1. Introduction

The 10 identified SGSs (see Meaburn 1981) plus an additional one detected in an H I survey (Kim et al. 1997) are listed in Table 1 and marked in Figs. 1 and 5. These structures need very effective creation mechanisms such as collisions of high velocity clouds (HVCs) with the disk of the galaxy or stochastic self-propagating star formation (SSPSF), both explaining the ring of H II regions and the 'hole' in the H I layer. According to the favorite SSPSF model, star formation will 'eat' its way from the initial point to all directions through the interstellar medium, creating a big cavity and a thick outer shell of neutral hydrogen ionized at the inner edge by the early-type stars (O-B2). Thus one should see a clear age gradient from the centre to the rim of about 15 Myr for structures extending 1.4 kpc like LMC 4.

This work (see Braun et al. 1997, hereafter Paper I) presents a photometric study of the stellar population inside the biggest SGS LMC 4. The found ages for this and other huge structures (see Fig. 5) hint at another plausible explanation: large-scale star formation triggered by the bow-shock (de Boer et al. 1998, hereafter Paper II).

2. Age Determination in LMC 4

The ages we derived for the stars in the 'J'-shaped region inside LMC 4 lie in the interval from 9 Myr up to 16 Myr (∼13 Myr) and are the same as the ages determined for NGC 1948, NGC 2004, LH 72 north, LH 63, and LH 60 at the border of this SGS (see Sect. 5 in Paper I for details and references). The reddening is of the order of a ¹/₁₀ magnitude.

There exist some examples of younger star groups likely having been built in secondary star forming processes and one can also find age gradients on small scales (< 150 pc), but seeing coeval stars on scales larger than 1 kpc is striking.

3. Luminosity Function and Mass Function in LMC 4

4. History of LMC 4

5. Bow-Shock Triggering Scenario

For the Magellanic System we can expect an effect by the motion of the LMC through the halo of our Galaxy. The first hint is the steeper density gradient of H I at the leading eastern edge of the LMC and Magellanic Stream extending to the west as a tidal tail of the clouds (see left part of Fig. 5). But could this affect the superstructure in the north-east?

The superstructures in the LMC (see right part of Fig. 5) are located at the outskirts. Furthermore we know that the dark cloud (DC), 30 Dor and LMC 4 are located at the rear side of the LMC fitting to the direction of the LMC motion. Taking the rotation of the LMC into account, we would expect star formation triggered by the compression of the gas due to the bow-shock at the leading edge (with a total relative velocity ∼450 km s^-1; see p. 125, de Boer 1998 and Paper II), which changed its position from the north to the east according to the clockwise rotation. If we look at the ages of superstructures and its traversed distance (see Fig. 6 and Table 3), we get a good correlation by the age determinations fitting to the rotation of the galaxy (Paper II).

If star formation is triggered by the bow-shock at the leading edge (now at the SE), one can predict ages and compare them with derived ones. To further support this model, we got a dataset at the 1.54 m Danish telescope at La Silla in December 1996 [ESO proposal 58.D-0149] pointing at N 70, LMC 2, N 214 and N 171. As a first preliminary result I present the CMD of N 70 which yields an age of ∼8 Myr (see Fig. 7). This value is in accordance with the expected age, but as star formation is going on all over the region of the LMC the huge features are better hints for triggering than the smaller associations and shells.

Additionally we got U, B, V photometry of the interior of SGS LMC 1 and LMC 7 at the 1.54 m Danish telescope at La Silla in January 1998 [ESO proposals 60.E-0234 and 60.D-0147] to get further constraints on the creation mechanisms for these prominent features of the youngest stellar generation and to enlighten the difference in the Hα appearance of the SGSs. Is this caused by an age effect or are the SGSs divided into two distinct groups (LMC 1-5 and LMC 6-9)?

6. Conclusions

References

• Braun J.M., 1996, Diploma Thesis, University of Bonn
• Braun J.M., Bomans D.J., Will J.-M., de Boer K.S., 1997, A&A 328, 167 [astro-ph/9708081] (Paper I)
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• de Boer K.S., 1998, in Proceedings of the Bonn/Bochum-Graduiertenkolleg Workshop 'The Magellanic Clouds and Other Dwarf Galaxies', Richtler T., Braun J.M. (eds.), Shaker Verlag, Aachen, p. 125 (these proceedings)
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