ATNF Science Highlights 2004
A wind bubble around a magnetar
Magnetar are pulsars with extremely high surface magnetic
fields, however, the difference of their origin is not
understood. Data from the Southern Galactic Plane Survey shows a
cavity in the interstellar medium around the magnetar 1E
1048.1-5937. These cavities are common around massive stars
which blow material away from their surfaces, but in the case of
1E 1048.1-5937, no nearby star massive enough to form such a
bubble could be found. This suggests that the bubble was formed
by the stellar progenitor of the magnetar, and that the
supernova occurred quite recently. It also suggests that the
difference between the origin of normal neutron stars and
magnetars is related to the progenitor's mass. From the known
distribution of stellar masses then follows that magnetars form
at a rate which is 10 times lower than that of normal pulsars.
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Discovery of pulsed OH maser emission stimulated by a pulsar
Pulsars are useful objects to study the interstellar medium
(ISM). Using the Parkes telescope, ATNF astronomers have
discovered pulsed OH maser emission in the direction of the
pulsar PSR B1641-45. The study unambiguously shows how the OH
emission varies with the pulsar emission, and is the first that
provides direct evidence for stimulated amplification of a
signal in the ISM. As the pulsar emission varies within a few
milliseconds, it is also the quickest maser variation yet
detected. The shape of the maser emission lines allows more
conclusions to be drawn about the geometry of the OH clouds
between the pulsar and us observers. When a pulse passes through
the clouds (on-spectrum), the lines are narrow, while they are
broad between the pulses (off-spectrum). The line of sight to
the pulsar, which is probed in the on-spectra, is very narrow,
and may be significantly different from the surrounding
medium. In the off-spectra, however, the entire cloud is probed,
and an average over a larger region is made.
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A "new" spiral arm for the Milky Way?
The shape and structure of the Milky way, especially the
distribution of its neutral hydrogen (HI), is not well
known. The recently completed Southern Galactic Plane Survey
(SGPS) has mapped the HI in our galaxy using the Parkes
telescope and the Compact Array, and the data show an extended
structure in the far outer galaxy. The velocity of HI can be
used to identify the location of HI, and the new feature was
determined to be between 17 kpc and 25 kpc away, and to have a
height over the galactic disk of between 1.2 kpc and 1.7 kpc. The
location of the feature indicates that it may be a spiral arm,
and model calculations can reproduce structures very similar to
the one observed. Other interpretations like elliptical gas
orbits or a very smooth extended HI disk were found to be
inconsistent with the data. The measurements may even show the
gravitational effect of the Magellanic Clouds on the Milky Way.
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The Galactic All-Sky Survey (GASS)
Galactic neutral hydrogen (HI) is observed in all directions
of the sky. Whilst the large-scale structure of HI has been
mapped in detail, the finer structures have yet mostly been
ignored. The Galactic All-Sky Survey (GASS) uses the Parkes
multibeam receiver to image the HI south of declination 0°
with an angular resolution of 15 arcminutes, a velocity
resolution of 1 km/s and a sensitivity limit of 70 mK. GASS will
focus on the galactic halo and its interaction with the
disk. Recent studies show that the halo contains plenty of
small, cold HI clouds, the origin of which is unknown. GASS will
also shed light on high velocity clouds (HVCs). These clouds
orbit the Galaxy with high velocities inconsistent with galactic
rotation. They may be tidal debris, related to a galactic
fountain, and dark matter halos. GASS will be a sensitive,
high-resolution, unbiased survey, and will use more than 1500
hours of telescope time, and the first results look spectacular.
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The Delta Quadrant Survey
Star formation has long been a riddle, but recent models
suggest that turbulence on a wide range of spatial scales is
able to create and regulate star formation. To provide the
models with observational constraints, ATNF astronomers use the
Mopra telescope to map the 13CO (J=1-0) transition in
the giant molecular cloud RCW 106, located in the fourth
quadrant (hence "Delta Quadrant") of the Milky Way. The project
required the development of On-the-Fly mapping, which allows to
continuously scan across the sky without the need for discrete
telescope pointings. The data will be analysed to characterise
the turbulence in detail, and will be supplemented with
observations of the C18O line and with data from the
SPITZER space telescope. This will yield a picture of the
relationship between turbulence and star formation across a
region roughly 100 pc in size.
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How to feed a starburst
Star formation is believed to happen when sufficiently cool
gas collapses under its own gravitational force, and the core
becomes dense enough for nuclear reactions. In some galaxies,
dubbed "starburst galaxies", the rate at which this process
happens is much higher than in our Galaxy, and the regions are
much smaller. Molecules are an excellent tracer of this process,
and are now observable with the Compact Array, and one very
prominent starburst galaxy is NGC253, which has been observed
with the new 3 mm receivers to find emission from HCN and
HNC. The ratio of the emission from these molecules gives
insight to the environmental conditions where they are formed,
with a high HCN/HNC ratio indicating higher temperatures through
conversion of HNC to HCN. However, towards the centre in NGC
253, where most stars are formed, the ratio unexpectedly
decreases. This can be understood as a self-shielding process
which keeps the molecular gas cool and allows it to further
collapse and form new stars.
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Highly-redshifted CO in galaxies
With the new 12 mm and 3 mm receivers coming into operation
on the Compact Array, new possibilities for the study of
molecules such as carbon monoxide and molecular hydrogen at
several redshifts have been created. Radio emission from
galaxies a few billion years after the Big Bang can give clues
about the formation of massive galaxies we still see today. The
most distant radio galaxy known to date is TN J0924-2201, at a
redshift of 5.2. Only the millimetre upgrade of the Compact
Array made it possible in 2004 to observe carbon monoxide in
this galaxy, with a total mass of about 100 billion solar
masses. This discovery also places an upper limit of 1.1 billion
years on the time scale for star formation in this
galaxy. Furthermore, the galaxy is peculiar in that it does not
have the amount of dust which would be expected from the amounts
of molecular gas, indicating that many similar objects still
remain to be found.
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