Probing the heart of the active galaxy NGC 6251, NASA's Hubble SpaceTelescope has provided a never-before-seen view of a warped diskor ring of dust caught in a blazing torrent of ultraviolet lightfrom a suspected massive black hole.
This discovery, which is reported in the September 10 issue ofthe Astrophysical Journal Letters, suggests that the environmentsaround black holes may be more varied than thought previously,and may provide a new link in the evolution of black holes in thecenters of galaxies.
"This is a completely new phenomenon which has never before beenseen. It blew my mind away," says Dr. Philippe Crane of the EuropeanSouthern Observatory, in Garching, Germany. "Before Hubble youcould never do this kind of research. We used a lightly exploitedfacility of Hubble: its extremely high resolution imagingcapability in the near ultraviolet provided by the Faint ObjectCamera (FOC), built by the European Space Agency."
Previously, black holes observed by Hubble have been largelyhidden from view because they are embedded inside a torus, adonut-shaped distribution of dust that forms a partial cocoonaround the black hole.
In galaxies previously studied, the intense light from super hotgas entrapped by the black hole's powerful gravitational fieldshines out from inside the "donut hole" of the torus and isrestricted to a narrow beam, like a searchlight.
But this is the first clear example of an "exposed" black holethat illuminates the surrounding disk. Because Hubble seesultraviolet light reflected on one side of the disk, astronomersconclude the disk must be warped like the brim of a hat.
Such a warp could be due to gravitational perturbations in thegalaxy's nucleus that keep the disk from being perfectly flat, orfrom precession of the rotation axis of the black hole relativeto the rotation axis of the galaxy.
The suspected black hole's mass has not yet been confirmed throughvelocity measurements of entrapped material, though yetunpublished Hubble measurements have been made with the FaintObject Spectrograph (FOS), prior to its replacement during the 1997Hubble servicing mission.
However, strong circumstantial evidence for the black hole isprovided by the powerful 3 million light-year-long jet ofradiation and particles emanating from the black hole's locationat the hub of the elliptical galaxy. The galaxy is located 300million light-years away in the constellation Virgo.
Hubble's sensitivity to ultraviolet light, combined with theexceptional resolution of the FOC which can see details as smallas 50 light-years across, allowed Crane and his team to look forstructure in the hot gas near the black hole at the base of thejet. Crane was surprised to see a peculiar finger-like objectextending from the nucleus, at right angles to the main jet.
Comparing the FOC image to a visible light image taken withHubble's Wide Field Planetary Camera 2 (WFPC2), Crane realized thefinger-like extension ran parallel to a 1,000 light-year-widedust disk encircling the nucleus. He concluded that the ultraviolet light must be reflecting off fine dust particles in a disk,or possibly the back wall of a ring. A ring-like structure wouldhave been shaped by a torrent of radiation coming from theexposed black hole, which would have plowed out a cavity aroundthe hole.
The Hubble astronomers are hoping to confirm ideas aboutscattering by looking at the disk's spectrum with ground-basedtelescopes. They will propose to use Hubble to look at severalother extragalactic jet sources which have dust.
Co-investigator: Joel Vernet (European Southern Observatory) * * * *
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).
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