The mass of the Milky Way

Thanks to the data collected by the Sloan Digital Sky Survey in the wake of stars produced from storage globular Palomar 5, it has been possible to estimate the mass of the Milky Way with unprecedented precision.
What would you think if your doctor told you that your weight is somewhere between 50 and 200 kg? Any patient could do better at home using a simple scale. However, there is a patient who is not able to obtain this result: the Milky Way. Although today we are able to peer deeper into space than we have done in the past, we have a range of uncertainty about the weight of our galaxy by about a factor of four. This is why researchers at the Department of Astronomy at Columbia University have developed a new method to obtain a more precise physical checkup for the Milky Way.


Credit: STScI / NASA

The Milky Way consists of about 100 billion stars, forming a stellar disk with a diameter of 100-200 thousand light years. The Sun is part of this structure and therefore, looking at the galaxy from the inside we see a huge densification of stars. The large number of stars that compose it and its enormous extension in the sky make it difficult to measure some fundamental values of the Milky Way, such as its weight.
An international team of scientists led by Andreas Küpper, a researcher at Columbia University, has used a group of stars that orbit outside the galactic disk to obtain a high accuracy measurement of the weight of the Milky Way. In a study recently published in The Astrophysical Journal, the team showed that these streams of stars produced by the disintegration of the globular clusters can be used not only to estimate the weight of the Galaxy, but also to determine the position of the Sun in it.
“Globular clusters contain from several thousand to several million stars born together at a time when the universe was still very young,” says Küpper. “These objects orbit the Milky Way and over billions of years, they disintegrate, leaving a trail along their journey. These trails of stars emerge clearly from the rest of the stars for their characteristics of density and consistency, a bit as the jet stream are distinguished clearly from the clouds. ”
The researchers used data from the Sloan Digital Sky Survey, which for 10 years has collected observations of the Northern Hemisphere, creating a highly detailed catalog of stars. The structure of stars used to test the new technology was produced from storage globular Palomar 5, and had already been discovered in 2001. Eduardo Balbinot, study co-author and researcher at the University of Surrey in England, he has revised the data of Sloan detecting density fluctuations in the data of Palomar 5.
“We observed that the fluctuations were very pronounced and arranged at regular intervals along the trail of stars,” said Balbinot. “Behaviour of this kind cannot be accidental.”


Credit: Sloan Digital Sky Survey

It is thanks to these fluctuations that researchers were able to obtain a measure of mass with unprecedented accuracy. Using the supercomputer Yeti of Columbia University have created millions of models for the structure of the Milky Way. From these models and the comparison between models and observations were able to deduce that the mass of the Milky Way, within a radius of 60,000 light years, amounted to 210 billion times the mass of the Sun with an uncertainty of 20 %. The peculiar structure and density fluctuations contributed significantly to exclude models for which the Milky Way too heavy or too light.
“An important step in this work is due to the robust statistical tools, the same used to study changes in the genome and to rank websites through search engines,” said Ana Bonaca, co-author of the work and researcher at Yale University. “This rigorous approach has allowed us to achieve the high precision measurement of the weight of the Milky Way.”
“There have been attempts in the past to get similar measures with other stellar, but the results were always quite ambiguous,” added Professor Kathryn Johnston, co-author of the study and head of the Department of Astronomy at Columbia Unversity. “Our result solves these ambiguities using the density distribution extremely peculiar that created Palomar 5 orbiting the Milky Way in the past 11 billion years.”
In the future, the researchers hope to be able to use structures similar to that produced by Palomar 5 to obtain a more precise measurement of mass and then to have an even more realistic model of the Milky Way. From this information, we will improve our knowledge about the formation and composition of our galaxy, the Milky Way and understand how it compares with other galaxies in the Universe.

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