|Annu. Rev. Astron. Astrophys. 1996. 34:
Copyright © 1996 by . All rights reserved
3.5. What Triggers Star Formation in the Magellanic Clouds?
The Magellanic Clouds show extensive evidence of mutual interaction. The extreme depth of the SMC (Caldwell & Coulson 1986, Mathewson et al 1986, 1988, Gardiner & Hawkins 1991), the inter-cloud bridge, and the Magellanic Stream all testify that the Clouds have had a tumultuous recent interaction history. Lin & Richer (1992), Muzzio (1988) argue that some MC clusters may have been lost to the Galactic halo on longer time scales. We have noted above (Section 3.4) that the unusual LMC cluster ESO121SC-03 may have formed originally in the SMC.
There is growing confidence that many of these global properties can be understood as originating in the tidal interactions of the Clouds with each other and with the Milky Way. Numerous workers (Murai & Fujimoto 1980, Lin & Lynden-Bell 1982, Shuter 1992, Gardiner et al 1994, Moore & Davis 1994, Heller and Rohlfs 1994, Lin et al 1995) have produced increasingly elaborate dynamical models that successfully account for many of the basic properties of the Magellanic Stream and the inter-cloud lridge and also properly account for the space velocity of the LMC (Kroupa et al 1994, Jones et al 1994). We have also seen in this review that the star-formation history of the Clouds are complex and distinct. Could tidal encounters between the MCs have provided the triggering mechanism needed to initiate this complex star-formation history? Interestingly, de Vaucouleurs & Freeman (1973), Freeman (1984) pointed out that Magellanic systems tend to come in pairs, as if tidal interactions may be necessary to form galaxies similar to the Clouds.
All dynamical models of the LMC / SMC system agree that the two galaxies had a particularly close encounter within the past 100-500 Myr (Gardiner et al 1994, Moore & Davis 1994, Heller et al 1994, Lin et al 1995). For example, Gardiner et al (1994) note that the minimum separation of the Clouds was about 5 kpc compared to the 18 kpc separation we see today. Moreover, the best recent models also demand that in order to account for many of the present-day features of the Magellanic system and the space motions of the Clouds, the two galaxies must form a long-lived (> 10 Gyr) binary pair. Thus, a tidal trigger for the complex star-formation histories of the Clouds is plausible since they probably have tidally affected each other all their lives. One particular model of Gardiner et al (1994) shows that the LMC / SMC separation has had two particuarly close encounters during the past 15 Gyr: the recent encounter 200 Myr ago, and a similarly close mutual passage 4 Gyr ago. The latter time corresponds very closely to when the LMC recommenced forming stars and clusters after its long hibernation (see Sections 3.3 and 3.4; Figure 1).
Before we can conclude that the star-formation histories of the Clouds are driven by tidal triggers, we must address at least three basic problems. First, the preferred model of Gardiner et al (1994) does not show any close encounters prior to the one 200 Myr ago, and in fact the Clouds have maintained a nearly constant separation prior to that time. Second, we must somehow account for the fact that the LMC and SMC have such different star-formation histories. One possibility is that the tidal effects of the LMC on the SMC have been sufficient to maintain an enhanced star-formation rate in the smaller galaxy, while only very close encounters of the SMC can trigger star formation in the more massive LMC. Finally, we lack a solid physical model that can translate tidal interaction into star-formation activity.
Recent HST imaging of actively interacting galaxies has enabled the identification of vigorous star and cluster formation (Holtzman et al 1992, Whitmore & Schweizer 1995). These results show that encounters between massive galaxies can and do induce vigorous star and cluster formation. Can the much milder encounters between small galaxies such as the MCs also trigger activity? The answer is important not only to understand the Clouds, but also to explain the complex star-formation histories of other satellites in the outer halo and the evolution of faint blue galaxies seen in deep surveys.