Galaxy’s hydrogen halo hides missing mass


Astronomers aiming to reconcile the observed and predicted masses of the Milky Way have discovered a vast cloud of neutral hydrogen surrounding our galaxy, writes Andrew Masterson.


What the Milky Way might look like to intergalactic alien astronomers: the spiral galaxy NGC 2683.
ESA/Hubble/NASA

The Milky Way is surrounded by a vast halo of diffuse hydrogen, new research has revealed.

The discovery of the halo, by astronomers Huanian Zhang and Dennis Zaritsky from the University of Arizona, confirms earlier predictions regarding the difference between the observed and theoretical masses of the galaxy.

The finding was made after analysing data obtained from 732,225 other galaxies, using the 2.5-metre telescope of the Sloan Digital Sky Survey, located at Apache Point Observatory in south-east New Mexico.

The suggestion of a massive hydrogen halo arose among astronomers to explain an apparent discrepancy in the ratio of conventional, or “baryonic”, matter to dark matter in the Milky Way.

Using widely accepted proportions of dark to baryonic matter, set against the calculated mass of the entire system, the observed baryonic portion came up light – missing about 50 per cent of the heft the calculations require.

Previous surveys have identified the existence of clouds of hydrogen in the Milky Way, but even under the most generous estimates they fail to account for more than a tiny percentage of the absent mass.

One of the major issues faced by scientists tackling the problem is that observing our own galaxy is particularly difficult because of the simple fact that it surrounds us. The exercise has been compared to describing an unfamiliar house while being trapped inside one of its rooms.

In an effort to resolve the problem, Zhang and Zaritsky combed through multi-spectral data obtained by the Sloan Survey, looking in particular at spiral galaxies roughly similar to the Milky Way.

Doing so, however, did not yield easy results, mainly because of the elusive nature of their quarry.

Hydrogen in space comprises single atoms, whereas on Earth they are twinned in molecules of hydrogen gas. Some 99.9% of the hydrogen in space is positively charged, but the 0.1% suspected of making up galactic hydrogen halos is neutrally charged – and therefore pretty much impossible to detect.

“You don’t just see a pretty picture of a halo around a galaxy,” Zaritsky says.

“We infer the presence of galactic halos from numerical simulations of galaxies and from what we know about how they form and interact.”

Based on such simulations, scientists predicted the presence of large amounts of hydrogen gas just hanging around the Milky Way.

Drilling through the data, Zaritsky and Zhang confirmed not only the existence of the gas, but also that its mass matches that missing in the baryonic-to-dark-matter equation.

“It’s like peering through a veil,” Zaritsky says.

“We see diffuse hydrogen in every direction we look. The gas we detected is not doing anything very noticeable. It is not spinning so rapidly as to indicate that it’s in the process of being flung out of the galaxy, and it does not appear to be falling inwards toward the galactic center, either.”

The study is published in Nature Astronomy.

  1. https://www.nature.com/articles/s41550-017-0103
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