Alien astronomers living in another galaxy might see the 100 billion stars of our Milky Way as little more than a small blur of light in the night sky. Yet if they analyse the frequencies of light in this faint speck, they might be able to work out what our galaxy is made of.
This raises the question: What does the Milky Way's chemistry look like from millions of light years away?

Research conducted by the Max Planck Institute for Astronomy in Germany and Yunnan University in China has provided an answer: Our galaxy is a peculiar galaxy, although not the only one of its kind.
The Milky Way stands out among other galaxies of similar size for its metal concentrations, which are very low at the centre, rising towards the midway point and then depleted at the outskirts of the galaxy.
By comparison, the metal distribution of other galaxies is much flatter, more like a pancake than a doughnut.
The researchers report that our galaxy "is not unique, but it is not common either". "The Milky Way may not have a typical metallicity distribution for a galaxy of its mass."

Astronomers use the word 'metal' to refer to all elements heavier than hydrogen and helium.
Hydrogen and helium nuclei formed about three minutes after the Big Bang, while electrons travelled to form atoms about 380,000 years later. The heavier elements are the product of billions of years of stellar evolution and require more time to appear.
Therefore, stars born earlier in the history of the Universe tend to have less metal than those born later.

The researchers compared the Milky Way to 321 galaxies of similar mass observed as part of the Mapping Nearby Galaxies (MaNGA) survey at Apache Point Observatory. They found that only one per cent of these galaxies matched the Milky Way in terms of metal distribution.
The researchers also compared the Milky Way with 134 galaxies created in the TNG50 Universe simulation, which models the evolution of tens of thousands of galaxies over a period of 13.8 billion years.
Only 11 per cent of the simulated galaxies were similar to ours.

Why is the Milky Way so strange?
There are several explanations. Perhaps older stars with lower metal content have exhausted all the resources in the centre of the galaxy. This means that few young stars are being born there, which would explain the metal decline in the centre.
Alternatively, the supermassive black hole at the centre of the Milky Way may have emitted radiation as it fed, making it harder for stars to form in the centre.

The lack of metal at the edges of the Milky Way could be due to our galaxy swallowing another galaxy with low metal content.
Another possibility is that the estimated size of the Milky Way's stellar disc is incorrect, which could be addressed in future surveys such as the WHT Enhanced Field Velocity Explorer (WEAVE) and the Sloan Digital Sky Survey V (SDSS-V), the researchers say.

"If we want to know whether the Milky Way is special or not, we need to find ways to compare our home galaxy with more distant galaxies," says Jianhui Lian, lead author of the study and astronomer.
"This has been an open question since astronomers realised a hundred years ago that the Milky Way is not the only galaxy in the universe."

 

Source: https://www.sciencealert.com/