Topic: Mid April Astronomy Bulletin

[Clive]

TRITON'S SUMMER AIR
ESO

Neptune has 13 known satellites, of which Triton is by far the
largest; it is about 2700 km in diameter (three-quarters the size of
our Moon), and is the seventh-largest moon in the Solar System.  An
initial infrared analysis, made with the VLT, of Triton's very thin
atmosphere confirmed the existence of methane, first detected by
Voyager 2, though the main constituent is nitrogen.  The observations
indicated that the atmosphere varies seasonally, thickening when
warmed, so the Sun still makes its presence felt on Triton, even from
so far away.  Triton actually has seasons, just as we do, but they
come round far more slowly. A season lasts a little over 40 years, and
Triton passed the southern summer solstice in 2000.  From the amount
of gas measured, researchers estimate that Triton's atmospheric
pressure may have risen by a factor of four since it was observed by
Voyager 2 in 1989, when it was still spring.  The atmospheric pressure
on Triton is now between 40 and 65 microbars -- 20,000 times less than
on Earth.


ACTIVE GALAXY'S PLUMES
NASA/Goddard Space Flight Center

Centaurus A is a disturbed-looking galaxy about 12 million light-years
away -- in the constellation Centaurus, naturally -- and was one of
the first celestial radio sources to have been identified with a
galaxy.  A hallmark of radio galaxies is that the emission arises in
huge double-lobed structures much bigger than the galaxies themselves.
Astronomers classify Cen A as an 'active galaxy', a term applied to
any galaxy whose central region radiates strong emissions at many
different wavelengths.  The power behind the emissions is supposed to
come from a black hole millions of times more massive than the Sun.
The black hole somehow diverts some of the matter falling towards it
into two oppositely directed jets that stream away from the centre.

If our eyes could see radio waves, Centaurus A would be one of the
biggest and brightest objects in the sky, nearly 20 times the apparent
size of the Full Moon.  What we can't see when we look at the galaxy
in visible light is that it lies nestled between much bigger plumes
radio-emitting gas.  Each plume is nearly a million light-years long.
The Fermi space telescope maps gamma-rays, radiation that typically
carries 100 billion times the energy of radio waves.  Nevertheless,
and to the surprise of many astrophysicists, Centaurus A's plumes show
up clearly in the satellite's data.  Not only do we see the extended
radio lobes, but their gamma-ray output is more than ten times their
radio output.

The gamma-rays arise from a process called inverse Compton scattering.
Centaurus A ejects magnetized particle jets moving at nearly the speed
of light.  Over the course of tens of millions of years, the jets have
puffed out two giant bubbles filled with magnetic fields and energetic
particles -- the radio lobes we now see.  The radio waves arise as
high-speed electrons spiral through the lobes' tangled magnetic
fields.  The entire Universe is filled with low-energy radiation --
radio photons from the all-pervasive cosmic microwave background, as
well as infrared and visible-light photons from stars and galaxies.
When a photon collides with a super-fast particle in the radio lobes,
it may receive such an energy boost that it becomes a gamma-ray.


GALAXY SURVEYS MAY MISS 90% OF WHAT THEY ARE LOOKING FOR
ESO

A deep survey with the VLT suggests that a large fraction of galaxies
whose light has taken 10 billion years to reach us have gone
undiscovered.  Surveys of remote galaxies are often made in the light
of the Lyman-alpha line, a strong emission line of hydrogen.  Most of
the Lyman-alpha light is absorbed within the galaxy that emits it.  In
the recent VLT experiment a well-studied area of the sky, known as the
GOODS-South field, was surveyed in Lyman-alpha light, following the
methodology of standard Lyman-alpha surveys, and also in the light of
H-alpha, another wavelength emitted by glowing hydrogen.  The
observations were specifically directed at galaxies whose light has
been travelling for 10 billion years (redshift 2.2).  The astronomers
concluded that surveys made in Lyman-alpha see only a small part of
the total light that is produced, since most of the Lyman-alpha
photons are destroyed by interaction with the interstellar clouds of
gas and dust.  The effect is much more significant for Lyman-alpha
than for H-alpha light.  As a result, many galaxies, a proportion that
may be as high as 90%, go undiscovered in Lyman-alpha.  That could be
seen as a warning to cosmologists, who have relied increasingly on
Lyman-alpha surveys to find and count the first galaxies to form in
the Universe.