A strange bar in the centre of the Milky Way

Through a very detailed analysis of a large stellar survey in the infrared (2MASS), a team from the OSU-THETA Franche-Comte, UTINAM Institute was able to study in detail the central regions of the Milky Way.By comparing a model of the Galaxy, that takes into account the distribution of absorbing dust and is able to simulate the observations by making hypothesis on the formation history of stars in the disk and the bar, the researchers showed that the Milky Way contains both a flattened spheroidal bulge and a long bar.

Comparison between a model of the Galaxy (top images) and observations (bottom images). The images on the left are three colour images in the visible whereas those on the left are in near-infrared bands. The observations in the middle are courtesy of A. Mellinger and the infrared data are courtesy of 2MASS.

These two populations  of stars are distinct both in their chemical composition (and therefore their star formation histories)  and by their orbits. They also show that the bar has an outwardly flared structure, which can occur if the disk stars are trapped in resonances due to the bar, thus disturbing their orbits. They explain why viewed from outside the bar appears like a box rather than as a spheroid. This is the first time such accurate characteristics  of the  Milky Way’s bulge and  bar have been obtained. New observations of stellar orbits and their chemical composition will be needed to fully clarify the mystery of the formation of these structures, the timescales of the formation of the bar as well as the age of bulge. This will be made possible through a large spectroscopic survey conducted by a European team on the VLT, the European Very Large Telescope located in the Atacama Desert in Chile, a survey that began in early 2012 and will last 3 years.Our Galaxy is known to be a typical spiral galaxy. We have known the general appearance of this galaxy for many years:  most stars form a thin disk, populated by stars of various ages between 0 and 10 billion years as well as gas and dust.This thin disk (with a thickness of about 1000 lightyears and a radius of 50 000 lightyears) is surrounded by a halo consisting mainly of old stars and dark matter. In the disk the youngest stars and the dust are concentrated along the spiral arms. The central regions of the Galaxy are harder to analyze but contain the highest stellar density. As the Sun is in the disk of the Galaxy, along with the dust and gas, it is very difficult to study the central regions, masked by the interstellar material that absorbs starlight in the visible wavelengths. Despite these challenges and through observations in the infrared, it has previously been possible to demonstrate the presence of a central bulge, with a large number of stars, similar to external galaxies of the same type, but several scenarios have been proposed to explain its formationand the question is still open.

Opinions differ between a scenario of quick formation early in the history of the Milky Way and  a scenario where, due to dynamic instabilities in the disc,  a bar is formed into an elongated shape, like a cigar or a rugby ball. The new model shows that the two scenarios coexist: there is an older type conventional bulge, at the same time a bar dynamically formed after the formation of the disc.

IRAP Contact

  • Douglas Marshall – IRAP – douglas.marshall@irap.omp.eu &  tel: +33 (0)5 61 55 87 53

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