Astronomers have used data from European Southern Observatory telescopes to create a three dimensional map of the central bulge of the Milky Way. The gigantic cloud at the center of our galaxy contains a staggering 10,000 million stars (or thereabouts) and resides around 27,000 light-years away. Despite the relative proximity of the area, prior to these new studies little had been confirmed concerning its origin and structure.
The main problem faced by astronomers when observing the core of our home galaxy is the obscuring cloud of dust and gas that sits between it and the Earth. Clouds such as this are a common obstacle for astronomers, and are particularly common in star formation regions where the scattered materials eventually come together to form new stars.
Solving the ProblemOnly a small amount of visible light makes it through the clouds for observation, meaning that astronomers must instead look at longer wavelengths of light, such as infrared, in order to obtain accurate and reliable data.
Rather than relying on more limited visible light data, the first new study looks specifically at near-infrared light from the region's red clump giant stars, using them as a standard candle (a term that refers to any type of object with a known absolute luminosity). Knowing the absolute luminosity of the star allows astronomers to calculate its distance based on its apparent luminosity, or how bright it appears from Earth.
Red clump stars are a good candidate for use as standard candles due their abundance throughout the Milky Way and beyond. A sufficient number of the star type exist close enough to the Earth for their luminosity to be calculated, while a large quantity also reside within the galactic bulge at the center of the galaxy.
The first team, from the MAX Planck Institute for Extraterrestrial Physics (MPE) in Germany, utilized data from the VISTA telescope's VISTA Variables in the Via Lactea Survey (VVV), identifying 22 million red giant stars.
Though the star type has been used to measure the galactic bulge in the past, the richness of the data from the VVV survey allows a much clearer picture to form, with astronomers able to identify stars up to thirty times fainter than the older readings.
The NGC4710 galaxy as captured by the Hubble Space Telescope. The galaxy's core exhibits the same x-shape as that of the Milky Way (Image: NASA/ESA)
"From this star distribution we can then make a three-dimensional map of the galactic bulge" says Ortwin Gerhard, lead author on the paper. "This is the first time that such a map has been made without assuming a model or the bulge's shape."
Though a detailed three dimensional picture of the galactic bulge was produced, the model itself relies partly upon the assumed symmetry of the region. Due to the incomplete nature of the VVV survey, the team was unable to survey the bulge in its entirety. Instead they used an 8-fold mirror system to create the full 3D image.
The study found the core to exhibit an x-shape from the side and a "highly elongated bar" from above. The galaxy is thought to have developed into this shape, having started out as a disk of stars. Scientists believe that billions of years ago the disk formed into a flat bar shape, which finally collapsed in the center to form the x-shaped or peanut-like shape found in the new study.
A second team, headed up by Chilean PhD student Sergio Vásquez undertook a similar study, but instead of measuring luminosity, it focused on the movement of stars in the region.
The study looked at two sets of images from the MPG/ESO 2.2-meter telescope, taken 11 years apart. The team measured the small shifts in movement of select stars in the bulge, combining that data with the movement of the same stars towards or away from the Earth, creating a 3D map of the structure of the bulge. The results corroborated the findings of the first study. "It all fits very well with the predictions from the state-of-the-art models," Vásquez says.
When we combine these two new studies, what we get is a much clearer picture of the structure of the Milky Way's galactic core. Gerhard commented on the significance of the new data looking forward, stating that “Ultimately we hope that the results presented will help toward a detailed understanding of the formation and evolution of the Milky Way Bulge, for which we can resolve constituent stars, and which we can therefore study in greater detail than bulges in eternal galaxies.”
For more on the Milky Way check out the VISTA telescope's staggering 9-gigapixel image released back in October 2012. The image provides a unique look at the central part of our home galaxy and contains more the 84 million stars.