Received 31st March 1998
Abstract.
We use the new Leiden/Dwingeloo Survey for a survey of
H I-velocity-bridges and found some velocity-bridges
in the Magellanic Stream.
In this HVC-Complex, there is always a correlation between a local maximum
of the low velocity gas and the appearance of a velocity-bridge.
In the Magellanic Streams Cloud MS V, we found the strongest correlation
between high velocity gas and gas at lower velocities.
The interpretation is that the Magellanic Stream is interacting with low
velocity gas in the Galactic halo.
1. Introduction
The Leiden Dwingeloo Survey (LDS) which was obtained with the 25 m
Dwingeloo telescope (Hartmann 1994; Hartmann & Burton 1995) covers
the entire sky for δ ≥ -30°.
It is the first 21 cm-line survey that was corrected for stray radiation
using the method of Kalberla et al. (1980).
This means that it can be used to search for very faint emission in extreme
regions of the spectra.
We undertook a systematic search for emission in the velocity ranges between
high velocity clouds (HVC) and intermediate velocity clouds (IVC) or between
IVC's and low velocity gas (LVC).
These three classes of clouds are defined as follows:
- Low velocity clouds (LVC's):
|vLSR| ≤ 25 km s-1.
- Intermediate velocity clouds (IVC's):
25 ≤ |vLSR| ≤ 90 km s-1.
- High velocity clouds (HVC's):
|vLSR| ≥ 90 km s-1.
See Wakker and van Woerden (1997); review of high velocity clouds.
2. The High Velocity Cloud MS V
The cloud complex MS V in the Magellanic Stream extends from Galactic
longitude l = 86° to l = 99° and from
Galactic latitude b = 45° to b = 61°.
The most negative velocities (ca. -360 km s-1) are located
around l = 95°, b = 54°
(Fig. 1).
Here is a strong positional correlation between HVC's and LVC gas.
Several HVC's in the same data-cube have a head-tail structure.
The faint tail is called velocity bridge (VB), because it often appears as
a connection between HVC and IVC gas (e.g. in HVC complex C, Pietz et al. 1996).
Velocity bridges are interpreted as a sign of an inelastic collision:
thinner material was stripped off and decelerated, therefore it appears at less
negative velocities.
It appears that HVC's with high integrated intensities are most likely to have
velocity bridges (at least in the Magellanic Stream).
The best one is visible around l = 95°,
b = 55° (Fig. 2).
[Click here to see Fig. 1 and 2!]
3. Interpretation
The Magellanic Stream is likely to have its origin in the Magellanic Clouds
and seems to be falling into the Milky Way (Fujimoto & Murai 1984).
While the LMC has a distance of about 50 kpc, the cloud MS V could
be much nearer (maybe 30 kpc).
Since we find that
- the HVC MS V shows a velocity bridge,
- the LVC gas has two components:
at -8 km s-1 and at +3 km s-1,
and
- the HVC gas is in good positional correlation with the intensities
of LVC gas
we have evidence for an interaction between the Magellanic Stream and
corotating gas in the upper halo.
This kind of correlation is visible in other parts of the Magellanic Stream,
too.
That implies that there is much more neutral hydrogen with normal galactic
velocitites at z ≥ 30 kpc than previously assumed.
A more systematic investigation is necessary to justify such an interpretation.
References
- Fujimoto M., Murai T., 1984, Proc. IAU Symp. 108, p. 115
- Hartmann D., 1994, PhD Thesis, University of Leiden
- Hartmann D., Burton W.B., 1995, An atlas of Galactic Neutral Hydrogen
Emission, Cambridge University Press
- Hartmann D., Kalberla P.M.W., Burton W.B., Mebold U., 1996,
A&AS 119, 115-151
- Kalberla P.M.W., Mebold U., Reich W., 1980, A&A 82, 275
- Kerp J., Mack K.-H., Egger R., Pietz J., Zimmer F., Mebold U., Burton W.B.,
Hartmann D., 1996, A&A 312, 67-73
- Pietz J., Kerp J., Kalberla P.M.W., Mebold U., Burton W.B., Hartmann D.,
1996, A&A 308, L37-40
- Wakker B.P., van Woerden H., 1997, ARAA 35, 217-266
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| First version: | 13th | August, | 1998
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| Last update: | 04th | October, | 1998
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Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)