Michele De Leo
The Large and the Small Magellanic Clouds (LMC/SMC) constitute two of the most fascinating objects of our Local Group of galaxies. Due to their proximity to us and to their turbulent interaction history they provide natural laboratories to understand how galaxies form and evolve as well as how tidal interactions shape these processes.
Historically, the SMC has been less investigated than the LMC due to its greater distance from us and its apparently more disrupted state. Basic properties of the SMC such as its centre of mass, its line-of-sight depth and its dynamical state all remain highly debated.
With the aim of understanding the structure, dark matter content and dynamical state of the SMC, I present new spectroscopic data for ~2000 SMC red giant branch stars observed using the AAOmega spectrograph at the Anglo-Australian Telescope. These observations are complemented by further spectroscopic data from Dobbie et al. (2014) and proper motions from the Gaia DR2 catalogue to get a fully comprehensive view of the internal kinematics of the SMC. I show here that the SMC centre of mass is clearly offset from the velocity centre of its associated atomic hydrogen gas, demonstrating that the latter is likely to be far from dynamic equilibrium. The results presented are also the first unequivocal confirmation that the SMC is currently undergoing tidal disruption by the LMC. I find evidence of tidally stripped stars, projected along the line-of-sight, well within the innermost regions of the SMC. In order to gain further insights into how advanced is the state of tidal disruption in the SMC, I compare these findings to numerical models of the SMC/LMC system disrupting around the Milky Way. This work has been published in the MNRAS journal.
Galaxies constitute some of the biggest gravitationally bound structures in the cosmos, only surpassed by the 'clusters of galaxies', (large ensemble of galaxies). They hold important information about the baryonic matter (normal atomic matter) as well as about the underlying, invisible Dark Matter that accounts for approximately 85% of the matter in the universe.
Big galaxies, like our own Milky Way, have slowly formed over billions of years by “devouring” smaller galaxies in a process called 'galactic cannibalism'. These little systems called 'dwarf galaxies' come in all shapes and sizes, and provide the gas and chemicals needed to keep star formation going. Since they are the most numerous systems in our Universe and also constitute essential “building blocks” of larger galaxies, studying them is key to understand the evolution of all types of galaxies.
The Milky Way is surrounded by several of these smaller systems, orbiting around it like planets around the Sun. Among these satellites, the two biggest and closest to us are the Large and the Small Magellanic Clouds. While the scientific community recognise them with these names, they have been given different names by all the cultures of the Southern Hemisphere that observed them for centuries, as the indigenous inhabitants of Australia, South America and Southern Arabia.
The proximity of the Magellanic Clouds allows us to resolve their individual stars which, together with the fact that they have interacted a lot, makes them great laboratories to study the composition and evolution of galaxies.
The Small Magellanic Cloud in particular, being the less massive, is the one whose structure has suffered the most from the interaction with the Large Magellanic Cloud and still poses several questions regarding its shape, its internal motions, and its current structure.
In order to understand more about the above-mentioned galactic evolutionary processes, I combined data taken from the groundbased Anglo-Australian Telescope located at Siding Spring Observatory with data from the ESA satellite mission Gaia. This allowed me to have full 3D information about the motions of the target stars, knowing with good accuracy how they moved in all directions. Using these observations I studied the motions of several thousand stars inside the Small Magellanic Cloud. From the analysis of the observational data, I created maps showing unequivocally that the stars in the Small Magellanic Cloud are not orderly rotating but, instead, they are streaming towards the Large Magellanic Cloud. This result is important in the sense that orderly rotation is expected for stars inside a galaxy bound by its own gravity. The stars in the Small Magellanic Cloud not showing rotation suggest that the galaxy is not in equilibrium anymore and we need to rethink our understanding of the Small Magellanic Cloud as an extended, bound body.
I complemented the observational data with computer simulations in order to reproduce the past evolution of these gravitationally bound system. I run simulations of the Magellanic Clouds orbiting each other and orbiting, as a pair, the Milky Way.
These results taken together confirm that the stars in the Small Magellanic Cloud have been stripped from their orbits and dragged by the tidal influence of the bigger companion, the Large Magellanic Cloud. The analysis I performed indicates for the first time that this disruption operated by the Large Magellanic Cloud extends deep into the Small Magellanic Cloud.
This study has been published on the Monthly Notices of the Royal Astronomical Society.