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Astron. Astrophys. 354, 787-801 (2000)

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3. Results

3.1. The spectra

The absorption spectra are presented in Fig. 2a-t together with the 21 cm emission spectra in the direction of the continuum sources. The 21 cm emission data were taken from the survey of Luks & Rohlfs (1992, see also Fig. 3), which has a spatial resolution of [FORMULA] (=200 pc) and a velocity resolution of 1 km s-1. Fourier interpolation has been used to resample the spectra with channel centres and widths corresponding to the absorption spectra. When comparing the absorption component with the emission we take the two component model of Luks and Rohlfs into account. Their spectra in general show two main components, the principle velocity component (disk- or "D" component) of the LMC emission, which can be fitted well by a differentially rotating disk with a simple rotation curve and an inclination of 33o and a low velocity gas layer, the "L" component, which is mainly found at the east side of the LMC.

[FIGURE] Fig. 2a-t. See page 794

[FIGURE] Fig. 2a-t. See page 794

[FIGURE] Fig. 2a-t. See page 794

[FIGURE] Fig. 2a-t. Emission and absorption spectra toward the continuum sources. The upper panel shows the 21 cm emission spectrum measured with the Parkes telescope (brightness temperature in K) by Luks & Rohlfs (1992), the lower shows the absorption measured with the ATCA (relative absorption [FORMULA], times 100). At the bottom the interferometer phase as a function of frequency is given in degrees.

[FIGURE] Fig. 3. The positions of sources (survey 2 & 3) at the leading edge of the LMC are superposed on the contour map of the total H I column density (Luks & Rohlfs 1992) at the levels [FORMULA] = (2, 3, 4, 7, 10, 14, 18, 22, 29 and 32)[FORMULA]1020 cm-2

The 30 Doradus complex

There are three sources in the vicinity of the Tarantula nebula, a region of intense star formation including 30 Doradus (DEM263, N157), DEM271 (N159) and DEM284 (N160). It is located at the northern part of the giant H I cloud of high column density observed by Luks and Rohlfs (see Fig. 3). An exceptional velocity distribution of cool H I is found at all lines of sight toward the 30 Doradus complex.

Source MDM 68 (No. 12) is in the direction of the giant molecular cloud, about 14´.2 from DEM 271 (N159). It shows several deep absorption line components covering a velocity range similar to that of J0539-697 and J0540-697 in N159 (see survey 2).

Source MDM 56 (Nr. 8) has a projected distance of 500 pc from 30 Doradus and is lying in the direction of DEM233 (N150). The detection of a small molecular cloud and a compact H[FORMULA] knot toward MDM 56 indicates a young star forming region, still embedded in a dense cold cloud (Paper II). The absorption spectrum shows one very deep and broad line, which seems to be a blend of several narrower lines. It is offset from the emission ("L" component) by about 7 km s-1, indicating a very low spin temperature of the atomic gas of only 10 K (Sect. 4). No absorption is seen at the velocity of the "D" component in direction of this intrinsic source. This might support the result of Blondiau et al. (1997), who find the "L" component to be located in front of the "D" component. Source J0535-689 (survey 2), which is about 200 pc south-west of MDM 56, shows absorption at similar velocities but with smaller equivalent width.

Source MDM 89 (No. 14) has a projected distance of 440 pc from 30 Doradus and is about 12´ north of DEM 299 (N165), an ionized gas ring of 80 pc diameter. The spectrum reveals a deep band of absorption, which might be a blend of several narrower lines, covering velocities from about 258 to 276 km s-1. The deep absorption components at 258 km s-1 and at 264 km s-1 have velocities similar to the ionized hydrogen near N165 (Caulet et al. 1982).

Supergiant shell LMC 4

Seven lines of sight of our H I absorption survey (No. 3, No. 4, No. 6, No. 7, No. 9, No. 10 and No. 11) are near to LMC 4, the largest supergiant shell in the LMC. The H[FORMULA] filaments and H II regions of this complex pattern, located in the northern part of the LMC, form an almost perfect ring with a diameter of 1.4 kpc (1.6o) centered at 05h31m, -66o 55´ (1950) (Meaburn 1980). The disk H I of the shell shows a ringlike structure, 1.8 kpc in diameter with a very pronounced central hole (Dopita et al. 1985). The energetics of the whole region strongly suggest that massive stars generate a local overpressure in the interstellar medium, which drives a shock in the intercloud medium, sweeping over and around dense H I clouds and so initiating new episodes of star formation (Dopita et al. 1985, Domgörgen et al. 1995). Dopita et al. found an expansion velocity of 36 km s-1. As an alternative formation mechanism of LMC 4, a model of HVC-disk collision is discussed by Braun (1996). De Boer et al. (1998) propose that the supergiant structure finds its origin in star formation induced in the bow-shock formed at the leading edge of the LMC. Due to the rotation of the LMC this structure then moved aside.

