Proceedings of the Workshop
"The Magellanic Clouds and Other Dwarf Galaxies"
of the Bonn/Bochum-Graduiertenkolleg

The Kinematic Structure of the Supergiant Shell LMC 2

Sean D. Points

Astronomy Department, University of Illinois

Received 15th March 1998
Abstract. LMC 2 is the most spectacular supergiant shell in the Large Magellanic Cloud (LMC), having the most coherent filamentary structure and the highest X-ray surface brightness. We have obtained high-resolution echelle spectra of the Halpha line and Australia Telescope Compact Array aperture synthesis maps of the 21 cm line emission of LMC 2 to study its dynamics. The echelle spectra of LMC 2 reveal two major velocity components of the ionized gas at vhel∼250 and 275 km s-1. Four H I velocity components are detected toward LMC 2 at vhel∼250, 275, 300, and 320 km s-1. Neither the Halpha spectra nor the H I channel maps provide convincing evidence that LMC 2 is expanding.

1. Introduction

The supergiant shell LMC 2 has the brightest, most coherent filamentary structure of all known Large Magellanic Cloud (LMC) supergiant shells. Halpha images of LMC 2 show long, spectacular filaments in the NE quadrant and shorter, less coherent filaments in the southern part that form an arc extending toward the longer filaments. In addition to the filamentary structure, LMC 2 is also comprised of compact H II regions and diffuse patches of emission. LMC 2 is bordered on the west by a ridge of star formation regions (i.e., N 157, N 158, N 159, and N 160).

The kinematic structure of LMC 2 has been investigated by numerous groups with conflicting results. Caulet at al. (1982) studied the kinematic structure of the 104 K ionized gas of LMC 2 using Fabry-Perot interferograms. They suggest that LMC 2 is a quarter of a spherical shell expanding with a velocity of 30 km s-1 with respect to an ambient medium at 245 km s-1. Using the Parkes 64 m telescope, Meaburn et al. (1987) obtained profiles of the 21 cm emission line along three tracks in the LMC. They detect no evidence that any supergiant shells in the LMC are coherently expanding structures. Hunter (1994) obtained high and low resolution optical spectra of LMC 2. However, no expansion motion can be determined from these data because the slit positions are located near the eastern rim of LMC 2 where any expansion motion should be perpendicular to the line-of-sight. With the much improved spatial resolution of our Halpha and H I data, we may now re-examine the kinematic structure of LMC 2.

2. Kinematic Structure of LMC 2

We have obtained high-dispersion echelle observations along three long cuts across LMC 2 (Fig. 1) to examine the kinematics of the ionized gas. In the regions covered by the three echelle cuts. One of the gas components has a heliocentric radial velocity of 250±5 km s-1 and its peak intensity is strongest in the southeastern quadrant of LMC 2, especially where the echelle cuts cross bright filaments. We detect the other ionized gas component at vhel≥270 km s-1. The higher velocity component is brightest in the northwestern region of LMC 2, toward the cluster NGC 2100.

The ∼250 km s-1 component does not show any obvious systematic velocity variation along the three cuts and is observed both interior and exterior to the optical filaments defining LMC 2. The ≥270 km s-1 component has been interpreted as the receding hemisphere of LMC 2 (Caulet at al. 1982). However, the velocity structure along echelle cut E II shows that the ≥270 km s-1 does not converge to 250 km s-1 near the periphery of the shell and does not show a velocity variation conforming to that expected in an expanding hemisphere. Instead, the 250 and ≥270 km s-1 components, detected over the area of LMC 2, appear to be kinematically independent of each other. We do not see any evidence suggesting that LMC 2 is a coherently expanding structure.

We have extracted H I observations of LMC 2 from the H I mosaic survey of the LMC made with the Australia Telescope Compact Array (ATCA; Kim et al. 1997). The H I channel maps show that the neutral hydrogen toward LMC 2 is distributed in the form of long filaments and clouds, many of which have optical counterparts. The iso-velocity maps allow us to examine the kinematic relationship between different filaments. The long, coherent filaments that define the eastern boundary are detected in velocity channels between 230 and 255 km s-1 that is consistent with the velocities of these filaments we determined with our Halpha spectra. Filamentary H I structures in the northeast section of LMC 2, associated with very faint Halpha filaments, are detected in the velocity channels between 255 and 285 km s-1. In the northern region of LMC 2, some H I filaments are observed with heliocentric velocities between 290 and 310 km s-1. If LMC 2 were a spherically expanding structure, we expect to observe velocity extremes toward the center of the shell, not toward the periphery. Thus, the H I data provide further evidence that LMC 2 is not a coherently expanding structure.

[Click here to see Fig. 1!]

3. Conclusions

We have investigated the kinematic structure of the supergiant shell LMC 2 using high resolution echelle spectra of the Halpha line and aperture synthesis maps of the 21 cm emission line. The long-slit echelle spectra reveal the presence of two components of the ionized gas, but do not provide any convincing evidence that LMC 2 is a coherently expanding shell. Filaments of neutral gas seen in the aperture synthesis maps have close correlation with optical filaments. These filaments are seen in a range of velocity channels, contrary to what is expected from an expanding shell. Thus, it is unlikely that LMC 2 is an expanding structure.

References

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First version: 07thJuly,1998
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