Astronomy

Outflows from the first quasars

Black holes are best known for pulling matter in. But a distant supermassive black hole, observed as it was when the Universe was less than a billion years old, has been seen pushing gas out of its host galaxy.

Astronomers have long known of elliptical galaxies, which contain mostly old stars and are largely devoid of interstellar gas. But the finding1,2 in 2004 that such objects existed about 11 billion years ago, when the Universe was only 3 billion years old, was surprising — it hadn't generally been thought that such galaxies could have formed so early. The most popular explanation3 was that these ancient ellipticals once hosted the earliest quasars (accreting supermassive black holes), and that the energy released during this quasar phase was sufficient to blow out the galaxy's gas. Maiolino and colleagues4 now provide a significant boost for the quasar-outflow model in a paper published in Monthly Notices of the Royal Astronomical Society. The authors made the remarkable discovery that one such distant quasar, known as SDSS J1148+5251, which is seen as it was when the Universe was less than 1 billion years old, has just the sort of gas outflow required by these models.

The key to this story is the extreme environment at the centre of a galaxy. Most large galaxies, including the Milky Way, harbour at their centres black holes that have roughly a million times the Sun's mass, but in some cases the central black hole can be more than a billion times heavier than the Sun. These black holes are believed to have grown by accreting surrounding gas, a gradual process in which the infalling material is compressed into a disk and heated to such high temperatures that it comfortably outshines all the stars in the host galaxy. It is these accreting supermassive black holes that are known as quasars.

Although quasars are generally seen only as unresolved points of light, they have distinctive spectra characterized by broad ultraviolet and optical emission lines, which distinguish them from other astronomical sources. These emission lines are broadened by the Doppler effect that is associated with motion in the environment close to the quasar, revealing the extreme dynamics in the vicinity of the black hole. But the lines reveal little about the motion of the bulk of the interstellar gas farther out in the quasar's host galaxy.

The best way around this problem has been to try to measure emission lines associated with molecules that are not present in the immediate surroundings of the black hole. One possibility is to make observations at submillimetre wavelengths, at which there are several ionized-carbon emission lines. This method has been used to identify outflows from relatively nearby quasars (see, for example, ref. 5). Maiolino et al. adopted this approach, using one of the world's most sensitive millimetre arrays, the Institut de Radioastronomie Millimétrique Plateau de Bure Interferometer, to measure the shape — and thus the velocity profile — of an ionized-carbon emission line in the spectrum of SDSS J1148+5251. This light was emitted with a wavelength of 0.158 millimetres but was redshifted by the expansion of the Universe so that it reached Earth with a wavelength of 1.17 millimetres. The data showed not only a core line with a velocity width of a few hundred kilometres per second, as expected of material moving in a large galaxy, but also much broader 'wings' indicative of gas flowing out at speeds of up to 2,000 kilometres per second.

By adopting simple models to describe the geometry of the outflow (which the observations could not reveal), the authors found that the host galaxy of SDSS J1148+5251 was losing 10 solar masses of gas every day. Given that the total molecular-gas content of the galaxy had previously been estimated at 20 billion solar masses6, the galaxy would have had all of its gas blown out in about 6 million years — a mere instant in cosmological terms. And although the kinetic power of the outflow, some 2 × 1038 watts, might seem huge, it is less than 1% of the total power output of the quasar.

Overall, Maiolino and colleagues' data and interpretation paint a coherent picture of gas ejection from quasar host galaxies. However, given that quasars are fuelled by infalling material, it might seem surprising that they can also cause outflows. The explanation is that the light emitted by the quasar exerts a force (termed radiation pressure) on the surrounding gas, and in the extreme situation around a quasar this is strong enough to drive out all of the gas from the galaxy. The stars in the galaxy are so much denser than the gas that they are not noticeably affected, and the non-interacting dark matter between the stars does not experience any radiation pressure at all (Fig. 1).

Figure 1: Ejection of gas from a galaxy hosting a quasar.
figure1

Maiolino et al.4 have found a supermassive black hole (black circle) ejecting gas from its host galaxy. The white arrows show the spiral paths of material being accreted into the black hole, and the orange wavy lines represent photons emitted during this accretion process. Most of the photons escape the galaxy, perhaps to be seen by astronomers, but some impinge on clouds of gas (blue) in the galaxy, and this radiation pressure drives the gas out of the galaxy. The stars (yellow) and dark matter (grey points) are unaffected by the radiation.

Maiolino et al. also found some evidence that the outflow is visibly extended in their images, which would imply that it spans much of the galaxy. However, the tentative nature of this measurement, and the implication that this would be the largest such outflow ever measured, make this result speculative at best — a point that the authors are careful to make themselves. By contrast, the main finding that quasar SDSS J1148+5251 has been captured in the process of removing gas from its host galaxy seems quite robust, both because of the remarkable data and because of the existence of a compelling theoretical model.

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Correspondence to Daniel Mortlock.

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Mortlock, D. Outflows from the first quasars. Nature 489, 42–43 (2012). https://doi.org/10.1038/489042a

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