Researchers at the University of California, Santa Cruz (UCSC), believe clouds of dust, rather than twin black holes, can explain the features found in active galactic nuclei (AGNs). The team publish their results today (14 June) in a paper in Monthly Notices of the Royal Astronomical Society.
Many large galaxies have an AGN, a small bright central region powered by matter spiralling into a supermassive black hole.
When these black holes are vigorously swallowing matter, they are
surrounded by hot, rapidly-moving gas known as the "broad-line region"
(so-called because the spectral lines from this region are broadened by the rapid motion of the gas).
The emission from this gas is one of the best sources of information
about the mass of the central black hole and how it is growing. The
nature of this gas is however poorly understood; in particular there is
less emission than expected from gas moving at certain velocities. The
breakdown of simple models has led some astrophysicists to think that
many AGNs might have not one but two black holes in them.
The new analysis is led by Martin Gaskell,
a research associate in astronomy and astrophysics at UCSC. Rather than
invoking two black holes, it explains much of the apparent complexity
and variability of the emissions from the broad-line region as the
results of small clouds of dust that can partially obscure the innermost
regions of AGNs.
Gaskell comments: "We've shown that a lot of mysterious properties of
active galactic nuclei can be explained by these small dusty clouds
causing changes in what we see."
Co-author Peter Harrington,
a UCSC graduate student who began work on the project as an
undergraduate, explained that gas spiralling towards a galaxy's central
black hole forms a flat "accretion disk",
and the superheated gas in the accretion disk emits intense thermal
radiation. Some of that light is "reprocessed" (absorbed and re-emitted)
by hydrogen and other gases swirling above the accretion disk in the
broad-line region. Above and beyond this is a region of dust.
"Once the dust crosses a certain threshold it is subjected to the
strong radiation from the accretion disk", said Harrington. The authors
believe this radiation is so intense that it blows the dust away from
the disk, resulting in a clumpy outflow of dust clouds starting at the
outer edge of the broad-line region.
The effect of the dust clouds on the light emitted is to make the
light coming from behind them look fainter and redder, just as the
earth's atmosphere makes the sun look fainter and redder at sunset.
Gaskell and Harrington developed a computer code to model the effects of
these dust clouds on observations of the broad-line region.
The two scientists also show that by including dust clouds in their
model, it can replicate many features of emission from the broad-line
region that have long puzzled astrophysicists. Rather than the gas
having a changing, asymmetrical distribution that is hard to explain,
the gas is simply in a uniform, symmetric, turbulent disk around the
black hole. The apparent asymmetries and changes are due to dust clouds
passing in front of the broad-line region and making the regions behind
them look fainter and redder.
"We think it is a much more natural explanation of the asymmetries
and changes than other more exotic theories, such as binary black holes,
that have been invoked to explain them," Gaskell said. "Our explanation
lets us retain the simplicity of the standard AGN model of matter
spiralling onto a single black hole."
Media Contacts
Tim Stephens
University of California, Santa Cruz (UCSC)
United States
Tel: +1 (831) 459 4352
stephens@ucsc.edu
Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7292 3979
Mob: +44 (0)7802 877 699
rmassey@ras.ac.uk
Dr Morgan Hollis
Royal Astronomical Society
Tel: +44 (0)20 7292 3977
Mob: +44 (0)7802 877 700
mhollis@ras.ac.uk
Science Contact
Dr Martin Gaskell
University of California, Santa Cruz
mgaskell@ucsc.edu
Further Information
The new work appears in "Partial
dust obscuration in active galactic nuclei as a cause of broad-line
profile and lag variability, and apparent accretion disc inhomogeneities",
C. Martin Gaskell and Peter Z. Harrington, Monthly Notices of the Royal
Astronomical Society, Oxford University Press, in press.
Notes for Editors
More news from the University of California Santa Cruz.
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