Measuring the mass of our galaxy has always been a complex task: We can’t see it in full and most of it is made of dark matter. To weigh the Milky Way, it is necessary to understand how things move around it, and researchers at McMaster University have now come up with a new way to do just that.
The entire Milky Way system, everything within 850,000 light-years from its center, weighs between 1.3 and 1.5 trillion solar masses, with the stellar disk and dark matter being about 700 billion times the mass of the Sun. The Sun has a mass of about 2 billion billion billion tons, or about 330,000 times the mass of Earth.
The new estimation uses groups of stars, called globular clusters, as well as a variety of other satellites that surround the Milky Way. The orbits of these objects depend exclusively on the matter content of the galaxy, so by understating how they move, the researchers can calculate how much mass there is.
However, the novelty of the research, which will be published in the Astrophysical Journal, is the technique used to estimate the velocity of the objects. To obtain the total velocity, one usually measures two components: line-of-sight velocity (as it appears to move towards or away from us) and the velocity across the sky.
Unfortunately, not all globular clusters have both values available, so the astronomers constructed a statistical method and computer simulations to assess the true velocity based on the incomplete data set. The best values were calculated based on the most likely distribution of matter, both visible and dark, in the Milky Way.
The method is also versatile enough to allow for estimations within any distance from the center of the galaxy, which makes the results easy to compare with other studies. The author, graduate student Gwendolyn Eadie, is presenting these findings on May 31 at the Canadian Astronomical Society’s conference in Winnipeg.
In the paper, which can be read as a preprint on arXiv, she highlights future hopes to improve the model and refine its estimates. She also suggests that this method might be used to estimate the mass of other galaxies, although more work is necessary.