Received 16th March 1998
Abstract.
Despite a strong UV radiation field the Large Magellanic Cloud (LMC) shows
a relatively large abundance of cool H
I gas.
Neither CO- nor [C
II]-lines have been detected in most
of these regions in previous surveys.
The energy balance of these cool clouds, some of which are located in warm
surroundings, is still an open question.
The improved resolution and sensitivity of the
ISO telescope compared to previous
measurements offers the unique opportunity to study the heating and cooling
of these clouds in the LMC.
Here we present first results of an investigation of the dominant cooling
line, [C
II] (158 µm), toward cool
H
I clouds.
1. Observations
We searched for [C II] line emission at 158 µm
toward 13 lines of sight showing cool H I gas using
the Long-Wavelength Spectrometer (LWS, Clegg et al. 1996) on board the
Infrared Space
Observatory* (ISO, Kessler et al. 1996) with
angular resolution of 1.65'.
The sources have been selected from the H I absorption
surveys of Dickey et al. (1994) and Marx-Zimmer et al.
(1998a,b).
The positions are shown in Fig. 1.
The typical integration time was 9 minutes on each source, which provides
an rms of 0.8 Jy.
The velocity resolution is 260 km s-1.
*Based on observations with
ISO, an
ESA project
with instruments funded by ESA Member States (especially the PI countries:
France, Germany, the Netherlands and the United Kingdom) with the participation
of ISAS and
NASA
2. Results
The high amount of cool H I gas in the LMC with spin
temperatures between 4 K and 93 K and the non-detection of CO
emission toward most of these positions (Marx-Zimmer et al.
1998a,b)
rises the question, whether the gas is effectively cooled by collisional
excitation of ionized carbon, which might be present in the surroundings
of the cool H I clouds.
The 158 µm fine structure line of atomic carbon has an excitation
temperature of 92 K.
[C II] observations of Mochizuki et al. (1994) reveal
a mean ratio of [C II] to CO line intensities 18 times
larger than the value observed in the galactic plane.
However, most cool H I clouds do not show up in
the [C II] emission survey of Mochizuki et al.
Their beam size of about 12' is too large to detect compact clouds.
Our ISO-[C II]-observations toward cool atomic clouds
reveal emission of ionized carbon toward 6 out of the 13 lines of sight
(see Fig. 1).
Three of these do not show [C II] emission in the previous
survey of Mochizuki et al. due to a smaller beam filling factor.
There is no clear correlation of [C II] emission with
the spin temperature, Tspin, or the optical depth,
τH I, of the cool H I,
although the probability of finding [C II] emission
in direction of cool H I gas increases with
τH I.
We have compared the ISO-[C II]-data with our
SEST-CO-survey of cool H I clouds in the LMC.
No CO emission has been detected in direction of the [C II]
emitting gas.
CO seems to be completely dissociated in these regions.
In contrast, one H I cloud in the outer region of the LMC
shows CO but no [C II] emission.
The UV radiation field seems to be small in this direction.
The high fraction of cool H I gas in the LMC compared
to the Milky Way and the non-detection of the dominant cooling lines CO and
[C II] in most directions of the cool
H I suggest that the low temperature of the atomic phase
is not the result of a high cooling rate.
This indicates a low heating rate of gas in many directions of the LMC
despite a high UV radiation field.
One possibility for a lower heating rate of gas in the LMC compared to
the Milky Way might be a reduced number of small dust grains, since
photoelectric heating from small grains is expected to dominate the heating
of the cool neutral phase (Wolfire et al. 1995).
[Click here to see Fig. 1!]
References
- Clegg P.E., Ade P.A.R., Armand C., et al., 1996, A&A 315, L38
- Cohen R.S., Dame M.T., Garay G., et al., 1988, ApJ 331, L95
- Dickey J.M., Mebold U., Marx M., Amy S., Haynes R.F., Wilson W.,
1994, A&A 289, 357
- Kessler M.F., Steinz J.A., Anderegg M.E., et al., 1996, A&A 315, L27
- Marx-Zimmer M., Herbstmeier U., Zimmer F., Dickey J.M.,
Staveley-Smith L., Mebold U., 1998a,
Proceedings of the Workshop
"The Magellanic Clouds and Other Dwarf Galaxies",
p. ???
- Marx-Zimmer M., Zimmer F., Herbstmeier U., Chin Y.-N., Dickey J.M.,
Mebold U., 1998b,
Proceedings of the Workshop
"The Magellanic Clouds and Other Dwarf Galaxies",
p. ???
- Mochizuki K., Nakagawa T., Doi Y., Yui Y.Y., Okuda H., Shibai H.,
Yui M., Nishimura T., Low F.J., 1994, ApJ 430, L37
- Wolfire M.G., Hollenbach D., McKee Ch.F., Tielens A.G.G.M.,
Bakes E.L.O., 1995, ApJ 443, 152
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| First version: | 10th | August, | 1998
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| Last update: | 08th | October, | 1998
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Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)