Studying the dynamics of stars in the Large Magellanic Cloud

Scientists at the Leibniz Institute for Astrophysics Potsdam (AIP) have utilised data from the VISTA survey to study the dynamics of stars in the Large Magellanic Cloud.

Using a technique of repeated imaging observations to build a velocity map of stars in the central region of the Large Magellanic Cloud, the researchers from AIP – working in partnership with scientists from the VMC team – were able to confirm the existence of elongated orbits, which are at the backbone of the bar formation process.

The Large Magellanic Cloud

The Large Magellanic Cloud (LMC) is perceptible from the southern hemisphere as it is the largest and brightest satellite galaxy in the Milky Way. It comprises an abundance of stars that extend over a very diverse age range, from newly emerging stars, to stars as old as the known Universe.

The LMC is categorised as an irregular galaxy due to the fact it is distinguished by a single spiral arm and a bar which is offset from the centre of the disc.

“Stellar bar structures are a common feature in spiral galaxies. They are believed to form from small perturbations within the stellar disc that remove stars from their circular motions and force them on elongated orbits,” said Florian Niederhofer, who was the lead author of the study. “A specific type of these orbits are the ones that are aligned with the major axis of the bar. These are considered to be the ‘backbone’ of stellar bars and provide the main support of the bar structure.”

The VISTA telescope

The VISTA telescope was established to survey the southern sky at near-infrared wavelengths in order to examine sources that radiate favourably in this spectral domain, due to either their nature or the existence of dust.

By employing data from the VMC survey, the group has now discovered the first direct evidence for these orbits within the bar of the LMC. VMC is a multi-epoch survey of the Magellanic system and a public survey project of the European Southern Observatory (ESO), conducted between 2010 and 2018, with the intention of analysing the stellar content and dynamics of our closest extragalactic neighbours.

Novel research method

The researchers established an advanced technique to precisely ascertain proper motions of stars within the Magellanic Clouds. In a novel study, that has been published in the Monthly Notices of the Royal Astronomical Society, the technique was employed to central parts of the LMC.

From the measured values, the team computed the actual stellar motions within the frame of the LMC, generating comprehensive velocity maps of the galaxy’s internal velocity structure.

“The stunning level of detail in velocity maps shows how much our method has improved, compared with early measurements some years ago,” added Thomas Schmidt, co-author and doctoral student at AIP.

Much to the team’s surprise, their maps uncovered elongated stellar motions that adhere to the structure and orientation of the bar.

“Thanks to their close proximity of about 163,000 light-years, we can observe individual stars within the Magellanic Clouds using ground-based telescopes like VISTA,” explained Maria-Rosa Cioni, principal investigator of the VMC project and head of the Dwarf Galaxies and the Galactic Halo section at AIP. “Thus, these galaxies provide us with a unique laboratory to study in great detail the processes that shape and form galaxies.”

Dynamics of the stars

One exciting area of study was the dynamics of the stars, as they contain significant information about the evolution of galaxies. However, for a long time, the one-dimensional line-of-sight velocities of stars were the only way to attain dynamical information.

To develop the full three-dimensional velocities of the stars, it is essentials to know the proper motions of the stars, which are the apparent two-dimensional motions of the stars in the plane of the sky.

These motions can be acquired by studying the same stars multiple times over a given period, typically several years. The displacements of the stars are then determined regarding adjacent reference objects. These objects can be very distant background galaxies, which can be expected to be at rest, given their large distances, or stars with already known proper motions.

Precise data informs studies

As the motions of stars are minuscule when observed from Earth, accurate measurements are still challenging. At the distance of the Magellanic Clouds, the detectable motions of the stars are on the order of milli-arcseconds per year – one milli-arcsecond is about the size of an astronaut on the Moon as seen from Earth.

“Our discovery provides an important contribution to the study of dynamical properties of barred galaxies, since the Magellanic Clouds are at present the only galaxies where such motions can be investigated using stellar proper motions. For more distant galaxies this is still beyond our technical capabilities,” added Florian Niederhofer.

Overall, it took nine years of observation to amass enough images to quantify these tiny motions. “This unexpected measurement adds to a number of important results obtained by the VMC team,” concluded Maria-Rosa Cioni.

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