The SAGA saga: Putting the Milky Way’s ‘peculiarities’ into context
A trio of new studies suggests the Milky Way has a unique configuration of satellite galaxies surrounding it.
A long-term survey of small, “satellite” galaxies that orbit larger galaxies across the universe offers new insights into our own galaxy, the Milky Way.
Since 2012, the Satellites Around Galactic Analogs (SAGA) Survey has studied the regions around 101 galaxies that are similar to the Milky Way — finding hundreds of smaller, low-mass satellite galaxies orbiting around them. The discoveries provide crucial information about galaxy formation and evolution, and how the Milky Way compares.
Three new SAGA studies accepted for publication in The Astrophysical Journal suggest the Milky Way is both typical and an outlier.
Most of the satellites of the Milky Way have stopped forming stars, unlike many of the systems SAGA found. However, the number of observable Milky Way satellites is consistent with those in systems that SAGA surveyed.
“The Milky Way’s satellite population is a unique combination of small satellites containing only older stars, and its two largest satellites, which are actively forming new stars,” said Marla Geha, a professor of astronomy and physics in Yale’s Faculty of Arts and Sciences and co-founder of the SAGA Survey with Yao-Yuan Mao of the University of Utah and Risa Wechsler of Stanford University.
The trio of new studies represents the final release of SAGA Survey data, following earlier releases in 2017 and 2021.
“Our work is similar to character development in a movie,” Geha said. “The SAGA Survey results help us construct a backstory for the Milky Way, which in turn allows us to put the Milky Way’s peculiarities into context.”
In one of the new studies, the researchers highlight 378 satellite galaxies identified across 101 larger systems. The number of confirmed satellites per system ranged from zero to 13 — compared to four observable satellites for the Milky Way. (The Milky Way has about 60 known satellites, but most are too faint to be seen by the SAGA Survey.)
A mosaic of the images of 378 satellites across 101 Milky Way-like systems that the SAGA team has surveyed. The satellite images are sorted by their luminosity from left to right. (Credit: Yao-Yuan Mao (Utah), with images from the DESI Legacy Surveys Sky Viewer)
The study also suggests a correlation between the most massive satellites and the total number of satellites in a system. The researchers found that systems with the most massive satellite galaxies — similar in size to LMC, the largest satellite galaxy orbiting the Milky Way — tend to have more satellites overall.
Another of the new studies looks at some of the characteristics of the 101 Milky Way analogs and their satellites. The researchers found, for example, that satellite galaxies located closer to their host galaxy were more likely to have their star formation “quenched,” or suppressed. This suggests that environmental factors help shape the life cycle of small, satellite galaxies.
The third new study uses the SAGA Survey results to improve existing theoretical models of galaxy formation. Based on the number of quenched satellites in Milky Way-like systems, this model predicts quenched galaxies should also exist in more isolated environments — a prediction that in the coming years should be possible to test using spectroscopic surveys.
The researchers said all three studies may help astronomers better understand the role of dark matter halos in galaxy evolution. Dark matter halos are regions of space that contain dense patches of dark matter. All galaxies are believed to form within dark matter halos, and smaller galaxies can become satellites if they are caught up in the gravitational pull of a much larger host galaxy and drawn into its dark matter halo.
The SAGA research indicates that the way low-mass galaxies fall into the gravity well of a massive host may depend on more than just the host galaxy’s mass — perhaps also involving the environment around the galaxy, for example.
The SAGA Survey was supported in part by the National Science Foundation and the Heising-Simons Foundation. SAGA used data from more than a dozen public and private sky surveys and specialized instruments for the research.