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The results of a study, carried out by a group of Anglo-American researchers led by Michelle Collins (Cambridge's Institute of Astronomy), have been published in the Monthly Notices of the Royal Astronomical Society.
hey used the Keck telescope in Hawaii to identify the so-called "thick disk" in the nearby galaxy M31, in Andromeda. This structure, present in most spiral galaxies, is fundamental for understanding the evolution of these systems.
The disk component of a spiral galaxy is typically the most luminous, containing around 70% of the stars. The youngest of these are concentrated in the spiral arms, where compression waves trigger the collapse of interstellar gas clouds to form new stars.
This disk is itself made up of two components: the thick disk and the thin disk. The former is composed of relatively old stars with orbits that take them above and below the thin disk, whereas the thin disk is a more ordered structure that forms much later in the evolution of the galaxy.
The thin disk is then made up of younger stars, chemically different from those in the thick disk.
Separating these two components is not as easy as it might appear because of the need to measure the velocities of as many individual stars as possible, something only possible for our own galaxy (although here dust obscuration is a problem) and its closest neighbours.
The best candidate for this study was therefore the Andromeda galaxy.
From the results so far obtained, it remains likely that the huge amount of energy required to produce the thick disk has its origin in mergers of low mass satellite galaxies. This would help explain the different chemical composition seen between the thick and thin disk.
More detailed observations of these structures in the future will provide not only important information on the evolution of M 31 but also of our own galaxy, given their similarity.
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