ASTRONEWS
349 °^12 °(^34) °
(^57)
°
(^79)
28 °
2 °
30
5 °^
327
°^
(^26)
(^0) °
(^23)
(^7) °
(^215)
°
(^192) ° 170 °
147
°^
12
5 °
10
2 °
North
North-
northeast
Northeast
East
southeastEast-
Southeast
South-
southeast
South
South-southwest
Southwest
West-
southwest
West
West-
northwest
Northwest
northwestNorth-
East-
northeast
FAST
FAC T
WWW.ASTRONOMY.COM 19
WORLD WIDE WEB. A study published January 15 in Icarus
shows that the valleys left on Mars by flowing liquid water could
have been created even if the Red Planet was never “warm and
wet.” Instead, peak daily summertime temperatures just above
freezing on an otherwise cold and dry Mars could melt ice at the
edges of glaciers. Just a small amount of meltwater, year after
year, could have left behind the intricate network of valleys on
the planet’s surface. — A.K.
Active galactic nuclei (AGN) are
feeding supermassive black holes
in the centers of galaxies. For
decades, astronomers believed the
two most common types, Type I
and Type II, appear different only
because of their orientation. But
recent research by an international
team of astronomers suggests they
are intrinsically different, with Type I
AGN hosting “hungrier” black holes.
The research, published
September 28 in Nature, is based on
X-ray observations of 836 AGN
using NASA’s Swift Burst Alert
Telescope and complementary data
from 12 ground-based telescopes.
“Our new analysis of X-ray data from
NASA’s Swift Burst Alert Telescope
suggests that [the brighter] Type I
galaxies are much more efficient at
emitting energy,” co-author Richard
Mushotzky of the University of
Maryland said in a press release.
Astronomers envision AGN
as supermassive black holes
surrounded by an accretion disk
of material swirling inward, and a
larger, doughnut-shaped torus of
light-blocking gas and dust. To
explain differences between Type I
and II AGN, the unified model
currently used states that the tilt
of the torus affects the light we
receive and thus results in
classification differences.
“Our results suggest this has a
lot to do with the amount of dust
that sits close to the central black
hole,” said Mushotzky. “Type II gal-
axies have a lot more dust, and this
dust pushes against the gas as it
enters the black hole.” That slows
their feeding rate, which in turn
affects their classification.
If so, the unified model will need
modification and previous AGN
studies should be re-evaluated.
But, Mushotzky said, “By putting
us on a path to better understand
the differences between the galax-
ies that host Type I and Type II
active nuclei, this work will help
us better understand how super-
massive black holes influence the
evolution of their host galaxies.”
— A.K.
Voracious black holes lurk in some galaxies
How did Mars get its valleys?
TO DUST RETURN. Tabby’s Star may be surrounded by dust, accounting for the large dips in the star’s light.
This could explain why it dips in infrared but not in ultraviolet light.
ASTRONOMY
: ROEN KELLY
ALL ABOUT POSITION ANGLES
WHAT’S MY ALIGN? In astronomy,
position angles describe where the
secondary component of a binary
star is relative to the primary.
Position angles have a value
of 0° at north and progress
through east. When we
assign directional names
to them, however, there
are ranges. Here’s
a quick reference.
— Michael E. Bakich
Observers measure position
angles of double stars by
drawing a line from the brighter
star through the fainter one.
ENERGY EFFICIENCY. Active galactic nuclei are ringed by a doughnut-shaped torus of gas and dust. New research suggests
that the efficiency of the AGN and the amount of dust around it affect the light we see, rather than the orientation of the ring.
NASA/JPL-CALTECH/ARIZONA STATE UNIVERSITY
NASA/JPL-CALTECH