Researchers at the Leibniz Institute for Astrophysics Potsdam (AIP), in collaboration with scientists from the VMC team, have confirmed the existence of elongated orbits that play a crucial role in the formation of the bar structure in the Magellanic Clouds system using data from the VISTA survey (VMC). The team used repeated imaging observations to construct a velocity map of stars in the central region of the Large Magellanic Cloud, which allowed them to identify and study these elongated orbits. This discovery sheds new light on the mechanisms that shape the structure of galaxies and provides valuable insights into the evolution of the Magellanic Clouds system.
Observed orbits of stars within the central parts of the Large Magellanic Cloud. The stars in the central region, along the bar, follow elongated orbits which deviate from a circular shape (dashed contours) [Credit: AIP/F. Niederhofer, VISTA VMC Survey]
The Large Magellanic Cloud (LMC) is the most prominent satellite galaxy of the Milky Way and can be easily observed from the southern hemisphere with the naked eye. This galaxy is home to a variety of stars that span a wide range of ages, from newly formed stars to those as old as the universe itself. Despite being characterized by a single spiral arm and a bar that is offset from the center of the disk, the LMC is classified as an irregular galaxy.
Stellar bar structures are a common feature in spiral galaxies, and their formation is believed to result from small perturbations within the stellar disk that force stars onto elongated orbits. The backbone of these bars is formed by a specific type of elongated orbit that is aligned with the major axis of the bar. These orbits are critical to the support of the overall bar structure.
The VISTA telescope was developed to survey the southern sky at near-infrared wavelengths, making it ideal for studying sources that emit in this spectral domain due to their nature or the presence of dust. By using data from the VMC survey, the team of researchers has discovered direct evidence for these elongated orbits within the bar of the LMC. The VMC is a multi-epoch survey of the Magellanic system and is a public survey project of the European Southern Observatory (ESO), which took place between 2010 and 2018 with the aim of studying the stellar content and dynamics of our closest extragalactic neighbors.
The research team has developed a sophisticated method for accurately determining the proper motions of stars within the Magellanic Clouds. In a new study, which will be published in the Monthly Notices of the Royal Astronomical Society, this method was applied to the central parts of the LMC. Using the measured values, the authors were able to compute the actual motions of the stars within the frame of the LMC, resulting in detailed velocity maps of the galaxy’s internal velocity structure.
The researchers were amazed by the level of detail in the velocity maps, which demonstrated a significant improvement over early measurements conducted several years ago. The maps revealed elongated stellar motions that closely followed the structure and orientation of the bar, much to the researchers’ astonishment. Thomas Schmidt, a co-author of the study and a doctoral student at AIP, remarked on the impressive results of the team’s method.
The barred spiral galaxy NGC 1300, considered to be prototypical of barred spiral galaxies. Barred spirals differ from normal spiral galaxies in that the arms of the galaxy do not spiral all the way into the center, but are connected to the two ends of a straight bar of stars containing the nucleusat its center [Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)]
Maria-Rosa Cioni, the principal investigator of the VMC project and head of the Dwarf Galaxies and the Galactic Halo section at AIP, highlighted the unique advantage of observing individual stars within the Magellanic Clouds using ground-based telescopes like VISTA. Due to their close proximity of about 163,000 light-years, these galaxies provide a unique laboratory for studying the processes that shape and form galaxies in great detail.
The dynamics of stars are of particular interest as they hold valuable information about the formation and evolution of galaxies. However, for a long time, the one-dimensional line-of-sight velocities of stars have been the only source of dynamical information. These velocities can be readily measured by spectroscopic Doppler shifts, which depend on the effect of the observed light appearing bluer or redder depending on whether the star approaches or moves away from us.
To obtain the full three-dimensional velocities of stars, it is necessary to determine the proper motions of the stars, which are the apparent two-dimensional motions of stars in the plane of the sky. These motions can be obtained by observing the same stars multiple times over a given time period, typically several years, and determining the displacements of the stars with respect to nearby reference objects, such as distant background galaxies or stars with known proper motions.
However, precise measurements of these motions are still challenging as the observed motions of stars at the distance of the Magellanic Clouds are on the order of milli-arcseconds per year, which is incredibly small. It took nine years of monitoring to accumulate enough images to accurately measure these minuscule motions.
Florian Niederhofer highlights the importance of this discovery as it provides a vital contribution to the study of the dynamic properties of barred galaxies. The Magellanic Clouds are currently the only galaxies where such motions can be investigated using stellar proper motions, while for more distant galaxies, this is still beyond our technical capabilities. This unexpected measurement adds to the number of important results obtained by the VMC team, as noted by Maria-Rosa Cioni.
