New observations of a pair of colliding spiral galaxies hint at what might be in store for the Milky Way and the Andromeda galaxy if and when they collide.
The findings, described in July in the journal Monthly Notices of the Royal Astronomical Society, may also help to solve a puzzle plaguing the standard model of cosmology.
Around them are a horde of satellite galaxies. In the standard model of cosmology, such satellite galaxies are the leftover building blocks that were drawn together by a massive halo of dark matter and assembled the large galaxies long ago. According to theory, each dwarf should be orbiting its host galaxy through the dark matter halo on a random path, and together, they should create a cloud of satellites around the main galaxy, like a swarm.
Yet observations suggest that many galaxies don’t do this. Many of the Milky Way’s dwarf galaxies, for example, orbit it in the same direction along a narrow plane. Observations with the Hubble Space Telescope have shown that the dwarf galaxies of Andromeda — the closest major galaxy to the Milky Way — are also in their own co-rotating plane. The dwarf galaxies of other galaxies, such as Centaurus A — the product of a major merger between two large galaxies — also orbit in a plane.
The new observations of NGC 5713 and NGC 5719 have shown, for the first time, a cloud of dwarf satellites transitioning into a thinner orbital plane.
“We think that the satellites would likely have orbited randomly in more of a cloud than a disk prior to the massive galaxies’ approach,” Sarah Sweet, an astronomer at the University of Queensland in Australia, told Space.com. “Currently they are not so much in a plane but are certainly co-rotating, and in a flatter than isotropic distribution — more like a tube shape — around the center of mass of both galaxies.”
Fourteen dwarf galaxies have been confirmed in the NGC 5713/5719 system after having had their velocities measured to prove they are in orbit around their hosts. A further 18 candidates are awaiting confirmation, and there are probably many more that are too faint to have been detected. The observations indicate that the 14 confirmed dwarf galaxies are split into two subgroups associated with the two larger galaxies, but they also appear to be orbiting them in the same direction and in the same plane.
Sweet leads a survey program called Delegate, which involves researchers from Australian institutions investigating how representative the galaxies in our Local Group, particularly the Milky Way and Andromeda, are of galaxies elsewhere in the universe.
Our galaxy and Andromeda are heading toward each other. There’s still 2.5 million light-years between the two galaxies, but in 4 billion years or so, the two galaxies will come very close. A recent analysis concluded that although there’s only a 50% chance that Andromeda will collide with the Milky Way during their first pass in the next 10 billion years, gravity will eventually cause them to fall back onto each other and merge.
Astronomers are trying to figure out if that merger will resemble other mergers we see in the universe or if the Milky Way and Andromeda are different from other galaxies. Because NGC 5713 and NGC 5719 are farther ahead in the process of their collision, they make for a great case study.
The astronomers, led by Helmut Jerjen of the Australian National University, considered several explanations for why the orbits of the dwarf galaxies around NGC 5713 and NGC 5719 are transforming into a co-rotating, flattened plane. Their best explanation is that the two satellite systems, trailing after their host galaxies, fell onto each other along the Boötes strip, a great filament of dark matter in the cosmic web. As the two galaxies began to interact, the gravitational dynamics of the Boötes strip and the two galaxies led to the formation of a plane of satellites.
Cosmological simulations based on the standard model, which incorporates dark matter as a crucial ingredient, struggle to form galaxies with planes of satellites around them. That leaves two options: Either the models need to be updated, or the standard model of cosmology is wrong. While some researchers argue instead for an alternative theory of gravity, called modified Newtonian dynamics, to replace dark matter, others believe that the simulations are not accurate enough and require tweaking. The evidence from NGC 5713/5719 suggests that this might be the case.
However, the existence of planes of satellites around the Milky Way and Andromeda remains a puzzle, since the galaxies are still too far away from each other for a process similar to the one in NGC 5713/5719 to occur yet. There may have been other mergers in the past, however; the Hubble Space Telescope has observed signs that Andromeda experienced a significant collision and merger with another galaxy within the past few billion years. The Milky Way is more of a puzzle, though; its last large merger was between 8 billion and 11 billion years ago.
“Major mergers like this are one possible avenue for forming planes of satellites like we see around the Milky Way and Andromeda, but it is still something of a mystery because we do not see this sort of structure commonly in the best cosmological simulations,” Sweet said. “Other explanations could be that the dwarfs are formed in situ, or fall in along filaments, or a mixture of these.”
Hubble’s careful observations are gradually piecing together the motion of the dwarf galaxies around Andromeda, and it is estimated that another five years of study will enable astronomers to track back the orbits of those dwarf galaxies to see how they came to be in their current plane. And with continued observations of other galaxies, we can learn more about what makes our own galaxy tick.
