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.

Astronomers 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. The results suggest the conditions needed for slow galaxy interactions and mergers are more likely to occur in galaxies falling into a galaxy cluster compared to the general population of galaxies outside clusters.

And this may explain the mystery of how spiral galaxies in clusters are transformed over time into smooth disks: the slow-motion conditions needed for the transformation are indeed occurring among populations of galaxies falling towards the cluster centre.

Since infalling of galaxies into clusters was greater in the past, such interactions and mergers may have contributed significantly to the transformation of the past population of cluster spirals to lenticular galaxies in present-day clusters, explaining why clusters today have so few spirals and so many lenticular galaxies.

Figure 1. R band image of CGCG 538-043 in Abell 347 (z=0.019) obtained with the JKT. This image resolves the galaxy into an interacting pair of galaxies. Credit: C. Moss, C.F. Thomas, P.A. James, Astrophysics Research Institute, Liverpool John Moores Univ. [ JPEG ]

References:

• Moss, C., 2006, "Enhanced mergers of galaxies in low-redshift clusters", MNRAS, 373, 167
• Slow Motion Mergers in Galaxy Clusters Provide Conditions to Transform Spirals to Smooth Disks. RAS Press Release, 5th April, 2006 (issued at NAM 2006).
• Ker Than, "New Clues to How Galaxies Evolve", space.com.