The SAGA Survey has recently released three new research papers that shed light on the distinct characteristics of our Milky Way Galaxy. This follows their thorough examination of 101 satellite systems comparable to our own galaxy.
Is our Milky Way Galaxy truly unique? A group of scientists embarked on a quest over ten years ago to explore this question. Established in 2013, the Satellites Around Galactic Analogs (SAGA) Survey investigates galaxy systems akin to the Milky Way. Now, the SAGA Survey has unveiled three new research articles that enhance our understanding of what makes our Milky Way Galaxy one-of-a-kind after completing a survey of 101 similar satellite systems.
These “satellites” are smaller galaxies, both in mass and size, that orbit a larger galaxy, typically referred to as the host galaxy. Is our Milky Way Galaxy truly unique? A group of scientists embarked on a quest over ten years ago to explore this question. Established in 2013, the Satellites Around Galactic Analogs (SAGA) Survey investigates galaxy systems akin to the Milky Way. Now, the SAGA Survey has unveiled three new research articles that enhance our understanding of what makes our Milky Way Galaxy one-of-a-kind after completing a survey of 101 similar satellite systems.
Similar to how smaller satellites orbit Earth, these satellite galaxies are drawn in by the gravity of the large host galaxy and its surrounding dark matter. The Milky Way Galaxy is the host to several satellite galaxies, including the two largest ones, the Large and Small Magellanic Clouds (LMC and SMC). While the LMC and SMC can be seen with the naked eye from the Southern Hemisphere, there are numerous other dimmer satellite galaxies orbiting the Milky Way that require powerful telescopes to be observed.
The aim of the SAGA Survey is to analyze satellite systems around host galaxies that have stellar masses comparable to that of the Milky Way Galaxy. Yao-Yuan Mao, a faculty member at the University of Utah’s Department of Physics and Astronomy, co-leads the SAGA Survey alongside Marla Geha at Yale University and Risa Wechsler at Stanford University. Mao is the primary author of the first article in a series of three that have all been accepted for publication in the Astrophysical Journal. This series shares the latest findings from the SAGA Survey and makes its data accessible to researchers around the globe.
An outlier galaxy?
In the initial study led by Mao, researchers identified 378 satellite galaxies across 101 Milky Way-mass systems. The range of confirmed satellites per system varied from none to 13, while the Milky Way has four known satellites. Although the number of satellite galaxies in the Milky Way aligns with other similar systems, Mao noted, “The Milky Way appears to host fewer satellites if you consider the existence of the LMC.” The SAGA Survey has determined that systems with a significant satellite like the LMC typically have a higher total count of satellites, making our Milky Way seem like an exception.
One possible explanation for this difference between the Milky Way and SAGA systems is that the Milky Way has only recently acquired the LMC and SMC, in relation to the universe’s age. The SAGA article mentions that if the Milky Way Galaxy is indeed an older, slightly less massive host that has newly acquired the LMC and SMC, it would naturally have a reduced number of satellites, not counting the smaller satellites possibly brought in by the LMC and SMC.
This finding underscores the significance of understanding the relationship between the host galaxy and its satellites, especially when analyzing what we observe in the Milky Way. Ekta Patel, a NASA Hubble Postdoctoral Fellow at the University of Utah who is not part of the SAGA team, investigates the orbital histories of Milky Way satellites. Commenting on SAGA’s findings, Patel mentioned, “While we can’t yet study the orbital histories of satellites around SAGA hosts, the latest SAGA data release offers a tenfold increase in Milky Way-like systems that have an LMC-like companion than previously recorded. This major advancement provides more than 30 galaxy ecosystems for comparison with our own, which will be particularly beneficial in understanding how a substantial satellite like the LMC influences the systems they inhabit.”
What halts star formation in galaxies?
The second SAGA study, led by Geha, investigates whether these satellite galaxies are still forming stars. Understanding the factors that halt star formation in these small galaxies is crucial for studying galaxy evolution. The researchers discovered that satellite galaxies nearer to their host galaxy are more susceptible to having their star formation “quenched” or stopped. This indicates that environmental influences play a significant role in shaping the life cycle of small satellite galaxies.
The third new study is led by Yunchong (Richie) Wang, who earned his doctorate under Wechsler. This study leverages the results from the SAGA Survey to enhance current theoretical models of galaxy formation. The findings on the number of quenched satellites in these Milky Way-mass systems suggest that quenched galaxies should also exist in more isolated settings. This prediction is expected to be tested in the coming years with other astronomical surveys, such as the Dark Energy Spectroscopic Instrument Survey.
A contribution to the astronomy community
Alongside these exciting outcomes that deepen our understanding of galaxy evolution, the SAGA Survey team has made a significant contribution to the astronomy community. As part of their series of studies, the SAGA Survey team has published new distance measurements or redshifts for approximately 46,000 galaxies. “Finding these satellite galaxies is like searching for needles in a haystack. We measured redshifts for hundreds of galaxies to identify just one satellite galaxy,” Mao explained. “These new redshift measurements will empower the astronomy community to explore a diverse array of topics beyond just satellite galaxies.”
The SAGA Survey received support from the National Science Foundation and the Heising-Simons Foundation.
