Astronomers using ESO’s VISTA infrared survey telescope have discovered a dozen ancient stars, of a type known as RR Lyrae, in the vicinity of the center of our Milky Way Galaxy. This discovery provides the first direct observational evidence that the nuclear stellar bulge of the Galaxy contains ancient stars.
This image from ESO’s VISTA telescope shows the central part of the Milky Way. While normally hidden behind obscuring dust, the infrared capabilities of the telescope allow astronomers to study stars close to the Galactic center. Within this field of view astronomers detected several RR Lyrae stars. Image credit: ESO / VVV Survey / D. Minniti et al.
RR Lyrae stars typically reside in dense globular clusters over 10 billion years old. They are variable stars, and the brightness of each RR Lyrae star fluctuates regularly.
Dr. Dante Minniti, an astronomer at the Universidad Andrés Bello in Chile, and co-authors used ESO’s VISTA telescope, as part of the Variables in the Via Lactea (VVV) public survey, to search for RR Lyrae variables hiding in the central part of the Milky Way.
They found a dozen previously unknown RR Lyrae variable stars. Their discovery suggests that the bulging center of the Galaxy likely grew through the merging of primordial star clusters.
These stars may even be the remains of the most massive and oldest surviving star cluster in the entire Milky Way.
“This discovery of RR Lyrae Stars in the center of the Milky Way has important implications for the formation of galactic nuclei,” said co-author Dr. Rodrigo Contreras Ramos, of the Instituto Milenio de Astrofísica in Chile.
“The evidence supports the scenario in which the nuclear bulge was originally made out of a few globular clusters that merged.”
The theory that galactic nuclear bulges form through the merging of globular clusters is contested by the competing hypothesis that these bulges are actually due to the rapid accretion of gas.
The unearthing of these RR Lyrae stars is very strong evidence that part of the Milky Way’s nuclear bulge did in fact form through merging.
By extension, all other similar galactic bulges may have formed the same way.
“Not only are these stars powerful evidence for an important theory of galactic evolution, they are also likely to be over 10 billion years old — the dim, but dogged survivors of perhaps the oldest and most massive star cluster within the Milky Way,” the astronomers said.
A paper reporting this discovery is published in the Astrophysical Journal Letters.
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Dante Minniti et al. 2016. Discovery of RR Lyrae in the Nuclear Bulge of the Milky Way. ApJ 830, L14; doi: 10.3847/2041-8205/830/1/L14
