Astronomers know that mergers play a huge role in galaxy growth. Right now, the Milky Way is slowly consuming the Large and Small Magellanic Clouds. The evidence is a stream of gas called the Magellanic Stream that's about 600,000 light-years long. The Milky Way (MW) is stripping this gas from the clouds, which don't have enough mass to retain it. They're losing the gravitational tug-of-war with the much more massive MW.
The MW has consumed many other galaxies, too. The ESA's Gaia mission showed us that the MW consumed the Gaia-Enceladus-Sausage dwarf galaxy billions of years ago.
Our neighbour Andromeda is known to host many satellite dwarf galaxies and has grown through mergers in its past. New research examines its dwarf galaxies to try to understand the long, complex merger process. It's titled "The lives and deaths of faint satellite galaxies around M31," and it's been submitted to the Monthly Notices of the Royal Astronomical Society, and is currently available at arxiv.org. The lead author is Alex Merrow from the Department of Physics at Durham University.
Thanks largely to the ESA's Gaia mission, astronomers have a wealth of accurate proper motion measurements for more than one billion stars across the Milky Way. This allows them to recover the orbits of satellite galaxies back in time. Combined with the ability to spatially resolve individual stars at great distances, astronomers can detect populations of stars that share origins, even though they're no longer necessarily associated gravitationally. The researches point out that by using all of this data together, they can gain insights into how satellite galaxies are shaped by physical processes as they orbit their much larger hosts.
"The Local Group offers a unique testbed for galaxy evolution theories due to our unique vantage point within it," the authors explain. Previous research has examined how dwarf galaxies fall into the MW, and at what point during that process they're quenched, meaning they lack the gas to form new stars. The larger galaxy draws the gas away, which can sometimes trigger a massive burst of star formation in the larger galaxy. In a sense, the larger galaxy cannibalizes its smaller satellite and steals its new stars.
The researchers wanted to understand how satellite galaxies are quenched, a critical part of the merger process.
"We present predictions for proper motions, infall times and times of first pericentric passage for 39 of M31's satellite galaxies," the authors write. They estimated these properties based on cosmological N-body simulations and the properties of M31 itself. "We use these constraints on the satellites' orbital histories in conjunction with their published star formation histories to investigate the dominant environmental mechanisms for quenching satellites of M31-like hosts," they explain.
The results show that only the most massive satellites of M31 can maintain star formation for more than 3 billion years after their pericentre. The pericentre is the point where a satellite is closest to M31 and exposed to its powerful gravity. The authors say this indicates that "ram-pressure, tidal stripping and/or the shutoff of gas accretion reliably quenches dwarf galaxies with solar masses < 107.5 M⊙ upon becoming satellites of M31."
These lower mass satellites struggle to survive. The researchers say their results show that a large portion of them are quenched long before they really encounter Andromeda. "The majority of the remaining lower mass satellites appear to have been quenched significantly before their first pericentre passage, with some of the least massive quenching up to 10 Gyr prior," the authors write.
Some of them are quenched by reionization, where the gas is heated by UV radiation. Once its superheated, it acquires sufficient kinetic energy to escape from the dwarf galaxy. It's photoevaporated, or sort of boiled away from the satellite galaxy. But most of the low-mass satellite galaxies that are quenched very early on are quenched by what the researchers call 'pre-processing.' In this case, the satellite galaxy spent time near a different host galaxy with lower mass than Andromeda. This pre-processing can heat gas and remove it, leading to quenching.
The research team compared these results with what's known about the Milky Way. They found that the two galaxies have qualitatively different populations of satellite galaxies. "In particular, the Milky Way’s satellites have generally been satellites for longer and have been quenched more quickly following infall than M31’s satellite population," they write. Leaning on previous research, they point out that the Milky Way's satellites "mostly have old (> 11 Gyr) quenching times and/or old (> 9 Gyr) infall times, accounting for 76 per cent of the population."
But Andromeda's satellites have a wider and more even spread of both infall times and quenching times. This could be from the differences in observing and measuring the satellites of both galaxies. Or, it could suggest that the MW consumed its satellites earlier than Andromeda, with the notable exceptions of the Large and Small Magellanic Clouds.
"Across M31’s satellites, we see most quenching occuring before pericentre or even before infall," the authors explain. "This is consistent with any of internal quenching, pre-processing by other halos and/or the cosmic web or, in a few of the oldest cases, quenching by reionisation."
Astronomers have a lot more data available to them than even a few years ago, and it's allowing them to examine how massive galaxies quench and consume their satellites. Quenching is a fundamental part of merging, and this research lets astronomers get a handle on how it worka differently in different galaxies.
"The properties of M31’s satellites reflect the fact that environmental effects – rampressure, tidal stripping or the cessation of gas accretion – are reliable quenchers of low mass satellite galaxies in the Universe," the authors conclude.