A Complete Survey of Galactic Hydrogen near the Galactic Plane
Joachim Köppen Kiel Dec. 2013
Looking at the sky with our eyes shows bright and faint stars almost everywhere,
and also a less brilliant band of light, known as the Milky Way, which is composed
of many faint, hence far-away stars. The sky seen with radio waves looks quite
different from the familiar view: stars are not detected, but one observes the
emission of hot, warm, and also cool gas which is distributed between the stars.
At a frequency of 1420 MHz hydrogen atoms are able to emit in a spectral line.
Mapping the sky at this frequency reveals that hydrogen gas is mainly concentrated
in a thin belt across the sky which follows the middle of the Milky Way. This shows
us that the interstellar gas is organized as in our Galaxy in a thin disk, from
which the stars are born. The observed frequency of this line differs from the
known true frequency because of the motions of the gas. Thus the Doppler effect
permits to measure the velocities of these motions, and one finds that the hydrogen
gas is in nearly circular orbits around the Galactic Centre, along with all the
stars, including our Sun. A closer look at the data reveals that the gas is not
evenly distributed in space, but is concentrated in a spiral pattern: our Milky
Way is a spiral galaxy.
All these information can be obtained by a mapping of the sky at 1420 MHz - or
at a wavelength of about 21 cm. Since most of the emission is near the Galactic
Plane, it suffices to map the galactic latitudes from -10º to +10º. In the last
couple of years this had been done from the northern hemisphere, using a Small
Radio Telescope (SRT) of 2.3m diameter installed near Strasbourg. However, from
Europe one cannot observe the entire Galactic Plane. Fortunately, the survey
could be completed by observations from Australia, with a radio telescope of the
same type and size:
- the ESA-Haystack
radio telescope in Illkirch near Strasbourg. Being in the Northern Hemisphere,
it can cover the Galactic Plane from about 0º to 250 º in galactic longitudes.
Located on a unversity campus, its sensitvity is somewhat limited due to measures
to protect it against electronic interference. Several survey had been done,
but data from the
DataCube
survey is used, which was done in Nov/Dec 2012.
- the SRT at Grove Creek Observatory
near Syndey, New South Wales is ideally located on farmland far away from any
electric or electronic interference.
It covers the Plane in the longitude range from about 170º to 360° and then to
70º, with a better sensitivity and a much better stability. The data were taken
in three weeks in Nov/Dec 2013.
This applet shows the individual and combined results of the two surveys.
The GCO data (done with a system temperature of about 170 K) are rougly
calibrated in terms of antenna temperature. The ESA-Haystack data were
obtained with system temperatures of about 300 K; they have been scaled
to the GCO values.
The data are in a datacube of galactic longitude (from -180° to +180°
in steps of 2°), galactic latitude (from -10° to +10° in steps of 1°),
and radial velocity from -150 to +150 km/s.
False colour Maps of the antenna temperature as a function of any two
of the three parameters are shown for a fixed value of the third parameter.
Also, Plots in one of the parameter can be done, for fixed values of
the other two parameters.
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last update: March 2014 J.Köppen DF3GJ