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Galaxy formation

Caught in the act?

Nature volume 430, pages 979980 (26 August 2004) | Download Citation


Which came first, the stars and gas that make up a galaxy, or the giant black hole at its centre? Observations of a distant galaxy, caught as it forms, could help solve this chicken-and-egg problem.

Galaxies are thought to be surrounded by massive haloes of dark matter, each outweighing its galaxy by a factor of about eight. The visible part of a galaxy, occupying the inner 10% of the halo, consists of a mixture of stars and gas. Galaxies harbour a giant black hole at their centres, which in some cases is actively fuelled as it sucks in surrounding gas. In especially active galaxies, called quasars, the fuelling rate is so high that the radiation generated close to the black hole outshines the cumulative star-light from the entire galaxy. The sequence of cosmic events that leads to this configuration is still largely mysterious. How does gas condense into the central regions of the dark-matter halo? At what stage of the gas condensation process do the stars and the giant black hole light up? The unique observation of a distant quasar by Weidinger et al.1, reported on page 999 of this issue, offers fresh insight.

The formation of massive dark-matter haloes is dictated by gravity, and can be described by using ab initio calculations2. As the Universe expanded from its dense beginning, tiny inhomogeneities in the distribution of dark matter were amplified through the effects of gravity. Regions of space that were slightly denser than average had a higher gravitational pull on their surroundings; eventually, these regions stopped following the expansion of the rest of the Universe, turned around and re-collapsed on themselves. The resulting dense knots of dark matter — forming the intersections of a cosmic web of less-dense dark-matter filaments — are believed to be the sites at which galaxies lit up.

Dark matter thus dominates the formation of a galaxy, at least initially, and determines the gross properties of the galaxy population, such as their abundance, size and spatial distribution. But it is the trace amount of gas (mostly hydrogen and helium), pulled with the dark matter into the collapsed haloes, that forms the visible parts of galaxies and determines their observable properties. In particular, to condense to the core of the dark halo, the gas must cool continuously so as to deflate the pressure acquired by its compression. A fraction of the gas (typically 10% by mass) eventually turns into stars, and a much smaller fraction (typically 0.1%) into the central massive black hole3.

The composition of the gas inside the galaxy can be studied through the spectrum of radiation that it absorbs and emits. Primordial gas is essentially a pure mix of hydrogen and helium, but the spectra of all of the quasars discovered so far have shown the presence of various heavier elements (such as carbon, nitrogen, oxygen and iron). This indicates that the gas has been enriched by the nucleosynthetic yields from previous generations of stars. Even the most distant quasars, including those that existed about a billion years after the Big Bang (a mere 5% of the current age of the Universe), show a significant heavy-element content4. This suggests that vigorous star-formation is a necessary condition for any quasar activity. On the other hand, star formation seems to be occurring on relatively small scales, close to the galactic centre. A natural inference would then be the following sequence of events: the cosmic gas first contracts to the inner regions of the halo, and only then forms stars — but this is still before the formation (or at least activation) of any central quasar black hole.

Not necessarily so, according to Weidinger et al.1. They have detected the faint glow of hydrogen emission enveloping a distant quasar at a radius equivalent to about 100,000 light years — several times the size of the visible part of a typical galaxy. Such emission has a simple physical origin. The hydrogen atoms falling through the halo are ionized by the quasar's light, then recombine with electrons to become atoms again. Each recombination results in the emission of a so-called Lyman-α photon (a photon with energy equal to the difference between the ground and first excited states of a hydrogen atom). As a result, when viewed through a filter tuned to the Lyman-α frequency, a faint ‘fuzz’ can be seen to surround quasars5. This fuzz can serve as a diagnostic of whether or not a spatially extended distribution of infalling gas is present around the quasar6. If most of the gas has already cooled and settled at the centre of the halo, the extended fuzz would be absent.

Although the fuzz detected by Weidinger et al. is faint, it is as bright as would be expected if all of the hydrogen needed to make up a typical galaxy is still infalling6. As a result, the galaxy imaged by Weidinger et al. is likely to be still in its infancy, despite the fully formed appearance of its bright central quasar black hole. From the shape and kinematics of the fuzz, the authors were also able to confirm the presence of the dark-matter halo, which is accelerating the infall of the hydrogen gas, and to measure the halo's mass. Their value — 2–7×1012 solar masses — is in accord with independent estimates from spectral absorption features7 and from the abundance8 of other quasars of similar brightness.

Extended Lyman-α emission has previously been detected around quasars, but its interpretation as being due to a wide region of infalling gas was less convincing9,10,11. In contrast, Weidinger et al.1 have obtained a two-dimensional image of the source that clearly shows the extent of the emission, and a spectrum showing a velocity pattern consistent with infall. Extended emission is a relatively common phenomenon among so-called radio-loud quasars — a special subset believed to drive powerful gaseous outflows. However, the gas surrounding these sources is always enriched with heavy elements, whereas the fuzz detected by Weidinger et al. appears to be pristine (no heavy-element absorption or emission features are detected).

This result will undoubtedly prompt further theoretical modelling of the density and velocity distribution of the infalling gas. One outstanding question, for example, is whether the gas could have been delivered by a recent merger with another galaxy; such mergers stir up and spread gas over large regions, and might also be responsible for having activated the quasar in the first place. An observational census of similar quasars should be revealing, and feasible with a modest investment of time on modern telescopes. It could show whether prompt activation of a quasar, at an early stage in the assembly of the galaxy, while there is still a significant amount of cosmic hydrogen-gas infall, is the norm among galaxies, or whether it is a peculiarity of the galaxy examined by Weidinger and colleagues. In the meantime, we have a new piece in the chicken-and-egg puzzle, suggesting that, at least occasionally, the egg — the black hole — may come first.


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  1. Zoltán Haiman is in the Department of Astronomy, Columbia University, New York, New York 10027, USA. e-mail:

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