London: Scientists have discovered that there`s more than one way for older galaxies to stay young -- by cannibalising their smaller neighbours.
Astronomers studying an elderly elliptical galaxy, NGC 4150, have found that it is still producing stars despite its advanced age believed to be more than a billion years.
New observations with NASA`s Hubble Space Telescope revealed that NGC 4150 is keeping itself fat, sassy, youthful and reproductive by gobbling up a smaller neighbouring galaxy, or practicing "cannibalism" as scientists put it.
The new study, the researchers said, will help bolster the emerging view that most elliptical galaxies -- so called because of their oval shape -- have young stars, bringing new life to old galaxies.
"Elliptical galaxies were thought to have made all of their stars billions of years ago," said astronomer Mark Crockett of the University of Oxford and the leader of the Hubble observations.
"They had consumed all their gas to make new stars. Now we are finding evidence of star birth in many elliptical galaxies, fuelled mostly by cannibalising smaller galaxies," Crockett was quoted as saying by the Daily Mail.
"These observations support the theory that galaxies built themselves up over billions of years by collisions with dwarf galaxies. NGC 4150 is a dramatic example in our galactic
back yard of a common occurrence in the early universe," he added.
According to the scientists, the Hubble images showed turbulent activity deep inside the core of NGC 4150. Clusters of young, blue stars trace a ring around the centre that is
rotating with the galaxy.
The stellar breeding ground is about 1,300 light-years across, and long strands of dust are silhouetted against the yellowish core, which is composed of populations of older stars.
From a Hubble analysis of the stars` colours, Crockett and his team calculated that the star-formation boom started about a billion years ago, a comparatively recent event in
cosmological history. The galaxy`s star-making factory has slowed down since then, they said.
"We are seeing this galaxy after the major starburst has occurred," explained team member Joseph Silk of the University of Oxford.
"The most massive stars are already gone. The youngest stars are between 50 million and 300 to 400 million years old. By comparison, most of the stars in the galaxy are around 10 billion years old."
The encounter that triggered the star birth would have been similar to our Milky Way swallowing the nearby Large Magellanic Cloud.
"We believe that a merger with a small, gas-rich galaxy around one billion years ago supplied NGC 4150 with the fuel necessary to form new stars," said team member Sugata Kaviraj of the Imperial College London and the University of Oxford.
"The abundance of `metals`?elements heavier than hydrogen and helium?in the young stars is very low, suggesting the galaxy that merged with NGC 4150 was also metal-poor. This points towards a small, dwarf galaxy, around one-twentieth the mass of NGC 4150."