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Using the Magellanic Clouds to Understand the Interaction of Galaxies

Periodic Reporting for period 4 - INTERCLOUDS (Using the Magellanic Clouds to Understand the Interaction of Galaxies)

Reporting period: 2021-04-01 to 2022-03-31

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 influence their shape and the formation of their stars as well as the evolution of the galaxies. 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. 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 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 dust absorption within them. 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 refer to the spatial distribution and the motion of populations of stars with different median ages across the Large and Small Magellanic Cloud (LMC and SMC) as well as to the determination of the amount of dust present inside the galaxies. Results were disseminated via research publications and presentations at international conferences. We selected stars based on their observed flux within near-infrared filters and attributed ages to them from the comparison with the flux predicted from models of stellar evolution. We explored ages from a few million to several billion years and we distinguished between stars of the Magellanic Clouds and stars of the Milky Way along the line of sight. We produced maps of the spatial distribution of the Magellanic Clouds’ stars within the smallest bins to date that outline the morphological features of the galaxies most likely associated to dynamical interactions. In the SMC, the predominantly spheroidal shape traced by the old stars contrasts the triangular shape traced by young stars. In the LMC, young stars trace coherent structures, multiple spiral arms and clumps. Whereas older stars are overall more regularly and smoothly distributed. We also studied groups of young stars and characterised over 2500 of them, at different levels of significance, that range in size form a few to over 1 kpc. We derived other parameters such as surface density, number of stars, crossing time and velocity dispersion as well as relations among them which indicate that the fractal nature of these structures was probably inherited from the gas clouds from which they formed and their architecture was most likely generated by supersonic turbulence. 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. In the centre of the LMC we found that intermediate-age/old stars follow elongated orbits whereas young stars move instead along the filamentary bar structures. We measured the tangential motion also for stellar populations of the Magellanic Bridge. There the stellar density is not very high and to remove the influence 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. We developed a machine-learning-based method to distinguish between Magellanic Clouds and Milky Way stars in the outer regions of the LMC. There we found that stars in the south east of the galaxy rotate slower than the overall rotation, which could be due to a fraction of SMC stars present in the region that move opposite to the expected rotation. Finally, we developed a method to quantify the total dust content of the Magellanic Clouds. 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 Magellanic Clouds. We combined the most sensitive optical and near-infrared imaging observations of the LMC and 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 LMC/SMC, the dust within the LMC/SMC, the dust between the LMC/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 likely associated to different tracers.
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 predominantly on near-infrared images of faint stars in the dense stellar regions represent a unique opportunity to consolidate the results into a comprehensive understanding of the system. We have characterised the components of the galaxy (disc, bar, and tidal tails) with an unprecedented level of detail which will allow us to interpret, with the use of dynamical simulations, their history of interaction and to provide precise links to their history of star formation. Our results have also served to refine the sample of stars to be observed by future spectroscopic facilities like the multi-fibre spectrograph 4MOST.
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Tangential (left) and mean-subtracted (right) motion in the the Small Magellanic Cloud.
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