The Magellanic Squall: Gas Replenishment from the Small to the Large Magellanic Cloud

Mon. Not. R. Astron. Soc. 381, L16–L20 (2007) doi:10.1111/j.1745-3933.2007.00357.x The Magellanic squall: gas replenishment from the Small to the Large Magellanic Cloud Kenji Bekki1⋆ and Masashi Chiba2 1School of Physics, University of New South Wales, Sydney, NSW 2052, Australia 2Astronomical Institute, Tohoku University, Sendai, 980-8578, Japan Accepted 2007 June 25. Received 2007 June 20; in original form 2007 May 8 ABSTRACT We first show that a large amount of metal-poor gas is stripped from the Small Magellanic Cloud (SMC) and falls into the Large Magellanic Cloud (LMC) during the tidal interaction between the SMC, the LMC and the Galaxy over the last 2 Gyr. We propose that this metal- poor gas can closely be associated with the origin of the LMC’s young and intermediate-age stars and star clusters with distinctively low metallicities with [Fe/H] < −0.6. We numerically investigate whether gas initially in the outer part of the SMC’s gas disc can be stripped during the LMC–SMC–Galaxy interaction and consequently can pass through the central region (R < 7.5 kpc) of the LMC. We find that about 0.7 and 18 per cent of the SMC’s gas could have passed through the central region of the LMC about 1.3 Gyr ago and 0.2 Gyr ago, respectively. The possible mean metallicity of the replenished gas from the SMC to LMC is about [Fe/H] =−0.9 to −1.0 for the two interacting phases, if a steep metallicity gradient of the SMC’s gas disc is assumed. These results imply that the LMC can temporarily replenish gas supplies through the sporadic accretion and infall of metal-poor gas from the SMC. They furthermore imply that if this gas from the SMC can collide with gas in the LMC to form new stars in the LMC, the metallicities of the stars can be significantly lower than those of stars formed from gas initially within the LMC. Key words: galaxies: haloes – galaxies: kinematics and dynamics – Magellanic Clouds – galaxies: star clusters – galaxies: structure. age of ∼2 Gyr has a distinctively low metallicity of [Fe/H] =−0.8 1 INTRODUCTION among intermediate-age SCs (Geisler et al. 2003; Grocholski et al. Tidal interactions between the Large Magellanic Cloud (LMC), the 2006, hereafter G03 and G06, respectively). Santos & Piatti (2004, Small Magellanic Cloud (SMC) and the Galaxy have long been con- S04) investigated integrated spectrophotometric properties of old sidered to play vital roles not only in the dynamical and chemical and young SCs and found that several young SCs with ages less evolution of the Magellanic Clouds (MCs) but also in the formation than 200 Myr have metallicities smaller than −0.6. Three examples of the Magellanic stream (MS) and bridge (MB) around the Galaxy of these low-metallicity objects including Rolleston et al. (1999, (e.g. Westerlund 1997; Murai & Fujimoto 1980; Bekki & Chiba R99) are listed in Table 1. Given the fact that the stellar metallicity 2005, hereafter B05). Although previous theoretical and numerical of the present LMC is about −0.3 in [Fe/H] (e.g. van den Bergh studies on the LMC–SMC–Galaxy tidal interaction discussed exten- 2000, hereafter v00; Cole & Tolstoy 2005), the above examples sively the origin of the dynamical properties of the MB (e.g. Gardiner of low-metallicity, young SCs are intriguing objects. No theoretical & Noguchi 1996, hereafter G96), they have not yet investigated so attempts have been made to understand the origin of these intriguing extensively the long-term formation histories of field stars and star objects in the LMC, however. clusters (SCs) in the MCs. Therefore, long-standing and remarkable The purpose of this Letter is to show, for the first time, that the ob- problems related to the interplay between the LMC–SMC–Galaxy served distinctively low metallicities in intermediate-age and young interaction and the formation histories of stars and SCs remain un- SCs in the LMC can be possible evidence of accretion and infall solved (See Bekki et al. 2004a,b for the first attempts to challenge of low-metallicity gas on to the LMC from the SMC. Based on dy- these problems). namical simulations of the LMC–SMC–Galaxy interaction for the One of the intriguing and unexplained observations of SCs in the last 2.5 Gyr, we investigate whether gas stripped from the SMC as LMC is that an intermediate-age SC (NGC 1718) with an estimated a result of the tidal interaction can