Received 12th May 1998
Abstract.
The Leiden/Dwingeloo 21 cm line survey (Hartmann & Burton 1997) was
used to search for a correlation between the H
I column
density distribution and magnetic fields.
The structure of the local galactic magnetic field was estimated from
the polarization surveys of Mathewson & Ford (1970), the rotation measures
of extragalactic radio sources of Tabara & Inoue (1980),
Simard-Normandin et al. (1981), Broten et al. (1988), and
from pulsar measurements by Taylor et al. (1993).
No general correlation could be detected between H I
column densities and rotation measures.
A number of filamentary H I features were found which
correlate well with stellar polarization data of Mathewson & Ford (1970).
These filaments seem to be located within the local interstellar medium
(d ≤ 1 kpc).
1. Introduction
Interstellar dust, gas, magnetic fields, and cosmic rays are expected to be
interwoven and closely related to each other (Parker 1969).
H I, although neutral, is expected to be closely tied to
interstellar magnetic fields because even a low-level ionization of
ne ∼ 4·10-4 cm-3 is
sufficient to couple the gas to the magnetic fields.
Also grains may be considered to be tied to the magnetic field because
electric charges on their surfaces are caused by the photoelectric effect.
Visualizations of the Leiden/Dwingeloo 21 cm line survey (LDS,
Hartmann & Burton 1997) show a rich wealth of filamentary features, which
are highly suggestive of beeing associated with magnetic field structures.
Since the accuracy of the LDS is an order of magnitude better than previous
H I surveys, we decided to search for possible associations between gas,
dust and magnetic fields.
2. Rotation Measures and H I
Rotations measures (Tabara & Inoue 1980; Simard-Normandin et al. 1981;
Broten et al. 1988; Taylor et al. 1993)
have been compared with the H I column density
distribution.
We found no global correlation between the rotation measures
and H I column densities. The distribution of rotation
measures shows a patchy structure, which is also not visible in
the H I data.
We conclude that there is either no correlation between neutral and ionized
components in the interstellar medium or the magnetic field is dominated
by turbulence and field reversals.
3. Stellar Polarization and H I
Correlating stellar polarization data (Mathewson & Ford 1970) with
LDS H I maps, a large number of filaments can be found
which are highly suggestive of a possible correlation.
In summary, the results are:
For H I gas at high- or intermediate velocities
(|vLSR| > 11 km s-1),
there is no significant correlation with the polarization data.
Most of the stellar data are from distances of
d < 1 kpc.
High-velocity clouds are assumed to be at larger distances, therefore no
correlation is to be expected from such observations.
For intermediate-velocity clouds the distances are less certain.
Our result is then consistent with the assumption that intermediate-velocity
clouds do not belong to the local interstellar medium.
Channel maps at constant velocities may be used to study details of
the correlation between H I gas and stellar polarization.
Individual H I filaments which are associated with optical
polarization have a typical velocity range of 5 to
10 km s-1.
H I gas with such line widths indicate temperatures of
few hundred Kelvin.
We compared the filaments which are associated with stellar polarization with
far-infrad emission, e.g. at 240 µm (derived from COBE maps,
Hauser et al.) and found a less significant correlation from these data.
This indicates that the H I filaments are most probably
warm clouds (T > 30 K).
It is well known that radio loops I, III and IV are associated with
H I filaments (Berkhuijsen 1971).
In Fig. 1 the radio loops are
indicated.
Loops I and IV are clearly associated with H I filaments
as well as with stellar polarization.
We find no obvious correlation with loop III.
[Click here to see Fig. 1!]
References
- Broten N.W., Macleod J.M., Vallée J.P., 1988, Ap & SS 141, 303
- Berkhuijsen E.M., Haslam C.G.T., Salter C.J., 1971, A&A 14, 252
- Fich M., Blitz L., Stark A. A., 1989, ApJ 342, 272
- Hartmann D., Burton W.B., 1997, Cambridge University Press
- Hauser M.G., Kelsall T., Leisawitz D., Weila J. (eds.), COBE
Diffuse Infrared Background Experiment (DIRBE) Explanatory Supplement,
Ref. Pub. No. 97-A (Greenbelt, MD: NASA/GSFC) available in electronic
form from the National Space Science Data Center.
- Mathewson D.S., Ford V.L., 1970, Mem.R.Astron.Soc. 74, 139
- Parker E.N., 1969, Space Sci. Rev. 9, 651.
- Reach W.T., Wall W.F., Odegard N., 1998, ApJ, submitted
- Simard-Normandin M., Kronberg P.P., Button S., 1981, ApJS 45, 97
- Tabara H., Inoue M., 1980, A&AS 39, 379
- Taylor J.H., Manchester R.N., Lyne A.G., 1993, ApJS 88, 529
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