[The Galactic Center from NASA’s Chandra X-ray Observatory.Credit Credit: NASA/CXC/SAO, JPL-Caltech, MSFC, STScI, ESA/CSA, SDSS, ESO]
The center of the Milky Way, about 26,000 light-years from Earth, is a region quite complex that has been observed by some of the world’s most powerful telescopes in different forms of light, ranging from the X-rays to the infrared and radio. These images provide great detail and a lot of information about what is going on in the heart of our Galaxy.
One of these telescopes is the NASA Chandra X-ray Observatory, which in 2009 released an image of a region 900 by 400 light years in size of the center of our Galaxy[1]. The image, which is a mosaic of 88 Chandra pointings, is not the kind of X-ray image we are used to seeing at our visits with the doctor. This colorful image glowing in orange, green, blue, and purple is a color representation of the low-energy X-rays (red), intermediate-energy X-rays (green), and high-energy X-rays (blue) that allow us to explore many cosmic events. The energy of the X-ray light is about 100 to 40,000 times larger than the visible light, and in the center of the Milky Way, it is produced by a supermassive black hole, superheated clouds of gas, massive stars, neutron stars, and many more.
[Galactic Center from NASA’s Chandra X-ray Observatory with labels for each object discussed here. Credit: NASA/CXC/UMass/D. Wang et al.]
In this image, Sagittarius A* (Sgr A*) is inside the bright semi-circle near the center, which together with Sgr A East and Sgr A West, contributes to the production of the intense X-rays in this region. Probably the most spectacular of the three objects is Sgr A*, which is the 4 million solar masses supermassive black hole that marks the center of the Milky Way Galaxy. Sgr A* and that was observed in 2022 with a global network of synchronized radio observatories named the Event Horizon Telescope (EHT) - this was the second black hole observed by the EHT[2]. Sgr A East, on the other hand, is the remnant of a supernova (SN) that exploded about 10,000 years ago. In another study of this SN remnant, made also with Chandra, it was concluded that the explosion most likely was produced by a peculiar thermonuclear SN Type Iax, which as its name says, is not very common[3].
[X-ray data from Chandra (blue) and radio emission from the Very Large Array (red) Credit: X-ray: NASA/CXC/Nanjing Univ./P. Zhou et al. Radio: NSF/NRAO/VLA)]
Sgr A West is a spiral-shaped structure of gas that may be falling toward Sgr A*. The spiral shape can be more clearly seen in an image taken in radio with the Very Large Array (VLA) in New Mexico
[Sgr A West, which shows that the point source in the Galactic Center is surrounded by what appear to be spiral arms of gas that orbit this object. Credit: Roberts et al. NRAO / AUI ./ NSF]
Permeating the region is a diffuse haze of X-ray light from gas that has been heated to millions of degrees by winds from massive young stars, explosions of dying stars, and outflows powered by Sgr A*. Also, the area around Sgr A* contains several mysterious X-ray filaments, some of these likely represent huge magnetic structures that are interacting with streams of very energetic electrons that are produced by rapidly spinning neutron stars, something similar to the solar flares we observe on the Sun but a much bigger scale.
The two bright sources to the right and left of Sgr A, labeled as 1E sources, might be produced by binary objects, each containing a black hole or a neutron star. Also, there are many massive stars in the Arch and in stellar clusters labeled as DB, these soon will produce more supernovas, neutron stars, and black holes. Actually, the X-ray Thread, appearing as an elongated feature at the top right, might be a supernova too. Besides Sgr A East, there is another SN in this image called SNR 0.9-0.1. With other observations, astronomers found that this supernova also emits very high energy (>100 GeV) gamma-rays that are most likely produced by a pulsar wind nebula[4] - these types of pulsars are found in many SN remnants. There are also hundreds of unnamed point-like sources that scientists think are solo neutron stars or white dwarfs, which all light up the region. Finally, Sir A, B1, B2, and C, are regions of colder gas where stars are forming. These regions are normally too cool to be detected in X-rays, but the edges of these clouds have been heated, allowing Chandra to see their X-ray glow.