pass through the central region of the LMC and consequently can play a role in the star formation history of the LMC. Based on the results of the simulations, we ⋆E-mail: [email protected] discuss how the sporadic accretion/infall of metal-poor gas on to C 2007 The Authors. Journal compilation C 2007 RAS The Magellanic squall L17 Table 1. Examples of distinctively metal-poor stars and SCs for the LMC of 0.2Rs for the dE and the dI models. Rs is fixed at 1.88 kpc so that and the inter-Cloud region close to the LMC. almost no stellar streams can be formed along the MS or the MB. Many dwarfs are observed to have extended H I gas discs Object name Age [Fe/H] Reference (e.g. NGC 6822; de Blok & Walter 2003). The SMC is therefore NGC 1718 2 Gyr −0.80 ± 0.03 G06 assumed to have an outer gas disc with a uniform radial distribution, NGC 1984 4 Myr −0.90 ± 0.40 S04 Mg/Ms(= f g) and Rg/Rs(= rg) being key parameters that determine DGIK 975 41 Myr −1.06 ± 0.12a R99 the dynamical evolution of the gas. The rotating ‘gas disc’ is represented by collisionless particles in the present simulations, first aMean metallicity for C, N, Ma and Si with respect to the solar abundances. because we intend to understand purely tidal effects of the LMC– SMC–Galaxy interaction on the SMC’s evolution and secondly because we compare the present results with previous ones by G96 the LMC from the SMC (referred to as ‘the Magellanic squall’) can and Connors, Kawata & Gibson (2006) for which the ‘gas’ was rep- control the recent star formation activities of the LMC. resented by collisionless particles. Although we investigate models with different f g and rg, we show the results of the models with f = 1 and 3 and r = 2 and 4 for which the Magellanic stream 2 MODEL g g with a gas mass of ∼108 M⊙ can be reproduced reasonably well. We adopt the numerical methods and techniques of the simulations The baryonic mass fraction [f b = (Ms + Mg)/MSMC] thus changes on the evolution of the MCs used in our previous paper (B05): according to the adopted f g. Owing to the adopted rg = 2 and 4, a we first determine the most plausible and realistic orbits of the very little amount of stars in the SMC can be transferred into the MCs by using ‘the backward integration scheme’ (for orbital evo- LMC for the last 2.5 Gyr. lution of the MCs) by Murai & Fujimoto (1980) for the last 2.5 Gyr The initial spin of the SMC’s gas disc in a model is specified by and then investigate the evolution of the MCs using GRAvity PipE two angles, θ and φ, where θ is the angle between the Z-axis and the (GRAPE) systems (Sugimoto et al. 1990). The total masses of the vector of the angular momentum of the disc and φ is the azimuthal 10 LMC (MLMC) and the SMC (MSMC) are set to be 2.0 × 10 M⊙ angle measured from X-axis to the projection of the angular momen- and 3.0 × 109 M⊙, respectively, in all models. The SMC is rep- tum vector of the disc on to the X–Y plane. Although θ and φ are resented by a fully self-consistent dynamical model with the total also considered to be free parameters, models with limited ranges of particle number of 200 000, whereas the LMC is represented by a these parameters can reproduce the MS and the MB (e.g. Connors point mass. As we focus on the mass-transfer from the SMC to the et al. 2006). The gas disc is assumed to have a negative metallicity LMC, this rather idealized way of representing the LMC is not un- gradient as the stellar components has (e.g. Piatti et al. 2007). The −1 reasonable. The gravitational softening length is fixed at 0.1 kpc for gradient represented by [Fe/H]g(R) (dex kpc )isgivenas all models. The orbital evolution of the MCs depends strongly on their masses [Fe/H] (R) = αR + β, (1) and initial velocities for given initial locations of the MCs (B05). g Therefore we first run a large number of collisionless models and thereby find models that can successfully reproduce both the MS where R (in units of kpc) is the distance from the centre of the SMC, and its leading arm features. For those models, we investigate time- α =−0.05, and β =−0.6. These values of α and β are chosen evolution of the SMC’s gas particles that can finally infall on to the such that (i) the metallicity of the central region of the SMC can LMC. We use the same coordinate system (X, Y, Z) (in units of kpc) as be consistent with the observed one ([Fe/H] ∼−0.6; v00) and (ii) those used in B05.

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