Astronomers spar over stars

The prestigious science journal Nature published an article in May claiming that massive black holes quench star formation in galaxies. Last week, University of Groningen astronomer Peter Barthel published the exact opposite conclusion in the Astrophysical Journal Letters. So who’s right?

The clash is remarkable. The Nature paper was written by a consortium of 60 scientists and is based on measurements by the Herschel Space Telescope. Barthel published in the Astrophysical Journal Letters with only three colleagues, and based his conclusion on data from the very same telescope. What is more, he is on the scientific steering committee for the Herschel telescope.

The Nature paper describes how galaxies with very strong X-ray emission – signifying the presence of a massive black hole at their centre – have low rates of star formation. The authors argue that the strong radiation, produced by matter falling into the black hole, must blast the dust clouds away that could otherwise have given birth to new stars.

But Barthel found the opposite: in several dozens of galaxies that produced a lot of radio emission (also a sign of a massive active black hole), star formation increased with the amount of radiation. A bigger black hole at the centre of a galaxy, producing more radiation, coincided with more vigorous star formation. Such a black hole together with all the bright new stars creates real fireworks, Barthel said in a press release. In contrast, our own Milky Way has a modest black hole at its centre and the rates of star formation are not as high.

How can it be that the two papers reached such opposing conclusions? Barthel has some ideas, ‘The Nature group measured the observed radiation from the galaxies, without correction for possible attenuation. But star formation occurs in galaxies with a lot of dust and this absorbs a lot of the radiation.’ This could create the impression that galaxies with a lot of star formation are the ones that produce weak X-ray emission.

‘But my hunch is that these galaxies are more active. The radiation just doesn’t reach us, or is at least strongly attenuated.’

Barthel can see more technical issues with the Nature paper, but these are harder to explain. He therefore believes he and his three colleagues are right and the 60 Nature authors wrong. ‘Actually, last Monday a paper was submitted to an astronomy preprint server that is in full agreement with us. They also say there is no evidence that massive black holes reduce star formation.’ However, he adds, this paper is just a draft. ‘But it is also based on the same type of Herschel data, although they have used a bigger set.’

Isn’t this a bit of an embarrassment to astronomy? Barthel smiles, ‘Well, you know, this is how science progresses. You have conflicting hypotheses, they are published and then everyone can judge for themselves. Eventually, we will reach a consensus on what is really happening.’ But he is convinced he is right. Until proven otherwise. ‘We have a very interesting time ahead of us!’

A press release on the paper by Barthel can be found on the University website.