Received 16th March 1998
Abstract.
In a third 21 cm absorption line survey toward the Large Magellanic Cloud
(LMC) we have used the Australia Telescope Compact Array to study the cool
atomic gas near the supergiant shell LMC 4, in the surroundings
of 30 Doradus and in the direction of the eastern steep
H I boundary.
We have identified 20 absorption features toward nine of 20 compact continuum
sources.
The cool H I gas in the LMC is either unusually abundant
compared to the cool atomic phase of the Milky Way or the gas is clearly
colder (Tc≅30 K) than that in our Galaxy
(Tc≅60 K).
The properties of atomic clouds toward 30 Doradus and LMC 4 suggest
a higher cooling rate in these regions compared to other parts of the LMC,
probably due to an enhanced pressure near the shock fronts of LMC 4 and
30 Doradus.
In contrast, the atomic gas is predominantly in the warm phase at the eastern
steep H I boundary in spite of an expected compression zone.
1. Observation
The 21 cm absorption survey has been done toward 20 compact continuum
sources, which have been selected from our
ATCA snapshot survey
at 1.4 GHz (Marx et al. 1997).
The sources show peak flux densities between 21 mJy and 80 mJy and
are mainly in directions of the 30 Doradus complex, the
Halpha supergiant shell LMC 4 and in the direction of
the sharp H I edge at the eastern boundary of the LMC
(see Fig. 1).
Directions far distant from these three groups have been used as a reference
sample.
For the spectral line observations we used the single 6 km configuration
(6D) of the
ATCA*.
The resulting synthesized beamwidth is ∼7".
The velocity resolution is Δ v = 1.65 km s-1.
We have integrated two to seven hours on each source, depending on the
continuum flux density.
This provides an optical depth sensitivity στ between 0.1
and 0.25.
*The Australia Telescope is funded by the
Commonwealth of Australia for operation as a National Facility managed by
CSIRO
[Click here to see Fig. 1!]
2. Results
We have identified 20 absorption features toward nine of the 20 lines of sight.
The derived spin temperatures for the cool atomic clouds range from 8 K to
69 K.
The cool gas parameters, i.e. the optical depth τ, the "equivalent
width" (EW = ∫ (1 - e-τ) dv),
the spin temperature,
Tsp = Tem/(1 - e-τ)
at the centre of the absorption line and the cool gas fraction,
fc = 0.0182·Tc·EW/Nem,
have been compared for the different regions of the LMC taking the results
of Dickey et al. (1994, survey 2) into account.
Here Tem is the brightness temperature of the emission
observed by Luks and Rohlfs (1992) and Nem is the column
density of the H I emission along the line of sight.
The cool gas clouds toward 30 Doradus and LMC 4 differ from clouds
far from star forming regions and shock fronts in higher values of EW
and τ and in a higher fraction of cool gas.
The 30 Doradus complex shows an unusually large amount of cool
H I, about half of the atomic neutral hydrogen.
No other galaxy, known so far, possesses regions with that high fraction of
cool H I gas compared to the warm.
The cool gas in the vicinity of 30 Doradus shows a complex dynamic
structure even beyond the optical part of this giant star forming region.
Whereas all lines of sight toward the 30 Doradus complex show cool
H I clouds, only about half of the lines of sight
toward LMC 4 reveal H I absorption features.
Cool H I clouds have been even less frequently detected
in direction of the eastern steep H I boundary.
The number of detected cool H I clouds and their properties
suggest a higher cooling rate near LMC 4 and 30 Doradus compared to
the reference positions, caused by an enhanced density near shock fronts.
While the heating rate is only proportional to the number density
Γ∝n, the cooling rate is given by
Λ=Lcal n2,
where Lcal denotes the interstellar cooling function.
In spite of an expected compression zone at the east side of the LMC due to
the motion of the galaxy through the halo of the Milky Way (Mathewson et al.
1987), the atomic gas is predominantly in the warm phase toward the leading
edge.
The distribution of cool H I at the eastern steep
H I boundary might indicate that the compression of
gas decreases to the south of the boundary.
From the present H I absorption studies we conclude that
the fraction of cool gas in the LMC is rather determined by local conditions
of the ISM (e.g. H II regions, SNRs) than by the distance
from the gravitational centre.
The new data corroborate the earlier suggestion that H I
clouds in the LMC are either unusually cool
(<Tc>≅30 K) compared to the cool phase in
the Milky Way (<Tc>≅60 K), or that the cool
atomic phase of the interstellar medium is more abundant in the LMC
(fc = 35% for Tc = 60 K) relative to
the warm neutral medium than in our Galaxy (fc = 24% for
Tc = 60 K).
Even, if we exclude lines of sight toward the 30 Doradus complex and toward
LMC 4 we find, assuming Tc = 60 K, a similar
mixture of warm and cool interstellar phases compared to that in the Milky Way,
despite the completely different radiation field,
heavy element abundance and dust-to-gas ratio in the LMC.
References
- Dickey J.M., Mebold U., Marx M., Amy S., Haynes R.F., Wilson W., 1994,
A&A 289, 357
- Kennicutt R.C., Bresolin F.Jr., Bomans D.J., Bothun G.D., Thompson I.B.,
1995, AJ 109, 594
- Luks Th., Rohlfs K., 1992, A&A 263, 41
- Marx M., Dickey J., Mebold U., 1997, A&AS 126, 325
- Mathewson D.S., Wayte S.R., Ford V.L., Ruan K., 1987, Proc. Astr. Soc.
Aust. 7, 19
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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)