NEARBY galaxies harbour far fewer supermassive black holes than expected, according to the largest survey of these cosmic behemoths.
Black holes can be detected by looking for telltale X-rays emitted as matter gets sucked in. Previous surveys have shown that the universe is filled with a diffuse background of X-rays, suggesting that there are plenty of black holes out there. This X-ray background peaks at about 30 kiloelectronvolts (keV), which is at the higher end of the X-ray energy spectrum, and astrophysicists think that the energy is generated by dust-shrouded supermassive black holes at the centre of some galaxies. The dust surrounding these “active galactic nuclei” (AGNs) should absorb most of the low-energy X-rays, but not the 30-keV ones, leading to the observed blip in the cosmic X-ray background.
Volker Beckmann of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and his colleagues searched for AGNs by analysing X-ray maps compiled by the European Space Agency’s Integral satellite over nearly two years. The researchers looked out to a distance of about 300 million light years, but were in for a surprise. “We didn’t see the sources that are causing most of the 30-keV peak,” says Beckmann ().
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This could mean one of two things, he says. Either the dust-shrouded black holes responsible for the 30-keV peak are too faint to show up, or there are not enough of them in the nearby universe. It is the latter explanation that Beckmann finds more reasonable.
“Looking out to 300 million light years, the astronomers did not see the black holes that are causing the X-ray peak”
If the 30-keV X-rays are instead coming from more distant black holes, and hence a younger universe, astronomers have got their models wrong. “The picture of the evolution of AGNs and galaxies may be more complicated that we thought,” says Beckmann.
Angela Malizia, of the National Institute of Astrophysics in Bologna, Italy, a member of another team that is also sifting through the Integral data, says they too have come up with a shortfall. “It means we are missing something or maybe that we need to go to more sensitive instruments,” she says.
