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.

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Dozens tune in to historic crewed rocket launch, jointly streamed by the ASX and UTAT

ASX and UTAT co-moderate event, with rocketry lead answering audience questions

By: Adam A. Lam

A new era of space flight began with the blastoff of SpaceX’s Falcon 9 rocket on May 30, livestreamed to an audience of over 3 million — including over 25 participants tuning in to a watch party by the Astronomy & Space Exploration Society (ASX) and the University of Toronto Aerospace Team (UTAT).

The flight of NASA astronauts Bob Behnken and Doug Hurley to the International Space Station was the first in history undertaken with a spacecraft designed and built by a private company. The success followed a first attempt scrubbed due to weather on May 27 — also watched by over 45 participants in a first watch party on Zoom by the ASX and UTAT.

Leading up to the successful launch, Jacob Weber, the Hybrid Engine Propulsion Lead from UTAT’s Rocket Division, fielded questions from participants about the science and engineering of the rocket launch during both watch parties.

Posting questions in the Zoom chat, audience members asked about topics such as the impact of weather on launch safety, the extent of automation of the rocket’s launch, and why white vapour was billowing out of the spacecraft before liftoff.

Audience members learned about the importance of weather monitoring to avoid excessive wind shear during a launch, which could damage the rocket and capsule in flight. Weber also noted the importance of “favourable conditions downrange” for the astronauts’ recovery, in case of an aborted mission mid-flight.

Weber also explained that the rocket’s launch was fully automated. He noted that, similar to UTAT’s rocket, the liquid oxygen, which is used to burn rocket fuel, needs to be vented out as it heats up during the launch.

In addition to technical questions, participants also asked lighthearted ones, such as if Weber would go to space (“definitely”) and what he would say to an angry Martian telling him to get off his lawn (“I’m not sure what I’d say”).

Upon the successful launch of the rocket, the chat erupted with cheers, as the two astronauts with sci-fi-inspired spacesuits departed in the first launch from US soil in nine years. Behnken and Hurley eventually arrived safely at the International Space Station after a 19-hour flight.

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