2. THE MAGELLANIC CLOUDS

2.1. The Large Magellanic Cloud

At high Galactic latitude and a distance of only ~ 50 kpc, the Large Magellanic Cloud (LMC) is one of the best-studied galaxies in the Local Group. With 0.5 ^. 10⁹ M [36], the gaseous component (neglecting He) contributes ~ 9 % to the total mass of the LMC (5.3 ± 1.0 ^. 10⁹ M; [1]). The gas to dust ratio is four times lower in the LMC than in the Milky Way [39]. The total diffuse H₂ mass is 8 ^. 10⁶ M, < 2% of the LMC's HI mass and ~ 1/9th of the Milky Way's fraction [72]. The reduced H₂ fraction may imply enhanced destruction through UV photodissociation in low-metallicity environments or suppressed H₂ on dust grains that H₂ [72]. While high dust content is correlated with high H₂ abundances, H₂ does not trace CO or dust per se [72].

CO shows a strong correlation with HII regions and young (< 10 Myr) clusters, but only little with older clusters and supernova remnants (SNRs) ([19], cf. [2]). Massive CO clouds have typical lifetimes of ~ 6 Myr and are dissipated within ~ 3 Myr after the formation of young clusters. CO clouds exist also in quiescent areas without ongoing star formation; potential sites of future activity. Overall, the LMC clouds haver lower CO luminosities than in the Milky Way and higher gas to dust ratios [19]. Individual cloud masses range from a few 10⁴ M to 2 ^. 10⁶ M. With 4 to 7 ^. 10⁷ M the estimated molecular gas mass of the LMC amounts to 8 to 14% of its total gas mass [19].

HI aperture synthesis maps of the LMC have revealed an ISM with a turbulent, fractal structure that is self-similar on scales from tens to hundreds of pc [18], likely due to the energy input of OB stars and supernova explosions. The flocculent ISM consists of numerous shells and holes surrounded by broken HI filaments [36]. At very large scales supershells dominate. 23 HI supershells (i.e., holes with sizes that exceed the HI scale height) and 103 giant shells (sizes below the HI scale height) were identified [37]. Many of the giant shells interlock or collide with one another, or occur at the rims of supershells. They probably result from winds of recently formed massive stars in a propagating-star-formation scenario. Generally, the HI shells show little correlation with the optically dominant HII shells, which suggests that HI shells live longer than the OB stars that caused them initially [37]. HI associated with HII typically exceeds the size of the ionized regions.

The overall appearance of the HI disk of the LMC is symmetric, does not show obvious correlations with the optical bar, and reveals spiral features [36]. Its southernmost "spiral arm" connects to the Magellanic Bridge, the tidal HI overdensity between the LMC and the Small Magellanic Cloud (SMC).

Photoionization is the main contributor to the optical appearance of the ISM at ~ 10⁴ K in the LMC and other gas-rich, star-forming galaxies. The LMC has a total H luminosity of 2.7 ^. 10⁴⁰ erg s^-1. 30 to 40% are contributed by diffuse, extended gas [35]. Nine HII supershells with diameters > 600 pc are known in the LMC [48]. Their rims are marked by strings of HII regions and young clusters/OB associations. The standard picture for supershells suggests that these are expanding shells driven by propagating star formation (e.g., [47]). However, an age gradient consistent with this scenario was not detected in the largest of these supershells, LMC4 [15]. Nor are the supershells LMC1 [53], LMC2 [52], and LMC4 [16] expanding as a whole, but instead appear to consist of hot gas confined between HI sheets and show localized expansion. Supershells in several other galaxies do not show evidence for expansion either [52], nor for the expected young massive stellar populations [57].

More highly ionized gas can be effectively traced through ultraviolet absorption lines from species such as such as CIV (10⁵ K), NV (2^. 10⁵ K), and OVI (3 ^. 10⁵ K; these temperatures are valid in the likely case of collisionally ionized gas). CIV and OVI are detected along sight lines across the entire LMC, spatially uncorrelated with star-forming regions. Its velocities indicate that it is likely part of a hot, highly ionized corona of the LMC ([75], [31]).

Shock heating through fast stellar winds and, more importantly, supernova explosions are the primary creation mechanisms for hot gas with 10⁶ K [11]. Diffuse hot gas in supershells, however, contributes only 6% to the total X-ray emission from the LMC [54]. LMC2, the supershell east of 30Doradus, has the highest X-ray surface brightness of all the supergiant shells in the LMC. The second highest X-ray surface brightness comes from the yet unexplained extended "spur" south of LMC2. The largest contribution to the LMC X-ray budget comes from discrete X-ray binaries (~ 41%), followed by diffuse field emission (~ 30%), and discrete SNRs (~ 21%) [54].

