Huge Galaxies Expand by Devouring Smaller Ones

A new study reports that galaxies enlarge by devouring their smaller neighbors.

Simulation showing the distribution of dark matter density overlayed with the gas density. This image clearly shows the gas channels connecting the central galaxy with its neighbors. Image Credit: Gupta et al./ASTRO 3D/IllustrisTNG collaboration.

A clear understanding of precisely how massive galaxies achieve their huge size is lacking, mainly because they enlarge over a period of billions of years. However, at present, a research team headed by Dr Anshu Gupta from Australia’s ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) has developed a combination of modeling and observation, which offers a vital clue.

In a study published in the Astrophysical Journal, the researchers integrate data from an Australian project named the Multi-Object Spectroscopic Emission Line (MOSEL) survey with a cosmological modeling program used on some of the world’s largest supercomputers to have a glance of the forces that produce these ancient galactic beasts.

According to Dr Gupta, an analysis of how gases inside galaxies move enable finding the proportion of stars produced internally—and the proportion that is effectively devoured from elsewhere.

We found that in old massive galaxies—those around 10 billion light years away from us—things move around in lots of different directions. That strongly suggests that many of the stars within them have been acquired from outside. In other words, the big galaxies have been eating the smaller ones.

Dr Anshu Gupta, ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions

Since light takes time to move through the universe, galaxies located at farther distances from the Milky Way are observed at an earlier point in their occurrence. Dr Gupta and her colleagues discovered that observation and modeling of these faraway galaxies divulge very little variation in their internal movements.

We then had to work out why ‘older’, closer big galaxies were so much more disordered than the ‘younger’, more distant ones. The most likely explanation is that in the intervening billions of years the surviving galaxies have grown fat and disorderly through incorporating smaller ones. I think of it as big galaxies having a constant case of the cosmic munchies.

Dr Kim-Vy Tran, Study Second Author, ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions

Similar to Dr Gupta, Dr Tran is also based at the UNSW Sydney.

The research group—including researchers from other Australian universities and institutions in Canada, the United States, the Netherlands, Belgium, and Mexico—ran their modeling on an exclusively developed set of simulations called IllustrisTNG.

The goal of this multi-year, international study is to develop a range of large cosmological models of the formation of galaxies. The program is so huge that it must run concurrently on multiple powerful supercomputers in the world.

The modelling showed that younger galaxies have had less time to merge with other ones. This gives a strong clue to what happens during an important stage of their evolution.

Dr Anshu Gupta, ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions


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