BENGLAURU: A recent study by astronomers led by a team from Indian Institute of Astrophysics’ (IIA) has revealed that when a smaller galaxy flies past a much larger galaxy like the Milky Way, it can trigger formation of spiral arms in the larger galaxy, thereby changing its structure. “…These fade away with time. The study shows that galaxy fly-bys, which have not received adequate attention in astronomical studies, are as important as galaxy mergers in shaping the evolution of these galaxies,” the astronomers said. The results of this study, led by Ankit Kumar from IIA, which is an autonomous institute of the department of science and technology (DST), have been published in the journal ‘Monthly Notices of the Royal Astronomical Society’. “Most galaxies in our universe are clustered together. In a crowded environment, two galaxies can fly past each other or even merge with each other and that such phenomena affect the dynamics and morphology of these galaxies,” the astronomers said. As opposed to mergers, fly-bys are when galaxies come close, exert a tremendous gravitational pull on each other, and go away on their own paths. Both processes have played a crucial role in shaping galaxies through 100s of millions of years. Although mergers have been studied in detail using advanced computer simulations, not much work has been done on fly-bys so far, and present research shows that fly-bys are much more common in the local universe. “Ankit Kumar, a PhD student and his team have done a detailed study of fly-bys. Kumar, along with Prof Mousumi Das of IIA and Sandeep Kataria of Shanghai Jiao Tong University, investigated gravitational influence of each of the galaxies on the motion of stars of the other using state-of-the art computer simulations,” the DST said. They generated a disk galaxy similar to the Milky Way and simulated fly-bys while varying a range of parameters, like mass of the smaller galaxy, the closest distance they approach, the kind of bulge in the larger galaxy, and so on. They found that when the smaller galaxy passes by the massive galaxy, it triggers the formation of spiral arms in the disk of the latter. “We used powerful supercomputers at IIA to follow the motion of individual stars of a fly-by by generating realistic mock galaxies and evolving them in time,” Kumar, the lead author, said, adding that closer the distance between two galaxies in the fly-by, the stronger the spiral arms. A large fraction of stars goes into the formation of these spiral arms, and as a result, the disk of the galaxy becomes smaller and thicker. The DST added that when the smaller galaxy leaves the gravitational pull of the massive galaxy, spiral arms’ strength decreases. The astronomers further concluded that the long-term dynamical evolution of large galaxies can be strongly influenced by multiple fly-bys of smaller galaxies over its lifetime. “We saw that spherical bulges of stars inside larger galaxies are very stable against fly-by interactions because of their dense nature,” Prof Mousumi Das, said. However, a class of disk galaxies that have ‘flat bulges’ are indeed affected by close fly-bys.

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