By Achintya Rao
Observations of a cluster of galaxies not too far from home have revealed that the cannibalistic galaxy at the centre of the cluster might be the most massive one in our local universe.
Images of Abell 3827 from the Gemini Observatory. Gemini Legacy Image: R. Carrasco et al., Gemini Observatory/AURA
The galaxy ESO 146-IG 005 at the centre of the galaxy cluster Abell 3827 has been known for over a decade to be cannibalistic, but astronomer Rodrigo Carrasco says, “The magnitude of its appetite has not been fully appreciated.” Carrasco and his colleagues at the Gemini Observatory‘s South telescope in Chile used the recent discovery of the gravitational lens around the cluster to conclude that the galaxy is nearly 30 trillion times the mass of Sol, our sun.
Line diagram of light bending around a mass. Image credit: Michael Sachs
Gravitational lensing is observed when light from a distant star or galaxy bends around a closer star or galaxy, sometimes giving the illusion of being in more than once place at a time. Often, the image of the star or galaxy observed will appear elongated. The bending of light is caused by the distortion of local spacetime by a massive object (see images). These apparent images sometimes form what are called Einstein’s rings.
The gravitational lens around the cluster, located 1.4 billion light years away from us, gave astronomers a value for the mass of the central galaxy that was 10 times higher than estimates using X-rays. “Assuming our model is correct, this is by far the most massive galaxy known in our local universe,” Carrasco said.
The two background galaxies observed lie at distances of 2.7 billion light years (labelled A, in the first image) and 5.1 billion light years (labelled B) away from us.
How Einstein's rings are formed. Image credit: NASA
The results, to be published in The Astrophysical Journal Letters1, are very important, particularly to understand the growth of elliptical galaxies. These galaxies are thought to acquire most of their mass by merging with other galaxies or by consuming them over the course of time. Cannibalistic galaxies such as ESO 146-IG 005 can grow to tremendous sizes by consuming their neighbours.
Michael West, an astronomer from the European Southern Observatory, who first observed the cannibalistic behaviour in 1998, said, “This unabashed cannibal is something of a messy eater, with the partially digested remains of at least four smaller galaxies still visible near its centre. Eventually this galaxy will grow even bigger judging by the number of nearby galaxies already within its gravitational grasp.”
- E. R. Carrasco, P. L. Gomez, T. Verdugo, H. Lee, R. Diaz, M. Bergmann, J. E. H. Turner, B. W. Miller, & M. J. West (2010). Strong Gravitational Lensing by the Super-massive cD Galaxy in Abell
3827 The Astrophysical Journal Letters arXiv: 1004.5410v1 [↩]