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Which came first - the galaxy or the black hole?

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This news isn't exactly fresh, having been announced several weeks ago, and it's been reported in many places. However, it's quite important (if correct), because it suggests that the supermassive black holes that seem to exist at the centers of most galaxies formed much faster, initially, than the rest of the galaxy.

We've already discussed a little about galaxy formation: here.

The study for present discussion was done using radio telescopes, which could detect spectral sequences due to carbon monoxide gas in order to make estimates of mass in galaxies' central regions. The galaxies studied were all observed as they existed less than two billion years after the big bang (redshift ≳ 3). At that age, all galaxies that can be observed in enough detail with optical telescopes are quasars, in which so much light comes from the hyperactive central black hole that almost nothing can be inferred about the rest of the galaxy. Use of radio telescopes circumvented that problem.

In most sufficiently regular galaxies closer to us, most of the mass of stars and gas is concentrated in a central bulge. In such galaxies, representing the large majority of the universe's age of 13.7 billion years, the ratio of mass in the central bulges to the mass of the central black hole is remarkably predictable and averages about 700:1. But in the four early galaxies studied, the ratio was only about 30:1. Although the mass of the central black holes studied was nearly as large as that of galaxies closer to us, the galaxies themselves appeared to be much less massive.

This anomaly in the ratios does not necessarily imply the black holes formed before the rest of the galaxy, but it does suggest they at least grew more quickly in the early stages. There are not yet good theories for why this might happen, or at what point growth of the rest of the galaxy accelerates compared to growth of the black hole.

The internal workings of quasars are not well understood. It may be that quasars can form only when the ratio of central mass to black hole mass is low, as in the observed examples. Or maybe gas in the neighborhood of a very active quasar is dispersed by the intense radiation, so that it can collapse further only when the activity dies down somewhat. Conversely, perhaps, in cases where the mass ratio is higher, radiation from the black hole has diminished, or never was as strong, so that we have not been able to detect those cases.

Good reasons exist to think that very active black holes should generally disrupt formation of stars in the surrounding gas. But there are also reasons they might at some point assist in star formation. We'll need much more observational evidence, and theoretical calculations, before the picture gets clearer.

There are other problems, too. For instance, the mass of the galactic bulges were estimated from the motion of gas rather than stars (which could not be observed). Magnetic fields could have affected gas flow, resulting in an underestimate of total mass. It's also not clear how supermassive black holes could have formed by themselves, without the gas they presumably grew from also collapsing to form individual stars.

While it's interesting to have a little evidence that the black holes formed first, a claim based on observations of only four early galaxies is less than fully convincing. Many more data points are needed.

And assuming the relationship holds up, even tougher questions remain. How and why did the black holes themselves form? How did they grow so big so quickly? What is the relationship over time between the size of the black hole and the galaxy that forms around it? How do they affect each other's growth?

Black Holes Lead Galaxy Growth (1/6/09)
Earlier studies of galaxies and their central black holes in the nearby Universe revealed an intriguing linkage between the masses of the black holes and of the central "bulges" of stars and gas in the galaxies. The ratio of the black hole and the bulge mass is nearly the same for a wide range of galactic sizes and ages. For central black holes from a few million to many billions of times the mass of our Sun, the black hole's mass is about one one-thousandth of the mass of the surrounding galactic bulge.

"This constant ratio indicates that the black hole and the bulge affect each others' growth in some sort of interactive relationship," said Dominik Riechers, of Caltech. "The big question has been whether one grows before the other or if they grow together, maintaining their mass ratio throughout the entire process." ...

"We finally have been able to measure black-hole and bulge masses in several galaxies seen as they were in the first billion years after the Big Bang, and the evidence suggests that the constant ratio seen nearby may not hold in the early Universe. The black holes in these young galaxies are much more massive compared to the bulges than those seen in the nearby Universe," said Fabian Walter of the Max-Planck Institute for Radioastronomy (MPIfR) in Germany.

"The implication is that the black holes started growing first."

The next challenge is to figure out how the black hole and the bulge affect each others' growth. "We don't know what mechanism is at work here, and why, at some point in the process, the 'standard' ratio between the masses is established," Riechers said.

However, another report quoting one of the researchers points out that only four very early galaxies were included in the radio telescope study:

In the Young Universe, Black Holes May Have Formed First (1/7/09)
“These very distant black holes are already about as massive as they will ever get — about 1 billion solar masses — so the only thing left is for the galaxy to form around them,” says Carilli. One implication, he says, is that the turbulent activity associated with accretion onto these black holes “may have a profound effect on the formation of the host galaxy very early in the universe.” But Carilli emphasizes that his team has examined only four galaxies from these early times. It’s possible, he says, that this handful of galaxies may have unusually heavy supermassive black holes.

“We really need to generalize to more galaxies that are less extreme,” he adds.

Martin Rees lays out the case for skepticism of the results:
“The results are interesting, and an important clue to the growth and evolution of galaxies,” comments Martin Rees of the University of Cambridge in England. However, he adds, “I think it is over-interpreting the data to say that ‘black holes come first.’ Even at [early times] the bulge mass could typically be about 100 times larger than the mass of the hole.”

Rees suggests that the bulges and holes form concurrently throughout cosmic history, but that in the early universe “it may be easier for infalling gas to go all the way to the center of a galaxy, forming a black hole rather than condensing into stars on the way in.”

A key question that’s still unanswered, he adds, is whether galaxies must have a minimum mass in order to possess a central black hole. “This is relevant to the issue of how the 'seed' black holes form, and to the role of mergers … in building up galaxies,” says Rees.


Other news reports:


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