MDM 31 (No. 4) is to the south of LMC 4, at a projected distance of 1020 pc from the centre position and about 10´ south of the diffuse filament DEM 211. The absorption spectrum shows a deep absorption line ([FORMULA] = 1.64 [FORMULA] 0.13) at 281 km s-1. The velocity of this absorption feature is similar to that toward J0526-678 (survey 2), which is 160 pc north-west, but shows higher optical depth. The emission reveals two blended lines, which might be associated with the disk H I and the shell of gas ejected below the plane of the LMC, as analysed by Dopita et al. (1985). The peak of the absorption is offset by about 6 km s-1 from the strongest emission peak, the "D" component.

Source MDM 55 (No. 7) has a projected distance of 718 pc from the centre of LMC 4. It is located in a region of high column density at the north-eastern rim of DEM231 (N57C), a possible H II region at the ionisation front of the shell. The absorption spectrum shows a narrow and deep absorption line at 279 km s-1, the velocity of the disk component. The H I absorption line is similar to that of MDM 31 (No. 4). It shows the same high optical depth.

No absorption has been detected above the detection thresholds toward the other lines of sight in the direction of LMC 4. Source MDM 25 (No. 3) is close to the ionized edge of the shell. It is located south of DEM 192 (N51D) in a region of less H I column density compared to MDM 55. There might be a very weak absorption component at 290 km s-1, associated with the disk H I but the existence of this cloud has to be verified by further investigations. The sources MDM 64 (No. 11) and MDM 60 (No. 10) are in the direction of the ionized inner part of the giant shell, near the diffuse H[FORMULA] region DEM 257. The sources MDM 44 (No. 6) and MDM 58 (No. 9) are south of DEM 229 (N57) and DEM 245.

The eastern steep H Iboundary

Sources MDM 109 (No. 16), MDM 111 (No. 17), No. 18, MDM 113 (No. 19) and MDM 114 (No. 20) are in the direction of the relatively sharp boundary of the H I gas toward the east of the LMC. Fig. 3 shows the location of the sources on the contour map of the H I column density distribution. A compression zone is hypothesised in this direction of the LMC due to the ram-pressure produced by the motion of the LMC through the gas of the outer galactic halo (Mathewson & Ford 1984).

A detailed search for cool gas in direction of this H I boundary reveals H I absorption toward only one of our sources. Source MDM 111 (No. 17) is about 29´ south of J0552-682 (showing H I absorption in survey 2) and about 23´ south of the weak arc-like structure DEM 328. The spectrum shows two deep absorption lines at the velocity of the "D" component. The optical depth of the two lines is a factor of three higher than that toward J0552-682. Both lines of sight are near the cone-like structure at the leading edge, which is seen in the far infrared, and has been discussed by Braun (1996) as likely to be the result of a high velocity cloud impact.

Others

Source MDM 95 (No. 15) is about 14´ (200 pc) from source No. 14. It has a projected distance of 600 pc from 30 Doradus. The spectrum shows a small, deep absorption line at 296 km s-1, which is offset from the emission peak by about +15 km s-1, and two weak absorption lines at about 4 [FORMULA]. The weak line at 274 km s-1 covers a velocity range from 270 to 280 km s-1. An absorption feature of similar velocity is seen toward MDM 89 (No. 14). There might be some confusion from emission fluctuations at about 300 km s-1. The complex dynamic structure of the cool neutral ISM observed in the vicinity of the Tarantula nebula seems to dissolve near source No. 15. Although the line of sight is only about 160 pc more distant from 30 Doradus than No. 14, the fraction of cool H I compared to the warm (see Sect. 4.1) is a factor of 3 lower.

Source MDM 20 (Nr. 2) is about 6.6´ south-east of the diffuse object DEM157 (N128). The very noisy spectrum toward the weak continuum source (26 mJy) shows two tentative absorption lines at 240 km s-1 and 264 km s-1.

Source MDM 1 (No. 1) is about one degree south of the bar. It is in a region of little optical emission, 20.7´ away from DEM102, a small diffuse object. MDM 1 shows a tentative detection of absorption (4 [FORMULA]) at a velocity of 230 km s-1.

Source MDM 88 (No. 13) is about 16´ from the diffuse, arc-like structure DEM276 (N214D) and in the direction of the eastern rim of the giant molecular cloud. The absorption spectrum is very noisy in the velocity range of the emission. We are not confident about the weak lines at 264 and 288 km s-1 near the detection threshold, which are confined to only one channel.

Source MDM 32 (No. 5) , near to the rim of the filamentary and diffuse shell DEM208 (N204) does not show absorption above the detection threshold.

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Online publication: February 25, 2000
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