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Red glow at Milky Way centre from highly energised hydrogen

A red glow at the centre of the Milky Way has been detected for the first time and it could help astronomers discover what powers the centre of our spiral galaxy.

The red light shines out of an area known as the ‘Tilted Disk’ – named for its orientation – that sits in the central bar region of the Milky Way, astronomers said.

A team from the Embry-Riddle Aeronautical University, Florida used the Wisconsin H-Alpha Mapper (WHAM) telescope in Chile to make this dramatic discovery. 

The faint beacon is only just visible from Earth – peaking through a hole in the dust and is a telltale sign of ionised hydrogen gas – coming from newly forming stars. 

Being able to identify and measure this ionised gas allowed the astronomers to compare the centre of the Milky Way to other spiral galaxies more easily.  

The next stage is to find out the source of energy that is powering this newly discovered ionisation at the centre of the galaxy, according to the study authors.

The team say the source of this red beacon of light was found by comparing other colours of visible light coming from ionised nitrogen and oxygen. 

Co author Dr Lawrence Haffner said that without an ongoing source of energy, free electrons usually find each other and recombine to return to a neutral state.

The Embry-Riddle researcher said this happens in a relatively short amount of time.

‘Being able to see ionised gas in new ways should help us discover the kinds of sources that could be responsible for keeping all that gas energised,’ said Haffner.

His colleague Professor Bob Benjamin, of the University of Wisconsin-Whitewater, was looking at two decades’ worth of WHAM data when he spotted the ‘red flag’.

The peculiar shape poking out of the Milky Way’s dark, dusty centre was ionised hydrogen gas which appears red – and was moving in the direction of Earth.

The position of the feature couldn’t be explained by known physical phenomena such as galactic rotation, according to the research team.

Dr Haffner said: ‘Being able to make these measurements in optical light allowed us to compare the nucleus of the Milky Way to other galaxies much more easily.

‘Many past studies have measured the quantity and quality of ionised gas from the centres of thousands of spiral galaxies throughout the universe.

‘For the first time, we were able to directly compare measurements from our galaxy to that large population.’

Lead author Dhanesh Krishnarao, a graduate student at University of Wisconsin-Madison, leveraged an existing model to predict how much gas there should be.

Raw data from the WHAM telescope allowed him to refine his calculations until the team had an accurate 3-D picture of the structure.

The study, published in the journal Science Advances, also found that around half of the hydrogen has been ionised by an unknown source.

‘The Milky Way can now be used to better understand its nature,’ Krishnarao said. 

‘Close to the nucleus of the Milky Way gas is ionised by newly forming stars, but as you move further away from the centre, things get more extreme,’ he said.

Further from the centre the gas becomes similar to a class of galaxies called LINERs, or low ionisation (nuclear) emission regions.

The structure appeared to be moving toward Earth because it was on an elliptical orbit interior to the Milky Way’s spiral arms, the researchers found.

LINER-type galaxies such as the Milky Way make up roughly a third of all galaxies. 

They have centres with more radiation than galaxies that are only forming new stars, yet less radiation than those whose supermassive black holes are actively consuming a tremendous amount of material.

‘Before this discovery by WHAM, the Andromeda Galaxy was the closest LINER spiral to us,’ said Dr Haffner, adding that ‘it’s still millions of light-years away.’ 

‘With the nucleus of the Milky Way only tens of thousands of light-years away, we can now study a LINER region in more detail.

‘Studying this extended ionized gas should help us learn more about the current and past environment in the center of our Galaxy.’

The researchers now plan to figure out the source of the energy at the centre of the Milky Way that is driving this newly discovered beacon.

Being able to categorise the galaxy based on its level of radiation was an important first step toward that goal, according to Haffner.

‘In the next few years, we hope to build WHAM’s successor, which would give us a sharper view of the gas we study,’ he said.

‘Right now our map `pixels’ are twice the size of the full moon. WHAM has been a great tool for producing the first all-sky survey of this gas, but we’re hungry for more details now.’

The findings have been published in the journal Science Advances. 

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