Formation and evolution of the Magellanic Clouds - I. Origin of structural, kinematic and chemical properties of the Large Magellanic Cloud

Abstract

We investigate the dynamical and chemical evolution of the Large Magellanic Cloud (LMC) interacting with the Galaxy and the Small Magellanic Cloud (SMC) based on a series of self‐consistent chemodynamical simulations. Our numerical models are aimed at explaining the entire properties of the LMC, i.e. the observed structure and kinematics of its stellar halo and disc components as well as the populations of the field stars and star clusters. The main results of the present simulations are as follows. Tidal interaction between the Clouds and the Galaxy during the last 9 Gyr has transformed the initially thin, non‐barred LMC disc into three different components: central bar, thick disc and kinematically hot stellar halo. The central bar is composed of both old field stars and newly formed ones, with the two fractions being equal in its innermost part. The final thick disc has central velocity dispersion of ∼30 km s−1 and shows rotationally supported kinematics with Vm/σ0∼ 2.3. The stellar halo is formed during the interaction, and consists mainly of old stars originating from the outer part of the initially thin LMC disc. The outer halo shows velocity dispersion of ∼40 km s−1 at a distance of 7.5 kpc from the LMC centre and has a somewhat inhomogeneous distribution of stars. The stellar halo contains relatively young, metal‐rich stars with a mass fraction of 2 per cent. Repetitive interaction between the Clouds and the Galaxy has moderately enhanced the star formation rate to ∼0.4 M⊙ yr−1 in the LMC disc. Most of the new stars (∼90 per cent) are formed within the central 3 kpc of the disc, in particular, within the central bar for the last 9 Gyr. Consequently, the half‐mass radius is different by a factor of 2.3 between old field stars and newly formed ones. Efficient globular cluster formation does not occur until the LMC starts interacting violently and closely with the SMC (∼3 Gyr ago). The newly formed globular cluster system has a disc‐like distribution with rotational kinematics, and its mean metallicity is ∼1.2 higher than that of new field stars because of pre‐enrichment by the formation of field stars prior to cluster formation. The LMC evolution depends on its initial mass and orbit with respect to the Galaxy and the SMC. In particular, the epoch of the bar and thick disc formation and the mass fraction of the stellar halo depend on the initial mass of the LMC. Based on these results, we discuss the entire formation history of the LMC, the possible fossil records of past interaction between the Clouds and the Galaxy, and the star formation history of the SMC for the past several Gyr.

Ages and luminosities of young SMC/LMC star clusters and the recent star formation history of the Clouds

“…North is up, west to the right. Bekki & Chiba (2005) and Kallivayalil et al (2006a,b) show that the MW, the LMC, and the SMC have only interacted long enough to produce the Magellanic Stream. According to these models, the last close encounter between SMC and LMC occurred about 100−200 Myr ago.…”

“…The star formation rate decreases again when the LMC recedes from SMC, thus leading to episodic cluster formation. This increase in cluster formation some 100−200 Myr ago may therefore have been triggered by a tidal interaction with the neighboring galaxy (e.g., Gardiner & Noguchi 1996;Bekki & Chiba 2005;Kallivayalil et al 2006a,b). The youngest peaks, however, might have another origin.…”

“…Possible orbits of the Small Magellanic Cloud (SMC), the Large Magellanic Cloud (LMC), and of the MW have been modeled by several authors (e.g., Bekki & Chiba 2005). They find that it was difficult to keep the Clouds bound to each other for more than 1 Gyr in the past.…”

“…They find that it was difficult to keep the Clouds bound to each other for more than 1 Gyr in the past. The LMC and the SMC have been part of a triple system with the MW for at least 1 Gyr (e.g., Bekki & Chiba 2005;Kallivayalil et al 2006a,b). It is possible that the Clouds are not a bound system and that they are making their first passage close to the MW.…”

Magnetic fields in Local Group dwarf irregulars

“…Within our unbiased LG sample the only similar dwarf is IC 10 (p ≈ +0.5) with strong magnetic fields of 9.7 μ G. The LMC is located at a similar location in the p − f diagram. Its star-formation episodes are believed to be triggered by close encounters with the Milky Way over the past 4 Gyr (Bekki & Chiba 2005). In contrast to the aforementioned bottom-right quarter, the top-left quarter of the p − f plane is occupied by less evolved objects with a relatively weak star-formation activity (negative p) that evolve slowly, despite having enough gas to produce stars.…”