ASX Star Talk: Why are U of T astronomers investigating the mass of the Milky Way?
By: Adam A. Lam
What is the mass of the Milky Way galaxy, and why is it important to investigate?
Dr. Gwen Eadie, an Assistant Professor jointly appointed between the University of Toronto’s David A. Dunlap Department of Astronomy & Astrophysics (DADAA) and its Department of Statistical Sciences (DoSS), spoke on these questions at the Astronomy and Space Exploration Society’s first Star Talk of the year on July 8. She discussed her research team’s investigations into the universe guided by statistical studies.
The structure of our universe
The galaxies of our universe — which includes the Milky Way — cluster into the shape of strings, noted Dr. Eadie.
“To recreate this hierarchical structure in computer simulations of how the universe evolved, we’ve discovered that you have to include something called dark matter,” Dr. Eadie noted. Dark matter is a hypothetical, currently undetectable type of matter that has a gravitational effect on visible matter — but does not otherwise interact with it.
Astronomers believe that every galaxy in the universe, including the Milky Way, lives inside its own dark matter halo, continued Dr. Eadie. This structure of dark matter “represents a really large portion of the mass of the galaxy.”
By estimating the total mass of the Milky Way along with its total visible mass, Dr. Eadie explained, “you can subtract out the matter that we can see and figure out how much dark matter is there. And that might tell us something about the nature of dark matter itself.”
Challenges to measuring the mass of the Milky Way
One way to estimate the mass of the Milky Way, noted Dr. Eadie, is by studying kinematic tracers — in other words, studying the motion of tracer objects. A common kinematic tracer, she continued, is a globular cluster, or a cluster of stars.
A globular cluster moving through space has a velocity with two components — one from the observer’s line of sight and another in the plane of the sky, explained Dr. Eadie. However, astronomers do not always have measurements of both components, posing a challenge for calculations.
In addition, astronomers who measure this velocity do so from the reference frame of Earth. This is another problem, as they must translate these measurements to the reference frame in the centre of the Milky Way, in order to estimate the galaxy’s mass. A third issue is accounting for measurement error from instrumentation for scientists to avoid overconfidence with their results.
Astronomers also have different assumptions about the trajectories of globular clusters — whether they are more elliptical or circular —as they can take millions of years to complete an orbit, noted Dr. Eadie.
Finally, separate research teams also reported differing interpretations about the Milky Way’s mass, as they report these estimates using differing ranges from the galaxy’s centre.
A potential solution by Dr. Eadie’s research team
To tackle these problems of uncertainty, Dr. Eadie and her research team have developed a hierarchical Bayesian method — in other words, a way to apply probability theory to estimate the Milky Way’s mass.
“It allows us to include the measurement uncertainty and allows us to incorporate this incomplete data [of velocity components],” said Dr. Eadie.
Using data collected by the Gaia satellite, an observatory of the European Space Agency that has measured “the position and velocities of over two billion stars in the Milky Way,” Dr. Eadie and her collaborators successfully applied their model to provide an accurate estimate of the galaxy’s mass.
They reported their results in a research paper published in The Astrophysical Journal in 2018. “Doing this kind of analysis could help improve our interpretation [of the galaxy’s dark matter halo],” reflected Dr. Eadie, and help astronomers better evaluate the results of previous research papers about the Milky Way.
Studying dwarf galaxies to create a new estimate
Another focus of Dr. Eadie’s research team are the approximately 30 dwarf galaxies orbiting the Milky Way. Since they orbit the galaxy just like globular clusters, she noted that they “can also be used to measure the mass of the Milky Way.”
“If we can get their positions and velocities and then use a model for the gravitational potential, then we can infer how much mass is there,” she noted.
To create a new estimate, Dr. Eadie is collaborating with undergraduate Xander Dufresne and recent graduate Keslen Murdock at the University of Toronto; recent graduate Anika Slizewski and Dr. Mario Juric at the University of Washington; Dr. Robin Sanderson at the University of Pennsylvania; and Dr. Andrew Wetzel at the University of California, Davis.
“With this new data, we’ve been able to get a new mass estimate for the Milky Way,” she said. “and we’re finding that with the dwarf galaxies, we actually get a much larger mass estimate than we were before.”
“We’re currently trying to figure out exactly why that is, and also why some of these dwarf galaxies seem to play a large role in determining what the [Milky Way’s] mass is.”
Locating ultra-diffuse galaxies with statistical methods
Another aspect of the universe under the focus of Dr. Eadie’s research team is the existence of ultra-diffuse galaxies, which are a relatively recent discovery by astronomers. Scientists are interested in studying them further, especially as they do not yet have a clear understanding of how their formation fits with the evolution of the universe, noted Dr. Eadie — along with why many appear to have large amounts of dark matter, while others appear to have little.
However, due to the ultra-diffuse galaxies’ low emittance of light, astronomers find it difficult to detect them with standard telescopes, she continued. While NASA’s Hubble Telescope can detect them more easily, Dr. Eadie noted that it is competitive to book time on the instrument. Astronomers must therefore narrow down where to point the telescope to find these galaxies for imaging.
To develop a solution to this challenge, Dr. Eadie is collaborating to apply spatial statistics to develop a model for discovering ultra-diffuse galaxies with incoming PhD student Dayi Lee at U of T’s DoSS; Dr. Roberto Abraham at the DADAA; and Dr. Patrick Brown at the DoSS and the Centre for Global Health Research at St. Michael’s Hospital.
