Nicknamed "Mayall's object", Arp 148 is a "peculiar", ring-shaped galaxy with a long-tailed companion. It is located in the constellation of Ursa Major, the Great Bear, approximately 500 million light-years away.
Credit: NASA
SYDNEY: The most common type of galaxy changed from a "peculiar" shape six billion years ago to a typical spiral shape today, and may indicate galaxies have a more violent past than previously believed.
A team of European astronomers took a 'snapshot' of galaxies today and in the past using the Hubble Space Telescope and the Sloan Digital Sky Survey, and compared the two.
They found that of 116 local galaxies sampled, 72% were spiral galaxies, such as our own Milky Way and our nearest neighbour, Andromeda.
Older galaxies more peculiar
Three percent of local galaxies were elliptical, like fuzzy blobs, while 15% were lenticular and 10% were what's classified as 'peculiar' — those that don't fit into the other categories in the Hubble tuning fork classification scheme developed by American astronomer Edwin Hubble in 1926.
But looking back in time (and space), the team found a very different picture. Six billion years ago, peculiar galaxies dominated with 52% of 148 distant galaxies.
Only 31% were the familiar spiral shape, while the other ranges of shapes remained about the same (4% of distant galaxies were elliptical, 13% lenticular).
Peculiar galaxies evolve into spirals
This new census, published in two recent papers in the journal Astronomy & Astrophysics, indicates that peculiar galaxies evolved into spirals through collisions and mergers.
This is counter-intuitive to what astronomers previously thought, which was that most galactic collisions produced elliptical galaxies.
"This means that in the last six billion years, these peculiar galaxies must have become normal spirals, giving us a more dramatic picture of the recent universe than we had before," said astrophysicist Rodney Delgado-Serrano, from the GEPI Laboratory at the Paris Observatory in Meudon, France, lead author of one of the papers.
The authors envisage that the history of a typical spiral galaxy such as Andromeda involved mergers of peculiar galaxies with gas-rich components, leading to the formation of giant spiral galaxies with discs and central bulges. As the gas cooled, the galaxies flattened out into a typical spiral.
This scenario means that our own Milky Way galaxy is atypical, however, as it seems to have been spared this violent past.
