In 2013, scientists started the Satellites Around Galactic Analogs (SAGA) Survey to find small galaxies orbiting larger ones, like the Milky Way. Recently, SAGA published three papers showcasing the Milky Way’s uniqueness after studying 101 similar satellite systems.
Satellite galaxies are small and low in mass. They are pulled in by the gravity of larger host galaxies and become part of the host’s dark matter halo.
The Milky Way has several satellite galaxies, with the Large and Small Magellanic Clouds (LMC and SMC) being the largest and visible to the naked eye from the Southern Hemisphere. Many faint satellite galaxies can only be seen through large telescopes.
Unraveling the mystery of the Milky Way’s satellite galaxies
The SAGA Survey aims to study satellite systems around other galaxies with stellar masses similar to the Milky Way.
In the first study, scientists identified 378 satellite galaxies in 101 systems similar to the Milky Way. The number of confirmed satellites in each system varied from zero to 13, while the Milky Way has four. Overall, the number of satellite galaxies around the Milky Way is similar to those in other Milky Way-mass systems.
The SAGA Survey found that systems with a large satellite, like the LMC, usually have more satellites overall, with the Milky Way being an exception.
One reason for this difference could be that the Milky Way only recently acquired the LMC and SMC in the universe’s timeline. According to the study, if the Milky Way is an older, somewhat less massive host that recently acquired the LMC and SMC, it would anticipate having a lower number of satellites, excluding the smaller ones that the LMC and SMC may have introduced.
This finding highlights the need to understand how host galaxies interact with their satellite galaxies, particularly when interpreting observations of the Milky Way.
Ekta Patel, a NASA Hubble Postdoctoral Fellow at the U but not part of the SAGA team, said, “Though we cannot yet study the orbital histories of satellites around SAGA hosts, the latest SAGA data release includes a factor of ten more Milky Way-like systems that host an LMC-like companion than previously known. This huge advancement provides more than 30 galaxy ecosystems to compare with ours. It will be instrumental in understanding the impact of a massive satellite analogous to the LMC on the systems they reside in.”
The second study, led by Marla Geha at Yale University, explored whether star formation is still occurring in these satellite galaxies. Scientists found that satellite galaxies- close to their host galaxies—tend to have their star formation “quenched” or suppressed. This suggests that environmental factors play a vital role in the life cycle of small satellite galaxies.
The third study, led by Yunchong (Richie) Wang- who obtained his doctorate with Wechsler, used SAGA Survey results to improve existing theoretical models of galaxy formation. Based on the number of quenched satellites in Milky Way mass systems, this model suggests that quenched galaxies should also be found in more isolated environments. This prediction can be tested in the coming years using other astronomical surveys, like the Dark Energy Spectroscopic Instrument Survey.
Milky Way is somewhat normal but not completely normal
Along with these three studies, the SAGA survey published new distance measurements, or redshifts, for about 46,000 galaxies.
Yao-Yuan Mao, a University of Utah faculty member in the Department of Physics & Astronomy, said, “Finding these satellite galaxies is like finding needles in a haystack. We had to measure the redshifts for hundreds of galaxies just to identify one satellite galaxy. These new galaxy redshifts will enable the astronomy community to study a wide range of topics beyond the satellite galaxies.”
Journal References:
- Yao-Yuan Mao, Marla Geha, Risa H. Wechsler, Yasmeen Asali et al. The SAGA Survey. III. A Census of 101 Satellite Systems around Milky Way-mass Galaxies. The Astrophysical Journal. DOI: 10.48550/arXiv.2404.14498
- Marla Geha, Yao-Yuan Mao, Risa H. Wechsler, Yasmeen Asali et al. The SAGA Survey. IV. The Star Formation Properties of 101 Satellite Systems around Milky Way-mass Galaxies. The Astrophysical Journal. DOI: DOI: 10.48550/arXiv.2404.14499
- Yunchong Wang, Ethan O. Nadler, Yao-Yuan Mao et al. The SAGA Survey. V. Modeling Satellite Systems around Milky Way-mass Galaxies with Updated UniverseMachine. The Astrophysical Journal. DOI: 10.48550/arXiv.2404.14500