Massive galaxies consume smaller ones, becoming larger in the process, according to a new finding. These collections of stars are rarely the birthplaces of new stars, but now grow by swallowing other galaxies.   

Astronomers studied 22,000 galaxies, examining how the age of galaxies affect star formation.

The Milky Way will collide with another large collection of stars, the Andromeda galaxy, about five billion years from now, after the Earth is destroyed by a dying Sun.  

"Technically, Andromeda will eat us because it's the more massive one," said Aaron Robotham, a radio astronomer working at the University of Western Australia, as part of the International Centre for Radio Astronomy Research (ICRAR).

The Milky Way is believed to be at a critical point in its evolution, as our home family of stars has slowed down collecting gas and dust, and is now growing primarily by swallowing other galaxies.

The Large and Small Magellanic Clouds, a pair of small galaxies near our own, will be consumed by the Milky Way in about four billion years.

"The Milky Way hasn't merged with another large galaxy for a long time but you can still see remnants of all the old galaxies we've cannibalized," Robotham said.

As massive galaxies grow from the collection of smaller groups of stars, gravity becomes more intense, drawing in even more small groups. It is still unknown exactly why such massive collections of stars do not continue to form new stars at the same rate as smaller galaxies.

"The topic is much debated, but a popular mechanism is where the active galactic nucleus basically cooks the gas and prevents it from cooling down to form stars," Robotham told the press.

Smaller galaxies become larger as they swallow dust and gas floating in deep space. As time goes on, many of the families of stars are consumed by larger galaxies, feeding their growth.  
 
The Anglo-Australian Telescope in New South Wales, Australia, was used to collect data on galaxies, as part of the Galaxy And Mass Assembly (GAMA) survey. This project involved 90 scientists and researchers, and took seven years to complete. Over 60 scientific articles have been published from the GAMA research, and three times that number are currently under development.

Robotham believes that, if the Universe does not collapse, the Universe may be populated by a few super-massive galaxies, tens of billions of years in the future.

The study of massive galaxies, and how they grow through the process of swallowing smaller collections of stars, was published in the journal Monthly Notices of the Royal Astronomical Society.