Astronomers at Liverpool John Moores University may have solved the mystery of how spiral galaxies in clusters are transformed over time into smooth disks. Results from a study of galaxy clusters confirms that the slow-motion conditions needed for the transformation are occurring among populations of galaxies falling towards the cluster centre.
Over the past several billion years the predominant shape of disc galaxies in clusters has changed from a spiral to a smooth disk. Theory suggests that this change occurs when two galaxies of unequal mass merge and gravitational effects pull gas to the galaxies nucleus, sweeping away the spiral structure and leaving behind a smooth, barren, thickened disk known as a lenticular galaxy. However, galaxies orbiting in clusters move at high speeds and in random directions, which should mean that conditions needed for these slow interactions rarely occur. Instead, multiple rapid encounters between galaxies, known as galaxy harassment, are dominant but these types of fast encounters cannot easily form the smooth disks.
The group from Liverpool John Moores compared eight examples of populations of galaxies falling towards the centres of galaxy clusters with control samples of galaxies far from the clusters. They found that the infalling galaxies in the cluster were predominantly distorted in shape and had a higher than normal rates of star formation. Between a half and three-quarters of these galaxies were very close by to another galaxy or appeared to be merging with a companion galaxy, which suggested that interactions and mergers are more common in galaxies falling into the cluster than in the control sample.
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