Using data from the VISTA Variables in the Via Lactea Survey (VVV) ESO Public Survey, astronomers have discovered a previously unknown component of the Milky Way Galaxy – a thin disc of young stars across the Galaxy’s bulge.
This diagram shows the locations of the newly-discovered Cepheids in an artist’s rendering of our Milky Way Galaxy; the yellow star indicates the position of our Solar System. Image credit: ESO / Microsoft WorldWide Telescope.
“The central bulge of the Milky Way is thought to consist of vast numbers of old stars. But the VISTA data has revealed something new and very young by astronomical standards,” explained Dr Istvan Dekany the Pontificia Universidad Catolica de Chile, a team member and the lead author on a paper in the Astrophysical Journal Letters (arXiv.org preprint).
Analyzing data from the VVV survey, Dr Dekany and co-authors found 655 candidate variable stars of a type called Cepheids.
These stars expand and contract periodically, taking anything from a few days to months to complete a cycle and changing significantly in brightness as they do so.
The time taken for a Cepheid to brighten and fade again is longer for those that are brighter and shorter for the dimmer ones. This precise relationship makes the study of Cepheids one of the most effective ways to measure the distances to, and map the positions of, distant objects in the Milky Way and beyond.
But Cepheids come in two main classes, one much younger than the other. Out of their sample of 655 Cepheids, the astronomers identified 35 stars as belonging to a sub-group called classical Cepheids – young bright stars, very different from the usual, much more elderly, residents of the central bulge of the Milky Way.
They gathered information on the brightness, pulsation period, and deduced the distances of these 35 classical Cepheids.
Their pulsation periods, which are closely linked to their age, revealed their surprising youth.
“All of the 35 classical Cepheids discovered are less than 100 million years old,” said Dr Dante Minniti of the Universidad Andres Bello in Chile, who is a co-author of the paper.
“The youngest Cepheid may even be only around 25 million years old, although we cannot exclude the possible presence of even younger and brighter Cepheids.”
The ages of these classical Cepheids provide solid evidence that there has been a previously unconfirmed, continuous supply of newly formed stars into the central region of the Milky Way over the last 100 million years. But, this wasn’t to be the only remarkable discovery from the survey’s dataset.
Mapping the Cepheids that they discovered, the team traced an entirely new feature in the Milky Way – an inner thin disk of young stars along the Galactic mid-plane, traversing across the bulge. This new component had remained unknown and invisible to previous surveys as it was buried behind thick clouds of dust.
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I. Dekany et al. 2015. The VVV Survey reveals classical Cepheids tracing a young and thin stellar disk across the Galaxy’s bulge. ApJ 812, L29; doi: 10.1088/2041-8205/812/2/L29
