An analysis of a three-dimensional map of galaxies and their velocities reveals the hitherto unknown edges of the large system of galaxies in which we live — dubbed the Laniakea supercluster. See Letter p.71
One of the greatest advances in cosmology has been the discovery of how matter and light are organized on scales larger than those of galaxies1,2. However, despite tremendous effort, astronomers have been unable to map in detail the large-scale cosmic structure in which the Milky Way resides. Now, on page 71 of this issue, Tully et al.3 report an analysis of data from a vast catalogue of galaxies that has allowed them to do just that.
The large-scale structure of the Universe is an intricate network of clusters, filaments and superclusters of galaxies, together with cosmic voids that are almost empty of galaxies. Superclusters are extended regions containing a large number of galaxies, but this concept is rather vague; researchers lack a robust, quantitative definition for them. Tully and colleagues have found a neat way of identifying the edges of superclusters by examining the motions of galaxies. In doing so, they have detected the boundaries of our home supercluster, which they have called the Laniakea supercluster. Their paper is supplemented by a beautiful movie (http://irfu.cea.fr/laniakea) that shows our supercluster and its dynamical connection to other neighbouring large-scale systems. The movie is essential for comprehending the complexity of cosmic structures.
Mapping the large-scale structure of the nearby region of the Universe is important for several reasons. First, it reveals details of the large-scale cosmic structures that surround the Milky Way. These details are nearly impossible to observe for systems far away from Earth. Second, the morphology of the nearby Universe is essential for a precise determination of cosmological parameters such as the density of dark energy4, which is thought to drive the acceleration of the expanding Universe. Third, examination of cosmic structures around the Milky Way will help us to understand how the Galaxy formed and evolved5, and galaxy-formation processes in general.
Tully and colleagues' study is based on data from the Cosmicflows-2 galaxy catalogue6. The authors combined existing measurements of the velocities at which galaxies recede from Earth — which are mainly caused by the cosmic expansion and provide an indirect estimate of how far away they are — with direct galaxy distance measurements from the Cosmicflows-2 data set. This enabled them to derive the 'peculiar velocities' of the galaxies, that is, their true velocity relative to a rest frame. The peculiar velocity is obtained by subtracting the contribution of the cosmic expansion, which is determined using the direct distance measurement, from the recession velocity.
Direct distance measurements of galaxies are extremely difficult to perform, and the lack of such data has limited this kind of analysis in the past. However, the use of peculiar velocities can provide information about cosmic structures that is otherwise hard to obtain. And in the present case, it allowed the extent, structure and dynamics of Earth's supercluster, as well as those of other nearby superclusters, to be determined. We can only imagine what other details and structures might be uncovered if additional direct-distance measurements of galaxies are carried out.
A noteworthy aspect of Tully and colleagues' study is the use of Wiener filtering7 — a nifty algorithm that translates an incomplete map of peculiar velocities of galaxies into a complete map of the underlying distribution (density field) and dynamics (velocity flow field) of matter (Fig. 1). It is this technique that allowed the authors to come up with a quantitative definition of a supercluster. According to their definition, a supercluster is a 'basin of attraction' in the velocity flow field. In other words, the boundaries of a supercluster are defined by the places at which the velocity flow field points in different directions on either side of the boundary. This is the first clear definition of a supercluster. The downside of it is that it requires dynamical information that is available only for the nearby Universe.
Tully et al. find several basins of attraction in our corner of the Universe, including Laniakea and the previously known Perseus–Pisces and Shapley superclusters. Laniakea has a diameter of 160 million parsecs (520 million light years), and is much bigger than already identified superclusters in our local neighbourhood. However, it is smaller than the largest superclusters that have been found in the more distant Universe8. It is a surprise that Laniakea was not spotted in previous astronomical surveys. It seems that measurements of the peculiar velocities of galaxies are essential for identifying the boundaries of some superclusters.
Of course, these results do not mark the end of mapping the Universe. Although Tully et al. used the best galaxy catalogue available, these data do not extend far enough in cosmic space to explain the motion of our Galaxy with respect to the rest frame of the cosmic microwave background — relic radiation from the Big Bang. The Universe must be mapped on a much bigger scale than that achieved here to fully understand what processes affected the formation of cosmic structures in our local Universe. This is a challenging task, but one that is worthwhile and that we must hope will be tackled using future surveys.
Finally, I praise the choice of the name Laniakea for Earth's supercluster. It is taken from the Hawaiian words lani, which means heaven, and akea, which means spacious or immeasurable. That is just the name one would expect for the whopping system that we live in.
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