Local scientists new way to weigh Milky Way

Local scientists have found a novel method to weigh the Milky Way galaxy with its missing baryons.

Astronomical observations have provided strong evidence that baryons, known as normal matter, only account for about 6 percent of the total mass of the Milky Way, a galaxy dominated by dark matter. But the Big Bang Nucleosynthesis theory predicts roughly 16 percent of the baryons in the universe.

Where are the missing baryons? They may hide themselves in the galactic halo.

Evidences showed that the Milky Way is surrounded by a huge hot halo of gas, with temperatures of million degrees. It possibly extends hundreds of thousands of light years. Its mass remains uncertain, but it’s widely believed that it is home to a large fraction of the mission baryons.

Research team led by Guo Fulai at Shanghai Astronomical Observatory of the Chinese Academy of Sciences has investigated the spatial temperature and density distributions of the halo gas, its observational signatures, and its interplay with energetic processes in the Milky Way during the past few years.

They found that when the halo gas stays in equilibrium, its temperature distribution is most sensitively affected by the mass of the galaxy. The higher the mass of the Milky Way, the higher the halo gas temperature.

By measuring the temperature of the hot halo, Guo’s team has managed to present a novel method to measure the mass of the Milky Way. They’ve derived a new constraint on the mass — it is 1.2 to 3 trillion times the mass of the sun.

Previous measurements of the Milky Way mass are mainly based on the observed rotational or random velocities of stars, globular clusters, and satellite galaxies in the disk and halo of the galaxy. Results can vary from 500 billion times to 2 trillion times the mass of the sun.

The new findings imply that large satellite galaxies of the Milky Way, such as the Magellanic Clouds and the Leo I dwarf spheroidal, are bound to the Milky Way.

They also imply that a large fraction of baryons is missing in the Milky Way, even when the hot halo gas is taken into account. These missing baryons may reside beyond the Milky Way halo or in a cool phase that has not yet been detected.

If the Milky Way mass is much lower than a trillion times the solar mass, its missing baryon problem would simply disappear.

They also imply that the so called “too-big-to-fail” problem in contemporary cosmology is still severe, potentially challenging the cold dark matter theory in the standard paradigm of cosmology.

The standard model of cosmology predicts that the Milky Way should have several satellite galaxies much denser and more massive than those currently observed. The inconsistency would have been resolved if the Milky Way mass is substantially lower than a trillion times the solar mass.

“The most important contribution of our work is perhaps that it provides an independent way to measure the Milky Way mass, which can be constrained much more stringently when combining with previous kinetic methods,” said Guo.

The study has been published in leading astronomical journal The Astrophysical Journal Letters.