First observation of the orbit of the accretion disk of a supermassive black hole

Astronomers have discovered hints of an accretion disk in the active galactic nucleus of galaxy III Zw 002 for the first time using the Gemini North Telescope, one half of the Gemini International Observatory, operated by NSF’s NOIRLab. These discoveries precisely constrain the size of the galaxy’s accretion disk and shed new light on its structure and behavior using two peculiar near-infrared emission lines.

Many galaxies include a supermassive black hole at their centers that is actively feeding in a tumultuous environment. These inconceivably dense objects are orbited by spinning accretion disks of gas and dust, which feed the black hole and produce copious amounts of energy across the electromagnetic spectrum, from high-energy gamma and X-rays to visible light, infrared, and radio waves. .

Studying accretion disks allows astronomers to better understand black holes and the development of the galaxies that host them. However, due to their large distances and small diameters, most accretion disks are difficult to observe directly. Instead, they gauge the size and behavior of the disk by studying its light emission spectra.

This method allowed astronomers to detect for the first time two near-infrared emission lines in the accretion disk of galaxy III Zw 002, setting a new limit for the size of these magnificent structures. The Gemini North Telescope is one half of the Gemini International Observatory, operated by the NSF’s NOIRLab.

As already mentioned, only two sources of accretion disks have been directly observed due to the large angular resolution of the Event Horizon Telescope. So how do astronomers determine if a supermassive black hole has a disk around it without access to a worldwide network of radio telescopes? A pattern of broad emission lines, known as a double-peak profile, contains hints of an accretion disk.

The gas on one side of the rotating disk is moving away from the observer, while the gas on the other is moving in the opposite direction. These relative motions lengthen and shorten the wavelengths of the emission lines, stretching and compressing them. This results in a broad line with two different peaks, one coming from each side of the rapidly spinning disk.

These double peak profiles are a rare phenomenon, as they only appear on sources that can be viewed almost head-on. The double peak has been found in the H-alpha and H-beta lines, two emission lines from hydrogen atoms that appear in the visible range of wavelengths, in the few sources where it has been observed. These lines give little information about the size of the entire accretion disk and originate in the inner region of the region of broad lines around the supermassive black hole. However, recent near-infrared observations have revealed a hitherto unknown portion of the outer broad-line region.

In this study, the scientists made the first unequivocal detection of two near-infrared double-peak profiles in the broad line region of III Zw 002. The Paschen-alpha (hydrogen) line originates from the inner region of the broad lines, and the O I (neutral oxygen) line originates on the outskirts of the broad line region, a region that has never been observed before. These are the first near-infrared double-peak profiles found, and they arose unexpectedly during observations made with the Gemini Near-Infrared Spectrograph (GNIRS).

According to Rodríguez-Ardila, “We did not know that III Zw 002 had this double peak profile, but when we reduced the data, we saw the double peak very clearly. We reduced the data many times, thinking that it might be an error, but each time, we saw same exciting result.”

“These discoveries help astronomers better understand the broad line region, while confirming the predicted existence of an accretion disk.”

“For the first time, the detection of these double-peaked profiles places firm limits on the geometry of a region that would otherwise be impossible to resolve. And now we have clear evidence for the feeding process of an active galaxy and its structure. internal.”

The scientists compared their observations with existing disk models. This provides them with parameters that give a clearer picture of III Zw 002’s supermassive black hole and the region of broad lines.

Depending on the model, the O I line originates at a radius of 18.86 light-days, and the Paschen-alpha line at a radius of 16.77 light-days, or the distance light must travel from the supermassive black hole. on an Earth day. In addition, the outer radius of the broad line region is predicted to be 52.43 light-days. The model estimates that the supermassive black hole at the heart of III Zw 002 is between 400 and 900 million times more massive than our Sun, and has an angle of inclination of 18 degrees with respect to observers on Earth.

source: The Astrophysical Journal Letters (2023). DOI: 10.3847/2041-8213/ace974