milky way

The Inner and Outer Milky Way Aren’t the Same Thickness, and that’s Surprising

At first glance, the universe and night sky seem largely unchanging. The reality is very different, even now, a gas cloud is charging toward the Milky Way Galaxy and is expected to crash into us in 27 million years. A team of astronomers hoping to locate the exact position of the expected impact site have been unsuccessful but have accidentally measured the thickness of the Milky Way! Analysing radio data, they have been able to deduce the thickness of the inner and outer regions and discovered a dramatic difference between the two. 

The team of astronomers from the US National Science Foundation’s Green Bank Observatory were attempting to study the Smith Cloud. This high velocity cloud of hydrogen gas is located in the constellation Aquila at a distance of somewhere between 36,000 and 45,000 light years. Previous studies from the Green Bank Observatory have shown the cloud contains at least 1 million times the mass of the Sun and measures 9,800 light years long by 3,300 light years wide. 

A false-color image of the Smith Cloud made with data from the Green Bank Telescope (GBT). New analysis indicates that it is wrapped in a dark matter halo. Credit: NRAO/AUI/NSF

The plan was simple enough, to observe the spot where the cloud is currently interacting with the Milky Way. The observation is tricky enough though as the cloud is on the far side of the Milky Way and there is a lot of stuff in the way! The team, led by Toney Minter used the 20m Green Bank Telescope to search for dust and emissions from hydroxyl molecules (composed of a hydrogen and oxygen molecule.)  What the team expected to see was a difference in composition in the region of the Milky Way interacted with the cloud which, should have very little dust and hydroxyl molecules. Clouds in the Milky Way tend to have both so a difference should be detectable. 

The Robert C. Byrd Green Bank Telescope. Credit: Jay Young.

Minter was candidly open about the study joking ‘I knew there was a low probability that I’d find what I was looking for—and I didn’t,. But this is all part of the scientific process. You learn from what you DO and DON’T find.’

Disappointingly the team did not detect any differences in composition but what they did find was equally as interesting. The study revealed information about the Milky Way itself and the structure of its inner regions. Minter and his team had to look through the Milky Way’s inner regions for their study and what they were able to determine was the thickness of the layer of molecules in the inner Galaxy. The information enabled them to deduce the scale height of the clouds of molecular gas in the inner Milky Way. The results showed that the layer of molecules in the inner region measured 330 light years thick while those in the outer parts measured twice as much, around 660 light years. 

The discovery still leaves questions unanswered. The observation certainly shows the difference in thickness between the inner and outer regions but it doesn’t give any clue as to what is driving the difference. Further observations are now required to follow up on this discovery to try and model the underlying process.  Of course one other question remains unanswered and that is the nature and mechanics of the Smith Cloud and how it will impact our own Galaxy. Far from being disappointed though, Minter stated ‘That’s why astronomy is exciting, our knowledge is always evolving’

Source : While Aiming for Massive Gas Cloud, Astronomers Spot Differences in Thickness of Milky Way Galaxy

Mark Thompson

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