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 Longitudes:
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 spiral arm.
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 arm.
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.
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 feature.
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!

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