5. The age of LMC 4
The ages of the various star groups of LMC 4 have been collected in Table 6. Note that the location of the various groups can be found in Fig. 1. For NGC 1948 we list only the newer age determination of Will et al. (1996) superseding the one by Vallenari et al. (1993).
The ages we derived for the 'J'-shaped region are in the range of 9 to 16 Myr. Also NGC 1948, NGC 2004, LH 72 north, LH 63, and LH 60 are 9 to 16 Myr old. This seems to suggest that most of LMC 4 was formed as an entity some 9 to 16 Myr ago. Much younger are LH 54 and LH 76, with an age of 5 Myr, but these associations are located on the very rim of LMC 4.
N 51, with LH 63, 60 and 54, shows an age gradient (Petr et al. 1994; Petr 1994), a clear hint at SSPSF on smaller scales. Much more recent starformation was triggered at the edge of LMC 4, after the entire interior was in existence for some 5 Myr (the time between formation of the LMC 4 interior and that of LH 54). It is interesting to note that 5 Myr is about the time the more massive stars need to become supernova.
Also LH 72 south is young, but it may have formed indeed more recently, in the aftermath of events in LH 72 north.
Note that the globular cluster NGC 1978, the rather conspicuous object near the northern rim of LMC 4 and having an age of approximately Gyr (Bomans et al. 1995), has nothing to do with the creation and/or evolution of SGS LMC 4. This statement is valid for all objects older than Myr.
© European Southern Observatory (ESO) 1997
Online publication: March 24, 1998