Astronomers have observed the outer edge of the disk of matter surrounding a feeding supermassive black hole for the first time.
These observations could help scientists better measure the structures surrounding these cosmic monsters, understand how black holes feed on those structures, and piece together how this feeding affects the evolution of galaxies that harbor such phenomena.
Feeding black holes are located at the hearts of regions of incredible brightness called active galactic nuclei (AGN). Directly around these black holes, which can be millions or even billions of times more massive than the Sun, is a rotating disk of gas and dust that is gradually fed into the central supermassive body.
The incredible gravitational influence of such supermassive black holes causes the matter in the accreting disks to reach temperatures of up to 18 million degrees Fahrenheit (10 million degrees Celsius). This causes the structure to emit radiation across the entire electromagnetic spectrum, from high-energy gamma rays and X-rays to visible light, infrared light and radio waves. These emissions from active galactic nuclei, also called quasars, can be so bright that they outshine the combined light from every star in the surrounding galaxies.
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However, even with this powerful output, because accretion disks are relatively small and many of them are located in incredibly distant galaxies, they are difficult to image directly. But alternatively, astronomers can use the full spectrum of light from the accretion disk to understand its physics and even determine its size.
This is the technique adopted by a team led by researchers from the National Institute for Space Research in Brazil. Denimara Dias dos Santos and Alberto Rodríguez Ardila studied the accretion disk of a distant quasar, III Zw 002, located at the heart of the galaxy Messier 106 (M 106). M 106 lives about 24 million light-years from Earth in the constellation Canes Venatic.
For the first time, the team saw near-infrared emission lines in the spectrum of light coming from the accretion disk of this quasar. These lines helped researchers determine the size of this plate-like structure from which the supermassive black hole, whose mass has been determined to be between 400 and 500 times the mass of the Sun, feeds.
“This discovery gives us valuable insights into the structure and behavior of the broadband region in this particular galaxy, highlighting the fascinating phenomena that occur around supermassive black holes in active galaxies,” Rodriguez-Ardila said. He said in a statement.
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Emission lines like the one the team studied occur when an atom absorbs energy and adopts what physicists call an “excited state.” Eventually, these atoms must return to their lowest energy state, or “ground state.” This drop to the ground state releases light that, since each element has a unique set of energy levels, has a wavelength and energy characteristic of the atom of a particular element.
This means that these emissions in the light spectra can help identify elements in the star, the planet’s atmosphere, and, in this case, in the accretion disk around the black hole.
Emission lines from stars and other sources take the form of thin ridges in the spectra, but the violent conditions around the supermassive black hole cause the accretion disk emission lines to adopt a different appearance.
As matter near the supermassive black hole accelerates to speeds approaching the speed of light, the associated emission lines broaden and take on shallower peaks. The region where these emissions come from is referred to as the accretion disk broadline region.
When one side of the accretion disc moves toward Earth, the other side moves away. This results in short wavelengths of light on the side rotating toward us and longer wavelengths of light on the side of the accretion disk moving away.
This is similar to what happens here on Earth when an ambulance heads toward you on a city street. The sound waves from the sirens combine, creating a short-wavelength sound and a high-frequency sound. As the ambulance moves away, the sound waves expand, and the frequency of the siren decreases.
This phenomenon is called the Doppler shift, and for light emerging from the accretion disk, it causes two peaks to appear – one on the side that is moving away from Earth and the other on the side that is moving quickly toward Earth.
When these broad, double-peaked emissions are seen coming from the inner region of the accretion disk, they give astronomers no hints about the size of the accretion disks. However, if these lines could be seen from the outer edge, they would be.
This team of astronomers has made the unambiguous discovery of two near-infrared, double-peaked profiles in the broadline region of III Zw 002, a line originating from hydrogen from an inner region of the broadline region disk and an oxygen-generating line at the outer boundary of this region.
The emission lines were found within data collected by the Gemini Near-Infrared Spectrograph (GNIRS), which is capable of observing the entire near-infrared spectrum simultaneously. This allowed the team to capture a single, clean, continuously calibrated spectrum of the quasar.
“We didn’t previously know that III Zw 002 had this double-peak appearance, but when we reduced the data, we saw the double peak very clearly,” Rodriguez-Ardila said. “In fact, we reduced the data several times thinking it might be wrong, but each time we saw the same dramatic result.”
This helped constrain the size of the accretion disk, as the team was able to see the hydrogen line coming from a distance of 16.77 light-days from the central supermassive black hole, while the oxygen line originates from a radius of 18.86 light-days.
Astronomers were also able to determine the size of the broadline region, estimating its outer radius to be 52.43 light-days. In addition, the team was able to calculate that the broad line region of the accretion disk is tilted at an angle of 18 degrees relative to Earth.
The team will continue to monitor quasar III Zw 002, watching its image change over time, as well as looking at using near-infrared light to study other active galactic nuclei.
The research was published in August in Astrophysical Journal Letters.
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