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Using the Magellanic Clouds to Understand the Interaction of Galaxies

Periodic Reporting for period 3 - INTERCLOUDS (Using the Magellanic Clouds to Understand the Interaction of Galaxies)

Reporting period: 2019-10-01 to 2021-03-31

This project is about the study of the Magellanic Clouds, two irregular dwarf satellite galaxies of the Milky Way, and their use to understand how dynamical interactions between dwarf galaxies influences their shape and the formation of their stars as well as the evolution of the galaxies. Furthermore, the Magellanic Clouds are under the process of being disrupted by the significantly more massive Milky Way. This is a standard merging event in the evolution of galaxies. The Magellanic Clouds are located relatively close to the Milky Way and this allows us to perform detailed observations from which we can derive the distance, motion, age and chemical composition of the stars. With the analysis of this data we will be able to view step-by-step important processes in the formation and evolution of galaxies (interactions and mergers).
The main objectives of this project are as follows. First, we would like to measure precise tangential motions of the stars and of the galaxies, i.e. velocities on the plane of the sky, from the position of stars observed in multiple images at near-infrared wavelengths. These images are obtained from panoramic observations of the Magellanic Clouds over about 10 years using the VISTA telescope from the European Southern Observatory as part of the VISTA survey of the Magellanic Clouds system (VMC). Then, we would like to build theoretical models to interpret the distribution of velocities and also the spatial distribution of stars across the galaxies. We would like to reconstruct the orbital paths of the galaxies on the sky and link them to changes in the shape of the galaxies as traced from the distribution of stars of different ages. To better characterise and measure the age and chemical composition of the stars we adopt observations at other wavelengths, obtained from other major survey projects. Multiwavelength images are an important tool to establish the memberships of stars to the Magellanic Clouds or to the Milky Way and also to estimate the influence of dust in the derivation of the stellar parameters. Ultimately, we would also use spectra to quantify the chemical composition and the component of the velocity along the line of sight. We are at the stage of preparing for major spectroscopic survey projects that will allow us to obtain spectra for many stars of the Magellanic Clouds sampling their components: discs, bars, tidal tails, and other features resulting from their dynamical history and history of star formation.
The main results obtained so far refer to the spatial distribution and the motion of populations of stars with different median ages across the Small Magellanic Cloud (SMC) as well as to the determination of the amount of dust present inside the galaxy.
We selected stars based on their observed flux within near-infrared filters and we attribute ages to them from the comparison with the flux predicted from models of stellar evolution. In particular, we explored ages from a few million to several billion years and we distinguished between stars of the SMC and stars of the Milky Way galaxy along the line of sight. We produced maps of the spatial distribution of the SMC stars within the smallest bins to date that outline the morphological features of the galaxy. We also recovered how star formation has progressed throughout the galaxy and concluded that the SMC formed most of its stars between 8 and 3.5 billion years ago. We derived the centre of mass of the galaxy, its location, and distance. The predominantly spheroidal shape traced by the old stars contrasts the triangular shape traced by young stars. These features are most likely associated to dynamical interactions with the companion galaxy, the Large Magellanic Cloud (LMC).
We also obtained individual distances to a specific type of stars, classical Cepheids, regularly changing their flux because of pulsation. These were used to establish that many Cepheids formed out of gas that was stripped from the interaction between the SMC and the LMC about 200 million years ago resulting in the formation of the Magellanic Bridge, a tidal feature connecting the two galaxies. These Cepheids are located away from the centre of the galaxy where instead there is a predominance of stars that were already in place before the interaction took place.
Furthermore, by accurately measuring the tangential motion of stellar populations within the central regions of the SMC we identified not only an overall flow motion towards the Magellanic Bridge but also a dynamical feature behind the core of the galaxy, this could be associated to a so-called counter-Bridge predicted from dynamical simulations reproducing the interaction between the SMC and the LMC. We measured the tangential motion also for stellar populations of the Magellanic Bridge. There the stellar density is not very high and to isolate Bridge stars from Milky Way stars we complemented near-infrared observations with distances obtained from the Gaia mission, currently charting the sky and providing accurate coordinates and distances.
Finally, we developed a method to quantify the total dust content of the SMC. Accounting for dust is one of the main uncertainties influencing most studies that use the observed flux of stars. In our method we used distant galaxies observed in the background of the SMC. We combined the most sensitive optical and near-infrared observations of the SMC stars and accounted for the several components of dust along the line of sight: the dust within the Milky Way, the dust between the Milky Way and the SMC, the dust within the SMC, the dust between the SMC and the background galaxies and the dust within the background galaxies. We obtained dust maps consistent in part with previous determinations using other methods and in part highlighting discrepancies associated to different tracers, for example stellar tracers embedded with the SMC galaxy.
Within the context of past and current studies, that have revealed the richness of the outermost regions of the Magellanic Clouds with streams and tails as well as new satellite galaxies, our investigations based on near-infrared images of faint stars in the central and dense stellar regions represent a unique opportunity to consolidate the results into a comprehensive understanding of the system. The second part of the project will focus on the Large Magellanic Cloud by performing similar studies on the spatial distribution and motion of populations of stars with different median ages. We expect to be able to characterise the components of the galaxy (disc, bar, and tidal tails) with an unprecedented level of detail, especially in the central regions that may harbour a black-hole. We plan to combine these results with those derived from the study of the Small Magellanic Cloud to interpret, with the use of dynamical simulations, the history of interaction of the galaxies and to provide precise links to their history of star formation. Furthermore, we will try to estimate stellar populations parameters using a Bayesian method applied to multi-wavelengths observations. Our results will also serve to refine the sample of stars to be observed by future spectroscopic facilities like the multi-fibre spectrograph 4MOST.
Tangential (left) and mean-subtracted (right) motion in the the Small Magellanic Cloud.