Galaxies are the fundamental building blocks of our visible universe, each containing billions of stars, like our Sun. They can broadly be classified into two types – elliptical and spiral. A typical spiral galaxy has two structural components: a central bulge in which stars move, more or less, on random orbits and a disk in which they move on approximately circular orbits like planets in our solar system. Understanding the formation and the evolution of these galaxies over the age of the universe, approximately 13.7 billion years, is at the heart of astronomical research.
Modern astronomy provides a fairly good understanding of how the universe has developed into its present state. Smaller galaxies are known to have merged and coalesced into more massive ones. More than half of the mass of the present day galaxies has been acquired in the last eight billion years.
Computer simulations based on theoretical models like these have been very successful at explaining the shape and structure of galaxies and the universe as we see through observations. Our computers are able to recreate the processes happening in the universe with very high accuracies.
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However, what has defied a successful explanation till now is the presence of large or massive spiral galaxies without a bulge at the centre, i.e., galaxies with only the disc component. The kind of processes involved in the formation of large galaxies, i.e., continuous mergers and accretion of matter from outside, should have led, at least some of the stars in the disc, all in constant rotational motion, to lose their angular momentum and fall to the centre of the galaxy, thereby creating a bulge. All computer simulations done to create massive spiral galaxies indeed create a bulge at the centre.
Even if it is assumed that these galaxies grew bigger in isolation, not through mergers, a central bulge seems inevitable because of the physical churning of the matter already inside. Thus, whether the galaxies evolved through mergers or in isolation, the understanding is that massive spiral galaxies should invariably have a bulge.
Our study involving statistical analysis of the data on galaxies, observed through the Hubble Space Telescope, has revealed that at present about 18 per cent of massive spiral galaxies are without a bulge. Interestingly, this proportion does not seem to have changed over last eight billion years which is roughly half of the known age of the universe.
Our deduction is based on the fact that a single exponential function is observed to fit the full light profile of these galaxies, from the centre to the edges. It indicates that the galaxy has a flat shape. Studying the light profile, we were further able to conclude that the average central brightness of the galaxy and the scale length have remained the same over the last eight billion years. However, the mass and size of these galaxies has increased substantially, by 40 and 60 per cent respectively, over the same time range.
This suggests that these galaxies have grown not by mergers but by accumulating matter mainly at the edges. That can possibly be the only explanation for the absence of bulge at the centre.
This hypothesis is able to account for massive bulgeless spiral galaxies as of now. However, it needs to be strengthened further by observations from future telescopes which would be able to probe the internal galaxy dynamics with better resolution.
Our work has been chosen as one of the most interesting recent results by American Astronomical Society. The post summarizing the work is available at: http://aasnova.org/2016/04/04/forming-galaxies-without-bulges/.
Sonali Sachdeva (Post-Doctoral Fellow) and Kanak Saha (Faculty) Inter-University Centre for Astronomy and Astrophysics, Pune, India.