Astronomers have observed the outer edge of the disk of matter surrounding a feeding supermassive black hole for the first time.
These observations can help scientists better measure the structures that surround these cosmic monsters, understand how black holes feed on those structures, and piece together how this feeding influences the evolution of galaxies that harbor such phenomena.
Feeder black holes lie at the heart 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 spinning disk of gas and dust that is gradually fed into the central supermassive body.
The incredible gravitational effect of such supermassive black holes causes the matter in the accretion 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 and radio waves. These emissions from active galactic nuclei, also called quasars, can be so bright that they outshine the combined light from each star in the surrounding galaxies.
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However, even with this strong output, because accretion disks are relatively small and many are located in incredibly distant galaxies, they are difficult to image directly. But as an alternative, 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. Dinemara Dias dos Santos and Alberto RodrÃguez Ardela studied the accretion disk of a distant quasar, III Zw 002, located in the heart of the galaxy Messier 106 (M 106). M 106 lives about 24 million light-years from Earth in the constellation of Canes Venatic.
The team saw, for the first time, near-infrared emission lines in the spectrum of light coming from the accretion disk of this quasar. These lines helped the researchers determine the size of the plate-like structure feeding from the supermassive black hole, which has been determined to have a mass between 400 and 500 times the mass of the Sun.
“This discovery gives us valuable insights into the structure and behavior of the broadband region in this particular galaxy, shedding light on the fascinating phenomena that occur around supermassive black holes in active galaxies,” Rodriguez-Ardela 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 causes light to be released, since each element has a unique set of energy levels, a wavelength and energy characteristic of an 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, the accretion disk around the black hole.
Emission lines from stars and other sources appear as thin bumps in the spectra, but the violent conditions surrounding a supermassive black hole cause the accretion disk’s emission lines to adopt a different appearance.
As matter near the supermassive black hole accelerates to velocities close to the speed of light, the associated emission lines widen and take on shallower peaks. The region from which these emissions come is referred to as the accretion disc broadline region.
When one side of the accretion disk moves toward Earth, the other side moves away. This results in short wavelengths of light on the side that rotate 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 is coming toward you on a city street. The sound waves from the siren combine, producing a short-wavelength sound and a high-frequency sound. As the ambulance moves away, the sound waves expand, and the siren’s frequency decreases.
This phenomenon is called the Doppler shift, and for the light leaving the accretion disk, it causes two peaks to appear – one on the side moving away from Earth and the other on the side moving rapidly toward Earth.
When these broadened, double-peaked emissions are seen coming from the inner region of the accretion disk, they don’t give astronomers any 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 unequivocally detected two double-peaked near-infrared profiles in the broadline region of III Zw 002, a line originating from hydrogen from the inner region of the broadline 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 know before that III Zw 002 had this double peak appearance, but when we reduced the data, we saw the double peak very clearly,” said Rodriguez Ardilla. “In fact, we reduced the data several times thinking it might be wrong, but each time we saw the same exciting result.”
This helped constrain the size of the accretion disk, as the team was able to see that the hydrogen line originates 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.
The astronomers were also able to determine the size of the broadline region, and estimated its outer radius to be 52.43 light-days. In addition, the team was able to calculate that the accretion disk’s broad line region is tilted at an angle of 18 degrees with respect to the Earth.
The team will continue to observe quasar III Zw 002, observing its changing image over time, as well as looking into using near-infrared light to study other AGNs.
The research was published in August Astrophysical Journal Letters.
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