The Milky Way's system of small, orbiting satellite galaxies is quite unusual, according to a new 12-year study of other galaxies in the local universe.
The Satellites Around Galactic Analogs (SAGA) survey is being conducted by a small group of astronomers to learn how the Milky Way and his small dwarf satellite entourage galaxies comparable to other galaxies.
'The Milky Way's satellite population is a unique combination of small satellites containing only older satellites starsand the two largest satellites, which are actively forming new stars," said Marla Geha, professor of astronomy and physics at Yale University and co-founder of SAGA, in a statement.
The two largest satellites are the Big And Small Magellanic Cloudsknown as the LMC and SMC in shorthand. These two satellites are by far the largest in the Milky Way family and are easily visible to the naked eye from the Southern Hemisphere. Most of the Milky Way's other 59 known satellite galaxies are extremely faint, making them necessary Hubble Space Telescope or large telescopes on the ground to detect them.
Related: Milky Way Galaxy: Everything You Need to Know About Our Cosmic EnvironmentSAGA conducted a census of 101 galaxies that are similar in size and mass to our Milky Way, and host a total of 378 satellite galaxies. The number of visible satellite galaxies per host galaxy ranged from 0 to 13. This is compared to the Milky Way, where SAGA (based on data from the Dark energy Spectroscopic Instrument, DESI) on the Mayall Telescope at Kitt Peak National Observatory in Arizona could detect only four satellites. The rest of our Milky Way's satellites are simply too faint for DESI to see.
"The Milky Way seems to host fewer satellites when you consider the existence of the LMC," said Yao-Yuan Mao, from the University of Utah and also co-founder of SAGA, in a statement.
That's because the trend discovered by SAGA is that host galaxies generally contain more satellite galaxies if they also have at least one Magellanic-type galaxy orbiting them.
However, galaxies without Magellanic types tend to have fewer satellites. One explanation is that the Magellanic Clouds are recent additions to the Milky Way family. For example, 2007 research by Gurtina Besla, now at the Steward Observatory in Arizona, found that the Magellanic Clouds are first-time visitors stuck by our Milky Way's gravity over the past three billion years and trapped in orbit. So before the Magellanic Clouds arrived, the Milky Way was not expected to have many bright satellite galaxies, based on the trends observed by SAGA. The assumption is that other Magellanic-type galaxies in other systems have formed around their host galaxy.
Moreover, Magellanic-type galaxies are themselves quite rare. Previous 2012 research led by Aaron Robotham of the University of Western Australia, as part of the Galaxy and Mass Assembly (GAMA) survey, concluded that only 3% of spiral galaxies similar to the Milky Way had Magellanic Cloud-type satellites.
In what is SAGA's final data release (the project's first two batches of data were produced in 2017 and 2021), astronomers also learned other things about dwarf satellites. For example, it found that the closer a satellite galaxy is to its host galaxy, the more likely the satellite's star formation rate will be zero, or close to it. The closer to the host galaxy, the deeper the satellite is in the host galaxy's gravity dark matter halo, and the closer it is to the radiation emitted by hot, young stars or supernova explosions that can remove star-forming gas from an orbiting satellite system.
Astronomers call the cessation of star formation in a galaxy "extinction," and SAGA's findings directly link these extinctions to the environment around the host galaxy. Most of the Milky Way's satellite galaxies are extinct, and that's at least partly why they're so faint: because they haven't managed to form many stars. SAGA's results also imply that extinct galaxies should be in more isolated environments, rather than in systems packed with other satellite galaxies with which they could interact, causing more star formation.
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But what does all this actually mean? Dwarf satellite galaxies exist within the vast halo of dark matter that surrounds all major galaxies. This halo acts as a gravitational platform for the formation of these galaxies. Dwarf galaxies are the building blocks of their larger host galaxies; the model of hierarchical galaxy formation described by the Standard Model of Cosmology, in which larger galaxies are composed of small galaxies, predicts that there should actually be many more dwarf satellites around the Milky Way than we currently observe. Where these missing galaxies are located remains a mystery, but by sampling and studying dwarf galaxies around other galaxies, we can learn more about the dark matter halos around other galaxies, their influence on galaxy formation and evolution, and where small satellite galaxies may be hiding.
There are three new research papers detailing SAGA's findings, led by Geha, Mao and Risa Wechsler of Stanford University in California. The papers have been accepted for publication in The Astrophysical Journal and are currently available as pre-prints: the Mao-led paper on the unusualness of the Milky Waythe Geha-led article on the extinguish satelliteand a third article about it modeling the data.