高清福利片

The first detailed cross-section of a galaxy broadly similar to the Milky Way, , reveals that our galaxy evolved gradually, instead of being the result of a violent mash-up. The finding throws the origin story of our home into doubt.

The galaxy, dubbed , turns out to have distinct 鈥榯hick鈥� and 鈥榯hin鈥� discs similar to those of the Milky Way. This suggests, contrary to previous theories, that such structures are not the result of a rare long-ago collision with a smaller galaxy. They appear to be the product of more peaceful change.

And that is a game-changer. It means that our spiral galaxy home isn鈥檛 the product of a freak accident. Instead, it is typical.

The finding was made by a team led by Dr Nicholas Scott and Dr Jesse van de Sande, from Australia鈥檚 ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions () and the School of Physics at the University of Sydney.

鈥淥ur observations indicate that the Milky Way鈥檚 thin and thick discs didn鈥檛 come about because of a gigantic mash-up, but a sort-of 鈥榙efault鈥� path of galaxy formation and evolution,鈥� Dr Scott said.

鈥淔rom these results we think galaxies with the Milky Way鈥檚 particular structures and properties could be described as the 鈥榥ormal鈥� ones.鈥�

This conclusion 鈥� published in 听鈥� has two profound implications.

鈥淚t was thought that the Milky Way鈥檚 thin and thick discs formed after a rare violent merger, and so probably wouldn鈥檛 be found in other spiral galaxies,鈥� Dr Scott said.

鈥湼咔甯＠�片 shows that鈥檚 probably wrong, and it evolved 鈥榥aturally鈥� without catastrophic interventions. This means Milky Way-type galaxies are probably very common.

鈥淚t also means we can use existing very detailed observations of the Milky Way as tools to better analyse much more distant galaxies which, for obvious reasons, we can鈥檛 see as well.鈥�

The research shows that UGC 10738, like the Milky Way, has a thick disc consisting mainly of ancient stars 鈥� identified by their low ratio of iron to hydrogen and helium. Its thin disc stars are more recent and contain more metal.

(The Sun is a thin disc star and comprises about 1.5 percent elements heavier than helium. Thick disc stars have three to 10 times less.)

Although such discs have been previously observed in other galaxies, it was impossible to tell whether they hosted the same type of star distribution 鈥� and therefore similar origins.

Dr Scott, Dr van de Sande and colleagues solved this problem by using the European Southern Observatory鈥檚 Very Large Telescope in Chile to observe UGC 10738, situated 320 million light years away.

The galaxy is angled 鈥渆dge on鈥�, so looking at it offered effectively a cross-section of its structure.

鈥淯sing an instrument called the multi-unit spectroscopic explorer, or MUSE, we were able to assess the metal ratios of the stars in its thick and thin discs,鈥� Dr van de Sande said.

鈥淭hey were pretty much the same as those in the Milky Way 鈥� ancient stars in the thick disc, younger stars in the thin one. We鈥檙e looking at some other galaxies to make sure, but that鈥檚 pretty strong evidence that the two galaxies evolved in the same way.鈥�

Dr Scott said UGC 10738鈥檚 edge-on orientation meant it was simple to see which type of stars were in each disc.

鈥淚t鈥檚 a bit like telling apart short people from tall people,鈥� he said. 鈥淚f you try to do it from overhead it鈥檚 impossible, but it if you look from the side it鈥檚 relatively easy.鈥�

Co-author Professor Ken Freeman from the Australian National University said: 鈥淭his is an important step forward in understanding how disk galaxies assembled long ago. We know a lot about how the Milky Way formed, but there was always the worry that the Milky Way is not a typical spiral galaxy. Now we can see that the Milky Way鈥檚 formation is fairly typical of how other disk galaxies were assembled.鈥�

ASTRO 3D director, Professor Lisa Kewley, said: 鈥淭his work shows how the Milky Way fits into the much bigger puzzle of how spiral galaxies formed across 13 billion years of cosmic time.鈥�

Other co-authors are based at Macquarie University in Australia and Germany鈥檚 Max-Planck-Institut fur Extraterrestrische Physik.

Declaration

Nicholas Scott and and Jesse van de Sande acknowledge support of Australian Research Council Discovery Early Career Research Awards funded by the Australian Government.听

ASTRO 3D

ASTRO 3D is a seven-year $40 million Centre of Excellence project funded by the Australian Government through the Australian Research Council. The centre began in June 2017 hosting more than 250 investigators and professional staff, mostly based at nine nodes: the Australian National University, Curtin University, Swinburne University of Technology, University of Melbourne, University of Sydney, Macquarie University, Monash University, the University of New South Wales, and University of Western Australia.