The EuropeanSpace Agency (ESA) Gaia mission, by combining new data from with observations made with the NASA/ESA Hubble Space Telescope, have found that the Milky Way weighs in at about 1.5 trillion solar masses within a radius of 129000 light-years from the galactic centre.
Themass of the Milky Way is considered as one of the most fundamental measurements astronomers can make about our galactic home. Previous estimates of the mass ofthe Milky Way ranged from 500 billion to 3 trillion times the mass of the Sun, being a subject of disagreement between researchers. This huge uncertainty arose primarily from the different methods used for measuring the distributionof dark matter – which makes up about 90% of the mass of the galaxy. Laura Watkins (European Southern Observatory, Germany), who led the team performing the analysis, considers that the inability to detect dark matter is the cause of that uncertainty.
Given the elusive nature of the dark matter, the team had to use a clever method to weigh the Milky Way, which relied on measuring the velocities of globular clusters – dense star clusters that orbit the spiral disc of the galaxy at great distances.
In words of Professor N. Wyn Evans (University of Cambridge, UK) says that "Most previous measurements have foundthe speed at which a cluster is approaching or receding from Earth, that is the velocity along our line of sight. However, we were able to also measure the sideways motion of the clusters, from which the total velocity, and consequently the galactic mass, can be calculated."
The group used Gaia's second data release as a basis fortheir study. Gaia was designed to create a precise three-dimensional map of astronomical objects throughout the Milky Way and to track their motions. Its second data release includes measurements of globular clusters as far as 65.000 light-years from Earth. The team combined these data with Hubble's unparalleled sensitivity and observational legacy. Observations from Hubble allowed faint and distant globular clusters, as far as 130 000 light-years from Earth, to be added to the study. As Hubble has been observing some of these objects for a decade, it was possible to accurately track the velocities of these clusters aswell.
The dark matter content of a galaxy and its distribution are intrinsically linked to the formation and growth of structures in the Universe. Accurately determining the mass for the Milky Way gives us a clearer understanding of where our galaxy sits in a cosmological context.
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