A new visualisation created using the Chandra X-ray Observatory and other telescopes allows scientists to study the centre of the galaxy
The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space observatory launched by NASA in 1999. The telescope’s high angular resolution of its mirrors mean that it is sensitive to X-ray sources 100 times fainter than any previous X-ray telescope.
The Earth is located about 26,000 light years, or about 150,000 trillion miles, from the centre of our galaxy, the Milky Way. And now, a new project has developed a new visualisation— complete with a 360-degree view — to the centre of the galaxy. This project, made using data from the Chandra X-ray Observatory and other telescopes, allows viewers to control their own exploration of the fascinating environment of volatile massive stars and powerful gravity around the supermassive black hole that lies in the centre of the galaxy, Milky Way (which scientists have been able to study using data from powerful telescopes that can detect light in a variety of forms, including X-ray and infrared light).
What role do Wolf-Rayet stars play?
The new visualisation builds on infrared data with the European Southern Observatory’s (ESO) Very Large Telescope (VLT) of 30 massive stellar giants called Wolf-Rayet stars that orbit within about 1.5 light years of the centre of the Galaxy, where powerful winds of gas streaming from the surface of these stars are carrying some of their outer layers into interstellar space.
The collisions between this outflowing gas and previously ejected gas from other stars causes shock waves, similar to sonic booms, which permeate the area. These shock waves heat the gas to millions of degrees, causing it to glow in X-rays. As such, the Chandra X-ray Observatory has been able to make extensive observations of the central regions of the Milky Way, providing critical data about the temperature and distribution of this multimillion-degree gas.
The role that Wolf-Rayet stars play in the cosmic neighbourhood at the Milky Way’s centre is as yet unknown, and is therefore an intense area of interest for astronomers, who want to gain a better understanding of how the stars interact with the supermassive black hole known as Sagittarius A*, which, although invisible, resides in the centre of the galaxy and has a mass equivalent to some four million Suns.
Offering a 360-degree movie that immerses the viewer into a simulation of the centre of our Galaxy, the Galactic Centre visualisation provides the viewer with a unique standpoint: the location of Sagittarius A*, where they are able to see some 25 Wolf-Rayet stars (visible as white, twinkling objects) which orbit the black hole as they continuously eject stellar winds (black to red to yellow colour scale) which collide with each other, with some of this material (yellow blobs) spiralling towards Sagittarius A*.
According to the team behind the new visualisation, this 360-degree video of the Galactic Centre is ideally viewed in virtual reality (VR) goggles, while it can also be viewed on smartphones using the YouTube app. Moving the phone around pans to show a different portion of the movie, mimicking the effect in the VR goggles. Finally, most browsers on a computer also allow 360-degree videos to be shown on YouTube (clicking and dragging the video enables the user to look around).
Two simulations are then included in the movie, each of which start around 350 years in the past and span 500 years. The first shows Sagittarius A in a calm state, while the second contains a more violent black hole which is expelling its own material, thereby turning off the accretion of clumped material (yellow blobs) that is so prominent in the first portion.
The effects of Sagittarius A*
The visualisation has been used by scientists to examine the effects Sagittarius A* on its surroundings, finding that the material the black hole pulls towards it clumps together before stretching due to the tidal forces it is being exposed to – and these forces grow as the material gets closer to the black hole.
It has also been found that Sagittarius A* occasionally expels material away from itself, which have the effect of clearing away some of the gas produced by the Wolf-Rayet winds.
A team of researchers, led by Christopher Russell of the Pontifical Catholic University of Chile, used the visualisation to understand the presence of previously detected X-rays in the shape of a disk that extend about 0.6 light years outward from Sagittarius A*. According to the team, the amount of X-rays generated by these colliding winds depends on the strength of outbursts powered by the black hole, as well as on the amount of time that has elapsed since an eruption occurred. Stronger and more recent outbursts result in weaker X-ray emission, they have argued.
The information provided by the theoretical modelling and a comparison with the strength of X-ray emission observed with Chandra has led Russell and his colleagues to determine that it is highly likely that Sagittarius A* had a relatively powerful outburst that started within the last few centuries. Moreover, their findings suggest the outburst from the supermassive black hole is still affecting the region around Sagittarius A*, even though it ended about one hundred years ago.
Russell presented the new visualisation and the related scientific findings at the 231st meeting of the American Astronomical Society in Washington, DC, USA, and some of the results are based on a paper by Russell et al. published in 2017 in the Monthly Notices of the Royal Astronomical Society.
Chandra X-ray Observatory
This article will appear in SciTech Europa Quarterly issue 26, which will be published in March, 2018