Galaxies grow bigger and puffier as they age: study

April 23, 2018, Australian National University
This is a long-exposure image from NASA's Hubble Space Telescope of massive galaxy cluster Abell 2744. It shows some of the faintest and youngest galaxies detected in space. Credit: NASA/ESA/STScI

A new international study involving The Australian National University (ANU) and The University of Sydney has found that galaxies grow bigger and puffier as they age.

Co-researcher Professor Matthew Colless from ANU said that in a young galaxy moved in an orderly way around the galaxy's disk, much like cars around a racetrack.

"All look like squashed spheres, but as they grow older they become puffier with stars going around in all directions," said Professor Colless, who is the Director of the ANU Research School of Astronomy and Astrophysics and a Chief Investigator at the ARC Centre of Excellence in All-Sky Astrophysics in 3D (ASTRO 3D).

"Our Milky Way is more than 13 billion years old, so it is not young anymore, but the galaxy still has both a central bulge of old stars and spiral arms of young stars."

To work out a galaxy's shape, the research team measured the movement of stars with an instrument called SAMI on the Anglo-Australian Telescope at the ANU Siding Spring Observatory.

They studied 843 galaxies of all kinds and with a hundred-fold range in mass.

The study, which is published in Nature Astronomy, was funded by ASTRO 3D at ANU and the ARC Centre of Excellence for All Sky Astrophysics (CAASTRO) at The University of Sydney.

Lead author Dr Jesse van de Sande, from The University of Sydney and ASTRO 3D, said that it was not obvious that galaxy shape and age had to be linked, so the connection was surprising and could point to a deep underlying relationship.

"As a galaxy ages, internal changes take place and the galaxy may collide with others," Dr van de Sande said.

"These events disorder the stars' movements."

Co-author Dr Nicholas Scott, from the University of Sydney and ASTRO 3D, said scientists measured a galaxy's age through colour.

"Young, blue stars grow old and turn red," he said.

"When we plotted how ordered the galaxies were against how squashed they were, the relationship with age leapt out. Galaxies that have the same squashed spherical shape, have stars of the same age as well."

Dr van de Sande said scientists had known for a long time that shape and age were linked in very extreme galaxies, that is very flat ones and very round ones.

"This is the first time we've shown shape and age are related for all kinds of galaxies, not just the extremes - all shapes, all ages, all masses," he said.

University of Sydney co-author Dr Julia Bryant, lead scientist for the SAMI instrument, said the team was still searching for the simple, powerful relationships like and age that underlie a lot of the complexity scientists see in galaxies.

"To see those relationships, you need detailed information on large numbers of galaxies," she said.

The Anglo-Australian Observatory (AAO) is building SAMI's successor instrument, Hector, which is designed to observe 100 galaxies at a time.

Explore further: Astronomers spun up by galaxy-shape finding

More information: Jesse van de Sande et al, A relation between the characteristic stellar ages of galaxies and their intrinsic shapes, Nature Astronomy (2018). DOI: 10.1038/s41550-018-0436-x

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2.3 / 5 (3) Apr 23, 2018
'Scientists Thought All Galaxies Had Dark Matter, but They Just Found One Without It'

"Dark matter fills 'empty' space, strongly interacts with visible matter and is displaced by visible matter."

The reason for the mistaken notion the galaxy is missing dark matter is that the galaxy is so diffuse that it doesn't displace the dark matter outward and away from it to the degree that the dark matter is able to push back and cause the stars far away from the galactic center to speed up.

It's not that there is no dark matter connected to and neighboring the visible matter. It's that the galaxy has not coalesced enough to displace the dark matter to such an extent that it forms a 'halo' around the galaxy.

A galaxy's halo is not a clump of dark matter traveling with the galaxy. A galaxy's halo is the state of displacement of the dark matter.
5 / 5 (2) Apr 23, 2018
Galaxies grow via mergers. Each new merger would have its own arbitrary orientation for the two galaxies vs the universe. Older galaxies would have had more randomly oriented mergers, which should result in more randomly orbiting stars, i.e. more "puffiness" - as mentioned very briefly in the middle of the article. It seems like one should a priori expect older galaxies to be more randomized.
Da Schneib
3 / 5 (2) Apr 23, 2018
The real question is how many middle-aged to old spirals there are. These galaxies would have had a lesser-than-normal number of collisions, if your conjecture is correct, @Erik. We'll need a lot of galaxies to be observed to find that out.
3 / 5 (2) Apr 23, 2018
mack, I don't know of ant way to produce a reasonable answer to your question.

How about instead asking "As our technology continues to improve. Enflessly refining the quality of our observations. How does that continuous correction of outdated perceptions, evolve to match the newer reality of the improved data collected?
not rated yet Apr 23, 2018
Galaxies grow bigger and puffier as they age: Sir As a student of science not as a scientist I think that galaxies grow puffier as most of the stars have completed their age might have turned into dust cloud after the explosion of neutron star and the forces binding the other stars might have been reduced thus it became puffier. Does it true.
Da Schneib
4 / 5 (4) Apr 24, 2018
@mac, at least you're now *asking* instead of *telling*.

You have noted that I did not draw any conclusions. It was deliberate. I think it's far too soon to draw any.

Let's see 10,000 galaxies analyzed this way. Then we have something to hypothesize on.

@rrwills, we have the technology. What we do not have is the instruments, and the sites. There are only a few sites in the world where ground telescopes of the size and reach needed are even worth installing; and at one of them the locals are having religious obsessions. The hottest thing going is the JWST and they've just delayed it again until 2020. A bunch of congresscritters in the US just sent a trillion dollars to blowing people up and are whining about a few million and then whining about delays because *they* tried to cheap out and didn't allow contingency funds to cover foreseeable delays. Want to get more data? Vote for congresscritters who don't have psychological problems with spending money on science.
1 / 5 (4) Apr 24, 2018
All galaxies look like squashed spheres, but as they grow older they become puffier with stars going around in all directions,"

Since they grow naturally from within by new matter being ejected from their supermassive core stars.

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