For decades, scientists have been puzzled as to what causes variation in the shapes of galaxies. Now, within days of each other, two teams have revealed the respective roles of spin and star formation in shaping these giant structures of hundreds of billions of stars.
Large galaxies are divided into those with a spiral shape, like our own Milky Way, and those described as elliptical.
"Massive elliptical galaxies are believed to be formed from collisions of disk galaxies," said Dr Ken-ichi Tadaki, of the National Astronomical Observatory of Japan, in a statement. "But, it is uncertain whether all the elliptical galaxies have experienced galaxy collision. There may be an alternative path."
Tadaki is first author of two papers in the same edition of the Astrophysical Journal that examine populations of galaxies 3 billion years after the Big Bang.
At the time, the galaxies were nearly flat disks, but contained huge concentrations of gas and dust towards their center, which were rapidly forming stars. The authors expect that this star formation will create bulges in the galaxies' centers so large that they will develop elliptical shapes.
"Here, we obtained firm evidence that dense galactic cores can be formed without galaxy collisions. They can also be formed by intense star formation in the heart of the galaxy,” Tadaki said. The authors compare star formation to yeast making the bread of galaxies rise.
Meanwhile, Dr Caroline Foster of the University of Sydney has found another factor shaping galaxies. When Foster measured the rate at which 845 galaxies are spinning, she concluded that among spiral galaxies, which have disks of stars, the faster-spinning ones have more circular disks.
In the Monthly Notices of the Royal Astronomical Society, Foster reports that rapidly spinning galaxies are more likely to be flatter and symmetric.
The rate of spin in turn reflects the pieces from which the galaxy formed. When smaller gas-rich galaxies come together, the process of dissipating that gas reduces spin. On the other hand, when most gas has already coalesced into stars, the merger is called dissipationless, and induces rapid spin.
Foster's conclusions were anticipated. Pizza chefs have known for centuries that making dough spin rapidly when it is tossed in the air produces a round, flat base. Foster told IFLScience her work was proof that new methods to measure galactic shape using measurements of star motion work, where simply looking at a two dimensional image does not. “If we'd got a different answer we would have said something is wrong in the method we are using to measure shape."
Now that the method has been verified, Foster intends to move on to examining the influence of other factors, such as age and the pull of neighboring galaxies.
