Australasian Science Magazine

The results of a computer simulation of a cold dark matter universe. Ordinary matter, in the form of stars and gas, is held to the dark matter via gravity and flows along filaments towards the largest local mass. The alignment of satellite galaxies around the Milky Way and Andromeda galaxies is a direct consequence of the filamentary nature of the universe.

By Stefan Keller

Astronomers have found a filament of ancient stars and galaxies that joins us to neighbouring clusters of galaxies and beyond to the vast interconnected universe.

Globular clusters hold hundreds of thousands of stars in a compact ball hundreds of light years across. From their colours and brightness these stars tell us that they are old – the majority formed at the same time as the Milky Way some 13 billion years ago.

Some globular clusters, however, are younger by several billion years than the majority. Understanding the creation of such star clusters has proven a problem.

The galaxy is known to have settled into its current disk-like shape at this time. This is expected to have drawn in the gas required to form such star clusters.

A study I have been conducting at the Australian National University (ANU) with Gary Da Costa and Dougal Mackey suggests a solution that is not of our galaxy. When we examined the distribution of the younger globular clusters around the Milky Way we saw that these clusters extend out to much greater distances compared with their older counterparts. Furthermore, the younger globular clusters are not scattered randomly about the Milky Way. Rather, they are confined to a plane that meets the disk of our galaxy almost face-on (Fig. 1).

The Milky Way has a flotilla of small satellite galaxies. When the arrangement in space of these satellites was examined, we found that these objects too are in a plane. Our findings show that the plane described by the Milky Way’s satellite galaxies and that of the globular clusters are identical.

How could such a cosmic coincidence arise? In our scenario, the younger globular clusters are the cores of cannibalised galaxies. Once a small galaxy comes too close to the Milky Way, the massive gravitational attraction of the Milky Way pulls the small galaxy to it. The smaller galaxy is flung around the Milky Way. With each orbit of the Milky Way, tidal forces thresh stars off the captured galaxy until all that remains is the core in the form of a globular cluster.

That globular clusters possess a range of ages suggests that the Milky Way has seen a protracted rain of smaller galaxies, each of which have eventually succumbed to the Milky Way’s gravity. As the Universe evolved, the frequency of such galaxy collisions diminished but remains an ongoing process.

A dramatic example of this is witnessed in the ongoing demise of the Sagittarius dwarf galaxy. The Sagittarius dwarf remained hidden against a backdrop of the Milky Way until its discovery at Australia’s Anglo-Australian Telescope. It was found as a grouping of stars travelling in a distinctly different direction to stars spinning in the Milky Way’s disk. On closer inspection the Sagittarius dwarf was found to be leaving a trail of debris that wraps around the entire Milky Way as this small galaxy is inexorably torn apart.

The tidal debris of Sagittarius is composed of two streams. This suggests that Sagittarius was originally a pair of galaxies that plunged together into the Milky Way’s gravity. At its heart, the Sagittarius dwarf possesses a central massive globular cluster. Once disruption of the galaxy is complete, the majority of Sagittarius’ stars will be dispersed within the Milky Way but the core globular cluster will remain to mark its demise.

Like a nurturing rain, the addition of small galaxies has added stars and gas to grow the Milky Way. As is evident by the plane of remnant globular clusters, the feeding of the Milky Way has occurred over much of the history of the Universe and from a preferred direction in the sky.

What does this mean for the distribution of matter in the Universe?

Let us also consider our nearest large neighbour, the Andromeda galaxy. Here we also see that Andromeda’s satellite galaxies form a plane. We find that the planes of satellites for the Milky Way and Andromeda systems are aligned along an axis that connects the two nearby galaxy clusters – the Virgo and Fornax clusters.

Such a cosmic alignment is, we argue, a natural consequence of our cosmology. We inhabit a universe in which 90% of matter cannot be seen – it is dark matter of nature unknown. We know that dark matter is there because its gravity changes the motion of stars and gas around it, but it does not interact in any other significant way with ordinary matter.

The paradigm of cold dark matter cosmology is a great success story of modern science. It has withstood several observational tests, from the cosmic microwave background formed shortly after the Big Bang to the explanation of the properties of how galaxies cluster at the present day.

Due to its dominance, dark matter is the medium that shapes ordinary stars and gas into galaxies that we see today. The gravity of dark matter draws with it the stars and galaxies like foam on the crest of a wave. Computer models of a universe of cold dark matter shows that material is driven into vast interconnected sheets and filaments that are stretched over enormous cosmic voids.

Gravity draws material over these interconnecting filaments towards the locally largest lumps of matter, with stars and galaxies drawn along for the ride. In this way the filaments act as cosmic umbilical cords that feed galaxies with matter, at first vigorously but at the present time only sporadically.

Such a cosmic filament extends between our neighbouring Virgo and Fornax galaxy clusters. It is from this filament that the Milky Way and Andromeda have been sustained by the inflow of small galaxies. The natural flow of smaller galaxies along this filament leads to the plane of satellite galaxies and younger globular clusters that we observe in the sky.

Our findings make a direct prediction that as-yet-undiscovered satellite galaxies of the Milky Way will trace the plane of satellites. There is no better place to look for these galaxies than in our own southern sky.

With this goal in mind, the ANU is operating the SkyMapper telescope. SkyMapper, led by Nobel laureate Brian Schmidt, is conducting the world’s first digital optical map of the southern sky. Matching results from the Northern Hemisphere, we expect that there are 10–20 unknown satellite galaxies of the Milky Way lurking in our skies.

These galaxies give us the clearest way with which to explore the cosmic filaments of dark matter that bind us to the fabric of the Universe.