How to weigh the Milky Way

SDSS
The Northern Hemisphere of the sky as seen by the Sloan Digital Sky Survey. Stellar streams stick out from the vast number of stars in this view, of which most lie within the Milky Way disk. The Palomar 5 stream is the densest of the stellar streams discovered so far and turned out to be a perfect scale and yardstick for our understanding of the Milky Way. (Credit: Ana Bonaca, Marla Geha and Nitya Kallivayalil with data from the Sloan Digital Sky Survey.)

The Milky Way consists of roughly 100 billion stars like our Sun, which form a huge stellar disk with a diameter of 100-200 thousand light years. The Sun is also part of this structure, hence, when we look into the sky, we look right into this gigantic disk of stars. The vast number of stars and the huge extent on the sky make it hard to measure fundamental quantities for the Milky Way – such as its weight. 

Together with my esteemed collaborators I used stars outside this disk, which orbit around the Milky Way in a stream-like structure, to weigh the Milky Way to high precision. In a study published in The Astrophysical Journal, we demonstrated that such streams, which are produced by dissolving globular clusters, can be used to measure not only the weight of our Galaxy, but can also be exploited as yardsticks to determine the location of the Sun within the Milky Way.

Globular clusters are compact groups of thousands to several millions of stars that were born together when the Universe was still very young. They orbit around the Milky Way and slowly disintegrate over the course of billions of years, leaving a unique trace behind. Such star streams stick out from the rest of the stars on the sky as they are dense and coherent, much like contrails from airplanes easily stick out from regular clouds.

We used data from the Sloan Digital Sky Survey, which scanned the sky of the Northern Hemisphere for about 10 years to create a comprehensive catalog of stars on the sky. The stream we tested the new technique on was produced by a globular cluster named Palomar 5, and had already been discovered in 2001 high above the Galactic disk. Coauthor Eduardo Balbinot from the University of Surrey in England revisited the Sloan data and detected density wiggles in the stream of Palomar 5. We found the wiggles to be very pronounced and regularly spaced along the stream. Such variations cannot be random.

It is in fact these wiggles that allowed us to gain the unprecedented precision of our measurements. Using the Yeti supercomputer of Columbia University, we created several million models of the stream in different realizations of the Milky Way. From these models and from comparing the wiggle pattern of the models to the observations, we were able to infer the mass of the Milky Way within a radius of 60,000 light years to be 210 billion times the mass of the Sun with an uncertainty of only 20%. The unique pattern of the density wiggles helped significantly to rule out models of the Milky Way, which were either too heavy or too skinny.

An important advance in this work was using robust statistical tools – the same ones used to study changes in the genome and employed by internet search engines to rank websites. Graduate student Ana Bonaca from Yale University was key in taking such a rigorous approach, which helped significantly in achieving the high precision in weighing the Milky Way.

Such measurements have been tried before with different streams, but the results were always quite ambiguous. Our new measurement breaks these ambiguities by exploiting the unique density pattern that Palomar 5 created as it orbited around the Milky Way for the past 11 billion years.

In the future, we aim at using more structures like the Palomar 5 stream to gain an even higher precision and to create the most realistic model of the Milky Way to date. From the improved precision we hope to learn about the formation and composition of our home galaxy, and to understand how the Milky Way compares with other galaxies in the Universe. So far, the results indicate that the Milky Way is a healthy patient – neither too skinny nor too heavy for its size.

Update: Our press release was picked up by the German press – BILD online, astronewspro-physik, Deutschlandfunk and Frankfurter Allgemeine. The English version of the press release kicked in a bit later: DailyMail, CNETScienceDaily, IFLSCIENCE!, mental_flossredOrbit, and several more! Entertaining is a reddit thread that got upvoted a lot and received 1710 points in the end.

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