The Andromeda Galaxy's Early Quenching of Satellite Galaxies
A new study published in the Monthly Notices of the Royal Astronomical Society sheds light on how the Andromeda Galaxy (M31) quenches and consumes its smaller satellite galaxies long before they actually fall into the larger host galaxy. The research, led by Alex Merrow from Durham University, leverages precise data from missions like ESA's Gaia to reconstruct the complex history of galactic mergers and their impact on galaxy evolution.
Galactic Mergers and Quenching
Mergers play a crucial role in the growth and evolution of galaxies over cosmic time. Our own Milky Way is currently in the process of slowly consuming the Large and Small Magellanic Clouds, as evidenced by the 600,000 light-year long Magellanic Stream of gas being stripped away. The Milky Way has also previously merged with other galaxies like the Gaia-Enceladus-Sausage dwarf galaxy billions of years ago.
Quenching, the process that shuts off star formation in a galaxy by removing or heating its gas, is a key aspect of the merger process. As Merrow and colleagues explain:
"The Local Group offers a unique testbed for galaxy evolution theories due to our unique vantage point within it. 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."
Studying Andromeda's Satellites
To understand the quenching and consumption of satellite galaxies, the researchers focused on 39 dwarf galaxies orbiting Andromeda. Using cosmological simulations and known properties of M31, they estimated the infall times, pericentric passage times, and proper motions of these satellites.
By comparing these orbital histories with the satellites' star formation histories, the team investigated the dominant environmental mechanisms responsible for quenching M31's satellites:
- Ram-pressure stripping: The removal of gas from a galaxy as it moves through the intergalactic medium
- Tidal stripping: The gravitational removal of stars and gas from a galaxy by a larger host galaxy
- Shutoff of gas accretion: The cessation of fresh gas infall that fuels ongoing star formation
Quenching Timescales and Mechanisms
The results showed that only the most massive of Andromeda's satellites can sustain star formation for more than 3 billion years after their closest approach to M31 (pericentre). Lower mass satellites with less than about 30 million solar masses are reliably quenched by ram-pressure, tidal stripping, and/or the shutoff of gas accretion upon becoming satellites of Andromeda.
Intriguingly, many of the lower mass satellites were quenched long before encountering M31 directly. As the authors note:
"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."
This early quenching can be attributed to reionization heating the satellite's gas until it escapes, or a process called "pre-processing" where the satellites spent time near a different, lower-mass host galaxy before falling into M31.
Comparing Andromeda and the Milky Way
When contrasting these findings with the Milky Way's satellite population, the researchers discovered notable differences. The Milky Way's satellites have generally been satellites for longer and were quenched more rapidly after infall. Previous studies found that 76% of the Milky Way's satellites have old quenching times over 11 billion years ago and/or infall times exceeding 9 billion years.
Andromeda's satellites, however, exhibit a broader and more uniform distribution of infall and quenching times. This could reflect observational differences, or suggest that the Milky Way consumed its satellites earlier than Andromeda did, with the key exceptions of the Magellanic Clouds.
Implications and Future Directions
This research highlights how the quenching of low-mass satellite galaxies is a crucial and reliable aspect of galactic evolution. The exact timing and primary mechanisms depend on the specific history and environment of the host galaxy. As stated by Merrow and colleagues:
"The properties of M31's satellites reflect the fact that environmental effects – ram pressure, tidal stripping or the cessation of gas accretion – are reliable quenchers of low mass satellite galaxies in the Universe."
With the wealth of new data from James Webb Space Telescope, ESO's Extremely Large Telescope, and other cutting-edge observatories, astronomers will be able to study the intricate dance of galactic mergers and satellite quenching across an ever-expanding range of cosmic history. This will yield vital insights into the formation and evolution of galaxies from the earliest epochs to the present day.