Finally, the LMC is the only external galaxy detected thus far in diffuse -rays, which are produced by (and directly proportional to) the interaction of cosmic rays (e.g., from supernovae) with the interstellar medium [50]. The integrated flux above 100 MeV is 1.9 ^. 10^-7 photons cm^-2 s^-1 [65].

2.2. The Small Magellanic Cloud

The SMC is the second most massive Milky Way companion (2 ^. 10⁹ M; [79]). With a total HI mass of 4.2 ^. 10⁸ M [66], 21% of its mass are in the ISM. The SMC's dust mass, on the other hand, is only 1.8 ^. 10⁴ M [67], and its average dust to gas mass ratio is 8.2 ^. 10^-5, a factor 30 below the Galactic value. The highest concentrations of dust are found in luminous HII regions. Cold gas appears to be mostly atomic rather than molecular due to the reduced dust abundance, fewer coolants, and a higher UV radiation field ([67], [14]), which help to photodissociate H₂. Less than 15% of the HI is in cold gas, which tends to be colder than in the Milky Way 40 K vs. 50 to 100 K [14]). The diffuse H₂ mass is 2 ^. 10⁶ M, ~ 0.5% of its HI mass and 1/9th of the Galactic value, similar to the reduced H₂ fraction in the LMC [72].

Three HI supershells (> 600 pc) and 495 giant shells were detected in the SMC ([68]; [66]). These shells appear to be expanding. Their rims coincide with a number of HII regions. Their centers lack pronounced H emission in good agreement with their dynamical ages of > 10⁷ years and the propagating star formation scenario proposed by [47], though detailed studies of the stellar age structure are lacking so far. As in the LMC, the ISM of the SMC is fractal [66], likely due to turbulent energy input. The idea that the SMC consists of multiple components that are distinct in location and velocity is not supported by the recent large-scale HI data and was probably an artifact of the complex shell structure of the SMC [68]. On large scales, areas of high HI column densities coincide with the luminous HII regions that form the bar and the wing of the SMC [66]. The distribution of stars younger than 200 Myr also traces these areas of recent massive star formation well [88]. Collisionally ionized gas with a few 10⁵ K forms a hot halo around the SMC and shows enhanced column densities toward star-forming regions [30]. Slightly enhanced diffuse X-ray emission has been detected along the SMC bar [64].

2.3. The Magellanic Bridge and Stream

The SMC has a distance of ~ 60 kpc from the Milky Way and ~ 20 kpc from the LMC. SMC, LMC, and Milky Way interact tidally with each other, which is reflected in, e.g., the HI warp of the Milky Way disk [76], in the thickening of the LMC's stellar disk [77], its elliptical extension toward the Milky Way [74], in the star formation histories of the three galaxies ([22]; [24]; [60]), and most notably in the gaseous tidal features surrounding the Magellanic Clouds.

The LMC and SMC are connected by the "Magellanic Bridge", an irregular, clumpy HI feature with a mass of 10⁸ M that emanates from both Clouds [55]. Cold (20 to 40 K) HI gas has been detected in the Bridge [38], and recent star formation occurred there over the past 10 to 25 Myr [13]. Higher ionized species with temperatures up to ~ 10⁵ K show an abundance pattern suggesting depletion into dust [40]. Interestingly, stellar abundances in the Bridge were found to be ~ - 1.1 dex [61], 0.4 dex below the mean abundance of the young SMC population, which is inconsistent with the proposed tidal origin 200 Myr ago [21]. However, it is conceivable that the Bridge formed from Magellanic Clouds material that mixed with an unenriched component [61], making cloud-cloud collisions a possible star formation trigger [40].

Additional tidal HI features include the leading arm (10⁷ M, 25° length, [55]) and the patchy, clumpy trailing arm (10° × 100°) of the Magellanic Stream, in which no stars have been detected so far [56]. The Magellanic Stream is detected in H due to photoionization by the Galaxy [4]. The abundance patterns of interstellar absorption lines are consistent with those in the SMC, and the H₂ detected in the leading arm may originally have formed in the SMC [63]. Based on their abundances, additional high-velocity clouds in the vicinity may have been torn out of the SMC [42].