New observations from the Hubble space telescope show a thin disc of material around the black hole at the centre of the spiral galaxy NGC 3147--the only problem? It shouldn't be there.
A Hubble Space Telescope image of the spiral galaxy NGC 3147 appears next to an artist's illustration of the supermassive black hole residing at the galaxy's core. (Hubble Image: NASA, ESA, S. Bianchi (Università degli Studi Roma Tre University), A. Laor (Technion-Israel Institute of Technology), and M. Chiaberge (ESA, STScI, and JHU); illustration: NASA, ESA, and A. Feild and L. Hustak (STScI)
The material around the supermassive black hole at the heart of NGC 3147 may defy current astronomical models, but it does present a unique opportunity to test Einstein's theories of special and general relativity.
Marco Chiaberge of the European Space Agency, and the Space Telescope Science Institute and Johns Hopkins University, both in Baltimore, Maryland, a member of the team that conducted the Hubble study, says: “We’ve never seen the effects of both general and special relativity in visible light with this much clarity.
The study's first author, Stefano Bianchi, elaborates: “This is an intriguing peek at a disk very close to a black hole, so close that the velocities and the intensity of the gravitational pull are affecting how the photons of light look.
“We cannot understand the data unless we include the theories of relativity.”
Black holes in galaxies such as NGC 3147 lack the gravitationally captured material to feed them. This results in the thin haze of in-falling material 'puffing up' like a doughnut in what is known as a gas torus --rather than flattening out like a pancake-shaped disc (known as an accretion disc).
This means that the thin disc encircling this particular black hole is something of a conundrum. It's almost as if this starving black hole--with a mass 250 million times that of the sun--is mimicking its better-fed counterparts in extremely active galaxies.
As Ari Laor of the Technion-Israel Institute of Technology located in Haifa, Israel explains, the finding bucks the expectations of the team entering into their study: “We thought this was the best candidate to confirm that below certain luminosities, the accretion disk doesn’t exist anymore.
"What we saw was something completely unexpected. We found gas in motion producing features we can explain only as being produced by material rotating in a thin disk very close to the black hole.” Defying current models
Artist's impression of the peculiar thin disc of material circling a supermassive black hole at the heart of the spiral galaxy NGC 3147, located 130 million light-years away. (ESA/Hubble, M. Kornmesser)
Current models predict that an accretion disc forms when ample amounts of gas are trapped by a black hole’s strong gravitational pull. This infalling matter emits lots of light, producing a brilliant beacon called a quasar, in the case of the most well-fed black holes. Once less material is pulled into the disk, it begins to break down, becomes fainter, and changes structure.
Rather than validating existing models regarding lower-luminosity active galaxies and their ill-fed black holes--the black hole the team chose seems to have raised more questions than answers.
Bianchi of Università degli Studi Roma Tre, in Rome, Italy, points out: “The type of disk we see is a scaled-down quasar that we did not expect to exist.
“It’s the same type of disk we see in objects that are 1,000 or even 100,000 times more luminous. The predictions of current models for gas dynamics in very faint active galaxies clearly failed.”
As the gas and plasma disc is deeply embedded in the intense gravitational field of NGC 3147's supermassive black hole, the light that it emits is changed in accordance with Einstein's celebrated theories of relativity. Thus, giving astronomers a unique chance to view dynamic processes close to the event horizon of a black hole.
As the material in the accretion disc whirls around the black hole at around 10% of the speed of light, a phenomenon called relativistic beaming occurs. This means that the light it emits is intensified as it whips towards us and dimmed as it hurtles away.
The Hubble Space Telescope (HST) floats gracefully above the blue Earth after release from Discovery's robot arm after a successful servicing mission. (NASA/ESA)
Using Hubble’s Space Telescope Imaging Spectrograph (STIS) the researchers produced a spectrograph to analyse the material in the accretion disc. The impressive resolution of STIS allowed the team to filter out contamination from starlight, isolating faint light from the black hole region. Spectrometry divides the light stars or other astronomical objects into characteristic wavelengths--this allows astronomers to determine characteristics such as chemical composition, temperature and because of the doppler effect--velocity.
Chaiberge adds: “Without Hubble, we wouldn’t have been able to see this because the black-hole region has a low luminosity.
“The luminosities of the stars in the galaxy outshine anything in the nucleus. So if you observe it from the ground, you’re dominated by the brightness of the stars, which drowns the feeble emission from the nucleus.”
The next steps for the team involve using Hubble to search for other accretion discs around low powered black holes in similar, less active, galaxies. As such they will discover if NGC 3147 is an odd outlier or if models of black hole feeding must be revised.
