhaps active galactic nuclei were being triggered when one of the giant clouds – which are up to 3,000 light years long and carry the mass of 1,000 suns – reached the black hole at the centre of a galaxy. The primordial universe used to be full of these hydrogen gas clouds and there was frequent interaction between them and early galaxies, he explains.

The clouds heat up as they pass into a galaxy. They become unstable and begin to collapse, triggering star formation. “All you need for star formation is some gas that is shocked and collapsing,” McKernan says.

“When the universe was formed it was a deluge of clouds. Now it is more like a drizzle.” Even so, these collisions do occur today.

The researchers showed that if a cloud managed to drift into the galactic centre without becoming too unstable, a large black hole could devour most of it and in the process become very active.

If the black hole were smaller, the researchers calculated that the galactic nucleus would be only relatively active but there would also be new star formation.

“It turns out that the rate of cloud impact that we calculate could explain the rate of black hole feeding in the local universe,” he says.

“Applying the calculations to our own galaxy, we should expect a cloud impact on the centre of the Milky Way around every 200 million years or so.”

Clouds impacting a galaxy but not at its centre might also become unstable and collapse to initiate star formation. If this were to happen close to our galactic neighbourhood, the result would be new stars in the sky and the possibility of new constellations, he says.

Of course, it takes millions of years to form a star so these would not light up very quickly.

Twitter: @dickahlstrom