Paper of the month:  A galaxy lacking dark matter

May 16, 2018 by RebecaBV

There are more than one hundred billion galaxies in the observable Universe. Given this enormous number and the fast rate at which they are discovered and catalogued nowadays, one would think that a new observation probably wouldn't have much scientific relevance. But sometimes we encounter an exciting surprise, as is the case of NGC1052-DF2 (DF2 for short), a galaxy studied by van Dokkum et al. in their recent Nature paper [1]. DF2 would be the first galaxy ever observed whose dynamics don’t require dark matter to be explained. It is categorised as an ultra-diffuse galaxy, which means that it is very spread out but very faint. This is clearly shown in Figure 1, an image of the galaxy captured by the Hubble Space Telescope. DF2 is indeed very faint and smeared, but a couple of bright spots, highlighted by white numerals in the image, stand out. These are believed to be extremely bright globular clusters, whose properties are interesting enough to have warranted a separate publication [2].

Fig 1. Hubble Space Telescope image of the ultra-diffuse galaxy NGC1052-DF2. The ten marked objects are thought to be extremely bright globular clusters.

van Dokkum et al. carefully studied the properties of the galaxy and the dynamics of its globular clusters and reached a striking conclusion: DF2 seems to contain very little or no dark matter. Given that dark matter is not directly observable, its presence must be inferred by indirect means. In the paper, the authors estimated the mass of the galaxy in two different ways. Firstly, the amount of radiant matter in a celestial body can be determined by measuring its brightness and the distance that separates it from us. Of course, this method only accounts for the mass in ordinary matter (that is, matter that interacts with light), but not dark matter. The second method that was used is more intricate, as it involves carefully measuring the redshift of the 10 globular clusters in DF2 in order to determine their velocity dispersion. The more matter present in the galaxy, the stronger the gravitational pull and the larger the velocities of its stars and globular clusters will be. In all galaxies studied before DF2, the second method yielded a much larger mass than the first one, implying that they contain a large amount of dark matter. However, in the case of DF2, both methods give very similar results, leaving very little room for the presence of dark matter, as can be seen in Figure 2.

Fig. 2. Comparison between the inferred amounts of ordinary matter and total matter enclosed in NGC1052-DF2. The orange line depicts the observed mass in luminous objects, while the black arrows show the upper limit on the total mass. The black lines correspond to theoretical calculations of the total mass if a certain amount of dark matter is assumed. Note that the observations leave very little room for dark matter, which in all other galaxies observed up to date is orders of magnitude more abundant than ordinary matter.

How did this galaxy come about? If it were formed in the earliest period of structure formation, it would be a remarkable counterexample to established cosmological models, which predict that all galaxies have a substantial dark matter fraction. The authors provide some interesting explanations that are consistent with the current paradigm. They suggest that it could have been expelled during the merger of two larger galaxies, or that it could have formed from ordinary gas influenced and then ejected by an exotic astrophysical object called a quasar. Alternatively, it could have arisen from gas initially destined for a much larger, neighbouring galaxy, which then fragmented during its journey. Although none of these mechanisms perfectly explains the characteristics of DF2, the associated astrophysics is complicated and so all the solutions are plausible. There is hope that the measurement of other galaxies similar to DF2 may shed light on its genesis. At the same time, other papers have cast some doubt on the underlying assumptions and uncertainties of the result, including the estimation of the distance of DF2 from us (which affects the inference of its radiant matter) and the suitability of using globular cluster dynamics to determine the gravitational mass of the galaxy [3]. Future measurements may resolve these open questions.

One of the most vigourous debates in the past three decades has been whether or not modifying gravity (particularly a class of theories called “MOND”) is a viable, or indeed preferable, alternative to dark matter. This is tackled by the authors, who claim that DF2 poses a challenge to MOND. They argue that since the galactic dynamics are consistent with the observed luminous matter, there is no sign of an alteration to gravity. While this is a reasonable argument, MOND advocates have responded that it does not account for subtleties and variations of MOND models. One rebuttal is that MOND is not necessarily ubiquitous, another is that the paper overstates MOND prediction for DF2’s dynamics. The debate will undoubtedly continue for some time. But perhaps the most significant development in our understanding of dark matter will have come from a galaxy where it’s nowhere to be found.

Text by Gonzalo Alonso Alvarez (UHEI) and Rupert Coy (CNRS)

References:

[1] van Dokkum, P. et al., A galaxy lacking dark matter, Nature 555 (2018) no.7698, 629-632

[2] van Dokkum, P. et al., Extensive globular cluster systems associated with ultra-diffuse galaxies in the Coma cluster, Astrophys. J. 844, L11 (2017).

[3] Laporte, Chervin F. P. et al., Measured and found wanting: reconciling mass-estimates of ultra-diffuse galaxies, Submitted to MRNAS.