By Rodrigo Contreras Ramos, MAS postdoctoral researcher
Globular clusters are among the most beautiful and valuable objects for astronomers. They consist of hundreds of thousands, sometimes millions of stars that were originated by the same gas cloud and virtually at the same time. Their stars are grouped in relatively small volumes (tens of light years), with high densities in their central region, as if they were gigantic swarms of bees. Being objects almost as old as the Universe itself, astronomers consider them key witnesses to understand the formation and evolution of our galaxy, since they have been present from the beginning of its history. In the Milky Way, about 160 globular clusters are known to orbit around its center as if they were electrons spinning around an atomic nucleus. Despite their importance, about a quarter of them have been poorly studied since by being in the direction of the galactic center, a big concentration of dust and gas interferes, making difficult to observe them clearly. Plus, there is another difficulty: the number of field stars towards the central region of our galaxy is so high that they are easily mistaken with those belonging to the globular cluster that we are interested in studying.
The situation is similar to the following: Imagine being in a hot air balloon over a main square with the mission of spying on a specific group of tourists who follow their tour guide. The task is not as easy as it seems, because that day the levels of smog on your city are so critical that you cannot see beyond a few meters away, and in addition, the square is full of people like never before. How to accomplish the mission? The first obstacle could be overcome using a camera with infrared vision, such as firefighters do when they go to a fire. All objects that have temperature emit electromagnetic radiation. A relatively cold object like the human body emits mostly infrared radiation, and as our vision is not sensitive to this type of “light” we need special instruments to detect it. But most important for our case is that infrared radiation, unlike visible light (which we do not emit but reflect), is able to penetrate the dust without problems. Thus, thanks to your “new eyes” you can see through the smog and distinguish the tour guide of that group of people that you are interest in. However, the number of people is so high that it becomes virtually impossible to know which tourists are following him. An intelligent solution could be to photograph repeatedly in the direction of the tour guide and then compare the images. You will see that, while all the people in the square move in a random way, the members of the group move coherently, all together. In this way you could not only identify each tourist that is part of the group, but also you could calculate the speed at which they move and in which direction.
This is exactly the work we have recently published (http://adsabs.harvard.edu/abs/2017arXiv170907919C) for the case of NGC 6544, a globular cluster located approximately 17,000 light years from us in the direction of the galactic center. Unlike the human body, stars are objects so hot that they emit most of their energy in the visible range, but part of their energy is also radiated in infrared. Thus, while the visible light of the stars is blocked by the curtain of dust that separates us from them, the infrared component arrives without problem to our instruments. Thanks to the sequence of images obtained by the VVV survey and to the infrared vision of the Vista telescope in Cerro Paranal, we have been able to identify and separate from the background contamination many of the stars belonging to this cluster, and from them infer its orbit around the Milky Way. Thanks to our results, we will now be able to study NGC 6544 in greater detail, star by star, and “listen” to their side of the story about the birth and life of our galaxy.
a) In the visual range, the globular cluster is blocked by the dust and gas of the disk that separates us from it. b) Observing in infrared we can distinguish our target, but still we notice that there is a great contamination of stars belonging to the field. c) Thanks to the method used in our work, the so-called proper motions technique, we have been able to reveal bona-fide stars members of the globular cluster NGC 6544.
* Note: The images used here do not really correspond to the globular cluster NGC 6544. They have been chosen with the intention of giving an illustrative example of our work.