New images from ESA’s XMM-Newton X-ray Observatory reveal some of the most intense processes taking place at the hearth of our Milky Way Galaxy.
X-ray view of the central region of our Milky Way Galaxy. This image was put together in a new study by compiling all observations of this region that were performed with ESA’s XMM-Newton, adding up to over one month of monitoring in total. The image combines data collected at energies from 0.5 to 2 keV (shown in red), 2 to 4.5 keV (shown in green) and 4.5 to 12 keV (shown in blue). It spans about 2.5 degrees across, equivalent to about 1,000 light-years. Image credit: ESA / XMM-Newton / G. Ponti et al.
The bright, point-like sources that stand out across the image above trace binary stellar systems in which one of the stars has reached the end of its life, evolving into a compact and dense object – a neutron star or black hole.
The Milky Way’s central region also contains young stars and star clusters, and some of these are visible as white or red sources sprinkled throughout the image, which spans 1,000 light-years.
Most of the action is occurring at the center, where diffuse clouds of gas are being carved by powerful winds blown by young stars, as well as by supernovae.
Sagittarius A*, a supermassive black hole in the center of the Galaxy, is also responsible for some of this action. The black hole has a mass of 4.3 million suns, and it is located within the bright, fuzzy source to the right of the image center.
While black holes themselves do not emit light, their immense gravitational pull draws in the surrounding matter that, in the process, emits light at many wavelengths, most notably X-rays.
In addition, two lobes of hot gas can be seen extending above and below Sagittarius A*.
Close-up of the region surrounding Sagittarius A*. The position of Sagittarius A* is indicated by the blue cross. The red ellipses show the position and extent of filamentary and diffuse X-ray emission features. The magenta dashed ellipses show the location and extension of the bipolar X-ray lobes. The black dashed ellipses indicate the position and orientation of the circumnuclear disc. Image credit: ESA / XMM-Newton / G. Ponti et al.
Astronomers believe that these lobes are caused either directly by the black hole, which swallows part of the material that flows onto it but spews out most of it, or by the cumulative effect of the numerous stellar winds and supernova explosions that occur in such a dense environment.
The large, elliptical structure to the lower right of Sagittarius A* is a super-bubble of hot gas, likely puffed up by the remnants of several supernovae at its center.
While this structure was already known to astronomers, a new study, published in the Monthly Notices of the Royal Astronomical Society, confirms for the first time that it consists of a single, gigantic bubble rather than the superposition of several, individual supernova remnants along the line of sight.
Another huge pocket of hot gas, dubbed the ‘Arc Bubble’ due to its crescent-like shape, can be seen close to the image center, to the lower left of Sagittarius A*.
It is being inflated by the forceful winds of stars in a nearby stellar cluster, as well as by supernovae; the remnant of one of these explosions, a candidate pulsar wind nebula, was detected at the core of the bubble.
The rich data set compiled in this study contains observations that span the full range of X-ray energies covered by XMM-Newton; these include some energies corresponding to the light emitted by heavy elements such as silicon, sulfur and argon, which are produced primarily in supernova explosions.
By combining the additional information present in the new data, astronomers obtained another, complementary view of the hearth of our Milky Way Galaxy, which reveals beautifully the lobes and bubbles described earlier on.
Milky Way’s center through the emission of heavy elements: the image combines data collected at energies that correspond to the light emitted by heavy elements such as silicon and argon, which are produced primarily in supernova explosions, as well as other narrow energy bands. It spans about 2.5 degrees across, equivalent to about 1,000 light-years. Image credit: ESA / XMM-Newton / G. Ponti et al.
In addition, this alternative view also displays the emission, albeit very faint, from warm plasma in the upper and lower parts of the image. This warm plasma might be the collective macroscopic effect of outflows generated by star formation throughout this entire central zone.
Another of the possible explanations for such emission links it to the turbulent past of Sagittarius A*.
Scientists believe that, earlier on in the history of our Galaxy, Sagittarius A* was accreting and ejecting mass at a much higher rate, like the black holes found at the centre of many galaxies, and these diffuse clouds of warm plasma could be a legacy of its ancient activity.
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G. Ponti et al. 2015. The XMM-Newton view of the central degrees of the Milky Way. MNRAS 453 (1): 172-213; doi: 10.1093/mnras/stv1331
