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Discovery in the Galactic Bulge

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Milky Way bulge
Artist's impression of the central bulge of the Milky Way. A recent study has discovered a new population of young stars in the bulge of our galaxy. [ESO/NASA/JPL-Caltech/M. Kornmesser/R. Hurt]

In our efforts to map our galaxy’s structure, one region has remained very difficult to probe: the galactic center. A new survey, however, uses infrared light to peer through the gas and dust in the galactic plane, searching for variable stars in the bulge of the galaxy. This study has discovered a population of very young stars in a thin disk in the galactic center, providing clues to the star formation history of the Milky Way over the last 100 million years.

Obscured Center

The center of the Milky Way is dominated by a region known as the galactic bulge. Efforts to better understand this region — in particular, its star formation history — have been hindered by the stars, gas, and dust of the galactic disk, which prevent us from viewing the galactic bulge at low latitudes in visible light.

The positions of the 35 classical Cepheids discovered in VVV data are projected onto an image of the galactic plane. The survey area is outlined by the blue lines. The galactic bar is marked with a red curve, and the bottom panel shows the position of the Cepheids overlaid on the VVV bulge extinction map. Click for a better look! [Dékány et al. 2015]

The positions of the 35 classical Cepheids discovered in VVV data, projected onto an image of the galactic plane. Click for a better look! The survey area is bounded by the blue lines, and the galactic bar is marked with a red curve. The bottom panel shows the position of the Cepheids overlaid on the VVV bulge extinction map. [Dékány et al. 2015]

Infrared light, however, can be used to probe deeper through the dust than visible-light searches. A new survey called VISTA Variables in the Via Lactea (VVV) uses the VISTA telescope in Chile to search, in infrared, for variable stars in the inner part of the galaxy. The VVV survey area spans the Milky Way bulge and an adjacent section of the mid-plane where star formation activity is high.

Led by István Dékány, a researcher at the Millennium Institute of Astrophysics and the Pontifical Catholic University of Chile, a team has now used VVV data to specifically identify classical Cepheid variable stars in the bulge. Why? Cepheids are pulsating stars with a very useful relation between their periods and luminosities that allows them to be used as distance indicators. Moreover, classical Cepheids are indicators of young stellar populations — which can provide information about the star formation history of the region.

New Population

Dékány and collaborators found a total of 35 objects that they believe to be young classical Cepheids. These stars, which were found to have a spread in ages below 100 Myr, span the center of the galaxy, bridging the innermost nuclear region and the larger bulge extent. Despite their distribution across the bulge, however, they make up a very thin disk, all lying close to the mid-plane of the galaxy.

This newly discovered component of the inner galaxy demonstrates that stars have been continuously forming in or near the central region of the galaxy over the last hundred million years. Future investigations of these stars will help determine if the star formation actually occurred in the central bulge, or if the stars originated further out and migrated inwards.

Citation

I. Dékány et al 2015 ApJ 812 L29. doi:10.1088/2041-8205/812/2/L29