Spiral Arms in 3D: our Galactic Gas Disk is Warped ...
Joachim Köppen Strasbourg 2010
The ESA-Haystack telescope can be used to map the distribution of neutral
hydrogen gas in the Milky Way. From the northern hemisphere we may observe
the Galactic Plane between Galactic Longitudes between l=0 and l=240,
almost 70 percent. Since most of the hydrogen emission is concentrated well
within 20° latitude of the Plane, it makes sense to restrict a systematic mapping
to the latitude range of -30° to +30°. As the antenna's beamwidth is about 6°,
recording spectra every 2° in latitude is a bit of oversampling, but it does not
harm to have more than sufficient data.
Having done such an observation of all latitudes at a couple of galactic longitudes,
we can produce false colour maps of the radial velocity and galactic latitude. For
example at l=90° looks like this:
There is a strong emission at 0 km/s, extending from about -10° to +10° in
latitude: this is the local hydrogen gas of the spiral arm of which our Sun is a
member. There are two concentrations at -40 and -75 km/s: these are two
spiral arms farther out from the Galactic Centre than the solar orbit. While the
feature at -40 km/s is in the Plane, the maximum of the -75 km/s arm is found
at latitude +2°! This is the evidence that the disk of our Galaxy is not flat,
but warped, like a hat whose brim is bent up on one side, but bent down at
the opposite side: If we assume the reasonable 'flat' rotation curve, the greater
radial velocity of -75 km/s indicates also a larger galactocentric distance than
for the -45 km/s feature.
Is that so? Let us follow what happens when we go through the Galactic
at l=20° we see that the radial velocities of gas in the
Plane have a wide span, from -30 to +80 km/s. Gas with
positive velocities is located in the inner disk, i.e. within
the sun's orbit around the Galactic Centre, as can be seen
from the Radial Velocity Tool.
at l=40° there is strong emission at +30 km/s, from an arm
of the inner disk. But also, there is emission at about -40 km/s,
already slightly above the Plane, which belongs to an exterior
at l=60° the strongest emission is at +10 km/s, from an arm
of the inner disk. At high latitudes, radial velocities approach
0 km/s, thus this is local gas which shows some motion away
from us, like an expanding shell. The exterior arm is now found
at -60 km/s.
at l=80° the strongest emission is at +5 km/s in the Plane. The arm of the inner disk is
no longer visible. Instead, there is emission from two exterior spiral arms, at -30 and
-70 km/s, resembling the map of l=90°. Both arms are above the Plane, at about +2° latitude.
at l=120° the three arms are still visible, as is the warping. The outermost
one is fairly faint.
at l=150° there is strong emission at -30 km/s from the exterior
in the direction of the anti-centre the spread of radial velocities
is rather narrow, about +/-20 km/s. Since motion in purely circular
orbits would produce zero radial velocities for all spiral arms, this
fact mainly indicates that the motion of the gas clouds indeed is
close to circular orbits. Note the asymmetry at high latitudes:
far above the Plane there seems little gas, but far below there
is an appreciable amount of gas, with slightly positive velocity.
The interpretation would be that in the solar vicinity there is little
neutral hydrogen in the northern hemisphere, but expanding gas
in the southern part.
at l=210° strong emission is seen at +10 km/s, which might be
the local expanding gas, also seen at high southern latitudes.
There is also quite a strong feature at +45 km/s, which would
be an external spiral arm in the Plane.
already at l=220° the picture has changed appreciably:
strong emission is seen at +15 km/s, perhaps local gas or
perhaps an outer arm, but a prominent outer arm is found
at +60 km/s.
It thus seems that near l=90° the outer disk is bent up a bit, and it seems
possible that at the opposite side it is bent down ... unfortunately, the
positions around l=270° are not observable from Europe, but the data
for l=230 are already highly suggestive!
at l=230° the structure has become more complex, with three or even
four features that can be distinguished: two outer arms at +75 and
+50 km/s, a strong component at +20 km/s which could be another
external arm and which extends below the Plane, and weaker emission
high above the Plane with +5 km/s. Most interestingly, the two outer
arms form almost a mirror image of the structures seen at l=90°:
the +75 km/s arm lies below the Plane and below the +50 km/s
In early 2012, we did a DataCube for longitudes between 60° and 110°, taking
latitude profiles between -10° and 10° in 1° steps.
Results are shown here. Use the
d-z map Button to show the deprojected view of the emission as
a function of distance and height above the Galactic Plane.
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last update: Feb. 2012 J.Köppen