This colder gas has been studied by other infrared and radio telescopes. GRAVITY provides an example of an observation of Sgr A taken in 2016[5]
[Interferometric GRAVITY observations of the Galactic Center. The star IRS 16C was used as a reference star, the actual target was the star S2. The orange cross is the position of the central black hole. Credit: ESO/MPE/S. Gillessen et al.]
or the image of Sgr B2, the giant molecular cloud as seen by the NSF's Very Large Array (VLA) radio telescope in New Mexico [6]
[The giant molecular cloud, known as Sagittarius B2 (North). This VLA image shows hydrogen gas in a region nearly 3 light-years across. Red indicates stronger radio emission; blue weaker. Credit: NRAO/AUI Archives]
Another spectacular view of the Galactic center was obtained by the MeerKAT Radio Telescope in South Africa, which penetrates the intervening dust that obscures the view of this region at other wavelengths. These observations add all sorts of information about the activity going on in the Galactic center.
[MeerKAT image of the Galactic Center shown with the Galactic plane running horizontally across the image. Credit: I. Heywood, SARAO]
The image is a mosaic from 20 separate observations using 200 hours of telescope time and covering an area of 6 square degrees (12 times the size of the Moon), which corresponds to about 1000 by 5000 light years in size[7]. The image reveals a lot of activity produced by the objects observed with Chandra but also shows us clearly the compact star-forming regions together with a large population of mysterious radio filaments. The energy of radio waves is about 0.0000001 times that of visible light and is at these energies where molecules, which survive only in very cold environments, like those in star-forming regions, emit photons (or packets of light) that we can observe. Here, the Sri B1 and Sri B2 are bright because these are dense molecular clouds, making it clear why we said before these were regions of star formation.
The high sensitivity of these images allowed a group of astronomers to use color to indicate the spectral slope of the emitted radio waves[8]. These colors, unlike the Chandra images, relate to what radio astronomers call spectral index which indicates what type of emission produced these radio waves; e.g. electrically charged particles, known as electrons, moving through magnetic fields, radio waves with a wavelength of 21 centimeters that atoms of hydrogen spontaneously emit, or emission from molecules like CO.
[MilkyWay-MeerKAT-spectra.jpg .Credit: SARAO / I. Heywood / J. C. Muñoz-Mateos]
The broad feature running vertically through the center of the image is a series of parallel radio filaments that follow the inner part of the Radio Bubbles. These bubbles are the pieces of an hourglass-shaped structure that extends 1,400 light-years and is symmetrical with respect to Srg A*. Astronomers speculate the bubbles were produced by a very powerful event occurring a few million years ago, very near the Milky Way's central black hole. Looking closer at this region reveals with much more detail the chaotic complexity of the heart of our Galaxy.
[The complex, cirrus-like emission from the Galactic Center’s super bubble. The Radio Arc is transversed by a complex of many parallel radio filaments. The bright dot near the center of this region is Sgr A*. Credit: I. Heywood, SARAO.]
Further to the right, appearing as a large bubble, there is the SN remnant G359.1-0.5 that seems connected with the Galactic plane by the longest radio filament known, which is called ‘the Snake.’ This SN remnant is not in the Chandra images, but in a closeup of this region taken by MeerKAT, we can also see ‘the Mouse,’ which is a runaway pulsar possibly formed and ejected by the SN event.
[Centre is the image of the SN remnant G359.1-0.5. To the left is ‘the Mouse’, To the upper right is ‘the Snake’. Credit: I. Heywood, SARAO.]
Clearly, there is so much to learn from these images. Astronomers will continue using these, and new observations to shed more light into the mysteries of the Galactic Center.
References:
[1] https://chandra.harvard.edu/photo/2023/archives/more.html
[2] https://eventhorizontelescope.org/blog/astronomers-reveal-first-image-black-hole-heart-our-galaxy
[3]https://chandra.harvard.edu/photo/2021/sgrae/
[4]F. Aharonian et al 2005, A&A 432, L25
[5]GRAVITY Image https://www.eso.org/public/images/eso1622b/
[6] https://www.nrao.edu/archives/exhibits/show/legacy-astronomical-images/item/33432
[7] https://www.sarao.ac.za/media-releases/new-meerkat-radio-image-reveals-complex-heart-of-the-milky-way/
[8] I. Heywood et al 2022 ApJ 925 165