Astrophysics

Distant galaxies lack dark matter

The masses of nearby spiral galaxies are dominated by invisible 'dark matter'. Surprisingly, galaxies in the distant Universe seem to contain comparatively little of it. See Letter p.397

In the 1970s, astronomers discovered that stars in the outer parts of galaxies seem to be moving too fast for the gravity due to their observed mass to keep them in orbit1. The researchers therefore suggested that most of the mass in these outer regions must be invisible. This 'dark matter' now underpins much of our understanding of the large-scale structure of the Universe, and also defines how galaxies form and evolve. On page 397, Genzel et al.2 report measurements of the rotation speeds of distant galaxies, extending the work that began nearly 50 years ago, but to a time when the Universe was approximately 20% of its current age. The authors' results suggest that these early galaxies were dominated by stars and gas, rather than dark matter, shedding light on how they formed and evolved.

The general process of galaxy formation and evolution is relatively well understood, but many of the details remain a mystery. Theoretical models3 suggest that galaxies form inside haloes of dark matter, and became stable structures owing to the influence of gravity. As the gas (which is mostly hydrogen) in these dark-matter haloes collapses, it separates from the dark matter. If the gas is not strongly disturbed during this process, it eventually becomes a stable disk in which stars can form.

During the epoch when today's massive galaxies formed most of their stars (about 10 billion years ago4), the stars and gas are thought to have been more concentrated than the dark matter. This is because gas interacts with its surroundings more strongly than dark matter does, and therefore loses energy more quickly. Consequently, the inner disks of distant galaxies should contain a larger fraction of stars and gas than those of nearby galaxies. However, there are few observational constraints on the distribution of dark matter, stars and gas in the inner disks of distant galaxies. Such constraints require the measurement of galaxy rotation curves, which describe the speed of stars as a function of their distance from the galactic centre.

Genzel and colleagues demonstrate that the rotation curves of distant star-forming galaxies can be measured with sufficient resolution to determine the composition of their inner disks. The authors use the K-band Multi-Object Spectrograph (KMOS) on the 8.2-metre Very Large Telescope in Chile to study six star-forming galaxies in the distant Universe. They use the motion of hydrogen gas in these galaxies to infer the shape of the galaxies' rotation curves. The authors' data set and the level of detail provided by the KMOS observations are remarkable, allowing some of the strongest constraints on the relative fraction of dark matter, stars and gas in such early galaxies.

The galaxies studied by Genzel et al. are ideal for two reasons. First, they seem to be forming stars at rates of 50–200 solar masses per year. This is typical of star-forming galaxies at this epoch5. Second, the total mass of the stars in these galaxies is similar to (or slightly higher than) that of the Milky Way6. The authors' galaxies are therefore expected to evolve into structures that are like the spiral and spheroidal galaxies we see today. Consequently, determining the composition of these galaxies provides a way of probing the pathway by which today's massive galaxies were assembled.

Away from the galactic centre, the rotation curves of nearby spiral galaxies become flat1, because dark matter dominates the mass of the galaxies' outer disks (Fig. 1a). By contrast, Genzel and collaborators show that the rotation curves of their galaxies, after rising to a peak, decrease rapidly with increasing distance from the galactic centre (Fig. 1b). This suggests that the fraction of dark matter in these distant galaxies is modest to negligible — the central regions seem to be dominated by stars and gas, and have little room for dark matter.

Figure 1: Galaxy rotation curves.
figure1

A, NASA/JPL-CALTECH; B, DATA SOURCE: NASA HUBBLE SPACE TELESCOPE

a, Nearby spiral galaxies such as Andromeda (shown here) are observed1 to have flat 'rotation curves' — away from the galactic centre, the rotation speeds of stars are approximately constant as a function of distance from the centre (red). This is in contrast to the rotation curves expected from the distribution of visible matter (yellow). To explain this discrepancy, astronomers proposed that the mass of galaxies is dominated by invisible 'dark matter' (blue). b, Genzel et al.2 report observations of six massive star-forming galaxies in the distant Universe (an example of such a galaxy, UDFJ033237-274751, is shown here). The authors find that these galaxies have rotation curves that, after rising to a peak, decrease with distance, suggesting that they contain relatively little dark matter.

The authors provide two possible explanations for their results. First, galaxies at these early times are known to be gas-rich owing to the continuous inflow of gas from the intergalactic medium. This inflow takes place down the long, thin filaments that make up the cosmic web — a major constituent of the large-scale structure of the Universe. Once inside the dark-matter halo, the gas can quickly lose angular momentum and 'pile up' in the central regions of the galaxy. Because this gas provides the raw fuel for star formation, the gas, and the stars that form from it, dominate these central regions. Second, because dark-matter haloes should be growing rapidly at this epoch, they might not yet be in a state of equilibrium. Therefore, regions of low dark-matter concentration could exist that do not conform to the density–size relations7 that govern the structure of dark-matter haloes in the local Universe.

Ruling out the possibility that Genzel and colleagues' galaxies are atypical will require a larger sample than that studied by the authors. Nevertheless, by measuring the contribution and spatial distribution of dark matter, stars and gas in distant star-forming galaxies, the authors' work is an important step towards identifying the dominant physical processes responsible for galaxy formation. In particular, their results shed light on how the star-forming, clumpy, irregular galaxies seen in the distant Universe transformed into the distinctive spiral galaxies, such as the Milky Way, that we see today.

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Correspondence to Mark Swinbank.

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Swinbank, M. Distant galaxies lack dark matter. Nature 543, 318–319 (2017). https://doi.org/10.1038/543318a

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