The Galaxy-Size Challenge: Measuring the Milky Way –“It’s Huge!”

 

 

3004611

 

What’s the mass of the Milky Way? The short answer, so far, is 7 X 1011 solar masses. In terms that are easier to comprehend, that’s about the mass of our Sun, multiplied by 700 billion. The Sun, for the record, has a mass of two nonillion (that’s 2 followed by 30 zeroes) kilograms, or 330,000 times the mass of Earth. “And our galaxy isn’t even the biggest galaxy,” says Gwendolyn Eadie of McMaster University.


Measuring the mass of our home galaxy, or any galaxy, is particularly difficult. A galaxy includes not only stars, planets, moons, gases, dust and other objects and material, but also a big helping of dark matter, a mysterious and invisible form of matter that is not yet fully understood and has not been directly detected in the lab. Astronomers and cosmologists, however, can infer the presence of dark matter through its gravitational influence on visible objects.

It is a galactic challenge, to be sure, but Eadie is getting closer to an accurate answer to a question that has defined her early career in astrophysics: what is the mass of the Milky Way?

 

Allsky-2mass

This panoramic view above from Caltech encompasses the entire sky as seen by the Two Micron All-Sky Survey (2MASS). The image has been reconstructed from a catalog of measured brightnesses of half a billion stars from the survey. This image is centered on the core of our own Milky Way galaxy, toward the constellation of Sagittarius. The reddish stars seemingly hovering in the middle of the Milky Way’s disc — many of them never observed before. These stars trace the densest dust clouds in our galaxy. The two faint smudges seen in the lower right quadrant are our neighboring galaxies, the Small and Large Magellanic Clouds.

The 2MASS survey uses near-infrared light at wavelengths longer than that of visible light. At these wavelengths, clouds of dust that obscure our view of more distant stars in our Milky Way galaxy become increasingly transparent. As a result we can see all the way to the very center of the galaxy, showing its overall structure far more clearly than is possible in visible light.

Eadie, a PhD candidate in physics and astronomy at McMaster University, has been studying the mass of the Milky Way and its dark matter component since she started graduate school. She uses the velocities and positions of globular star clusters that orbit the Milky Way.

The orbits of globular clusters are determined by the galaxy’s gravity, which is dictated by its massive dark matter component. What’s new about Eadie’s research is the technique she devised for using globular cluster (GCs) velocities.

The total velocity of a GC must be measured in two directions: one along our line-of-sight, and one across the plane of the sky (the proper motion). Unfortunately, researchers have not yet measured the proper motions of all the GCs around the Milky Way.

Eadie, however, has developed a way to use these velocities that are only partially known, in addition to the velocities that are fully known, to estimate the mass of the galaxy. Her method also predicts the mass contained within any distance from the center of the galaxy, with uncertainties, which makes her results easy to compare with other studies.

Eadie and her academic supervisor William Harris, a professor of Physics and Astronomy at McMaster, have co-authored a paper on their most recent findings, which allow dark matter and visible matter to have different distributions in space. They have submitted this work to the Astrophysical Journal, and Eadie will present their results May 31 at the Canadian Astronomical Society’s conference in Winnipeg.

Even after all this work, she says, she still loves looking into the night sky. In fact, she loves it more. “Every so often I think, ‘I’m measuring the mass of the Milky Way.’ That’s pretty neat.”

The Daily Galaxy via McMaster University

Image Credit: ESO

"The Galaxy" in Your Inbox, Free, Daily