What Makes the Milky Way a Special Galaxy? Team of Astronomers Publish New Findings

Is our galaxy, the Milky Way, a special place? A team of scientists set out on a journey to answer that question more than a decade ago. The Satellites Around Galactic Analogs (SAGA) survey, which began in 2013, studies galactic systems like the Milky Way. Now, the SAGA survey has published three new research papers on the topic. arXiv preprint server that provides us with new insights into the uniqueness of our own galaxy, the Milky Way, after completing a census of 101 satellite systems similar to the Milky Way.

These “satellites” are smaller galaxies, both in mass and size, that orbit a larger galaxy, usually called a host galaxy. Much like the smaller satellites that orbit Earth, these satellite galaxies are captured by the gravitational pull of the massive host galaxy and the dark matter surrounding it.

The Milky Way is home to several satellite galaxies, the two largest of which are the Large and Small Magellanic Clouds (LMC and SMC). While the LMC and SMC are visible to the naked eye from the southern hemisphere, there are many other fainter satellite galaxies orbiting the Milky Way that can only be seen with a large telescope.

The goal of the SAGA survey is to characterize satellite systems around other host galaxies that have stellar masses similar to those of the Milky Way. Yao-Yuan Mao, a faculty member in the Department of Physics and Astronomy at the University of Utah, co-leads the SAGA survey with Marla Geha of Yale University and Risa Wechsler of Stanford University.

Mao is the lead author of the first paper in a series of three that were all accepted by the Journal of AstrophysicsThis series of articles presents the latest findings from the SAGA survey and makes the survey data available to other researchers around the world.

An aberrant galaxy?

In the first study led by Mao, researchers found 378 satellite galaxies identified in 101 Milky Way mass systems. The number of confirmed satellites per system ranged from zero to 13, compared with four satellites for the Milky Way. Although the number of satellite galaxies in the Milky Way system is comparable to that of other Milky Way mass systems,

“The Milky Way appears to have fewer satellites when you consider the existence of the LMC,” Mao said. The SAGA study found that systems with a massive satellite like the LMC tend to have a higher total number of satellites, and our Milky Way appears to be an exception in this regard.

This apparent difference between the Milky Way and the SAGA systems is explained by the fact that the Milky Way acquired the LMC and SMC very recently, relative to the age of the universe. The SAGA paper explains that if the Milky Way is an older and slightly less massive host with the recently added LMC and SMC, then one would expect a lower number of satellites in the Milky Way system, not to mention other smaller satellites that LMC/SMC could have contributed.

This result demonstrates the importance of understanding the interaction between the host galaxy and satellite galaxies, particularly when interpreting what we learn from observing the Milky Way.

Ekta Patel, a NASA postdoctoral researcher at Hubble University who is not part of the SAGA team, studies the orbital history of the Milky Way’s satellites. Patel said of the SAGA results, “While we cannot yet study the orbital history of satellites around SAGA hosts, the latest SAGA data include a factor of ten more Milky Way-like systems that host a companion similar to the LMC than was previously known. This huge advance provides more than 30 galaxy ecosystems to compare with our own, and will be particularly useful for understanding the impact of a massive satellite similar to the LMC on the systems in which they reside.”

Why do galaxies stop forming stars?

The second SAGA study in the series is led by Geha and aims to determine whether these satellite galaxies continue to form stars. Understanding the mechanisms that would stop star formation in these small galaxies is an important question in the field of galaxy evolution.

For example, the researchers found 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 is led by Yunchong (Richie) Wang, who received his PhD from Wechsler. This study uses the results of the SAGA survey to improve existing theoretical models of galaxy formation. Based on the number of quenched satellites in these Milky Way-mass systems, this model predicts that quenched galaxies should also exist in more isolated environments – a prediction that should be able to be tested in the coming years with other astronomical surveys such as the Dark Energy Spectroscopic Survey.

Donation to the astronomical community

In addition to these exciting results that will improve our understanding of galaxy evolution, the SAGA team also brings a gift to the astronomy community. As part of this series of studies, the SAGA team has published new distance measurements, or redshifts, for about 46,000 galaxies.

“Finding these satellite galaxies is like looking for needles in a haystack. We had to measure the redshifts of hundreds of galaxies to identify a single satellite galaxy,” Mao said. “These new galaxy redshifts will allow the astronomy community to study a wide range of topics beyond satellite galaxies.”