X-ray emission from the Magellanic Clouds was first observed in a five-minute rocket flight from Johnston Atoll in the South Pacific on October 29, 1968. The LMC was detected as an ∼ 4 σ excess in two adjacent 5° bins, the flux was ∼ 1.5 × 10−9 erg cm−2 s−1 and the spectrum was slightly softer than that of the diffuse background (Mark et al. 1969). Two years later two source regions in the LMC were identified by Price et al. (1971) and emission from the SMC was recorded. The same year Leong et al. (1971) showed that the LMC emission could be resolved by the collimated detector system of the Uhuru satellite into three steady and one possible highly variable source; they were designated LMC X-1, X-2, X-3, and X-4. There was also a possible diffuse emission extending over much of the Cloud. The SMC emission was located to a single, highly variable source, called SMC X-1, in the Wing region. It was the first stellar X-ray source to be confirmed in an external galaxy.

Confirmation of the existence of LMC X-4 was presented in the second Uhuru catalogue (Giaconi et al. 1972).

The LMC sources X-1, X-2, and X-3 were confirmed by Copernicus satellite observations (Rapley and Tuohy 1974) and given more accurate positions. Also Markert and Clark (1975), using the OSO 7 satellite confirmed these three sources, defined an upper limit for LMC X-4, and introduced a fifth source, LMCX-5.

1. LMC = Large Magellanic Cloud

2. SMC = Small Magellanic Cloud

3. ISM = Interstellar Medium

4. BC = Bolometric Correction

5. Mboi = absolute bolometric magnitude

6. Lbol = bolometric luminosity

7. Teff = effective temperature

8. Γ = –x = Slope of the IMF

9. X,Y,Z = relative content of H, He, and heavy elements (M)

10. [M/H] = logN(M/H)star - logN(M/H)⊙

11. FWHM = Full Width at Half Measure

12. SSPSF = Stochastic Self-Propagating Star Formation

13. LF = Luminosity Function

14. IMF = Initial Mass Function

15. CMD = Colour Magnitude Diagram

16. HR diagram = Hertzsprung Russell Diagram

17. MS = Main Sequence

18. ZAMS = Zero-Age Main Sequence

19. AGB = Asymptotic Giant Branch

20. TP = Thermal Pulse

21. E-AGB = Early AGB

22. TP-AGB = Late AGB

23. RGB = Red-Giant Branch

24. SGB = Sub-Giant Branch

25. HB = Horizontal Branch

26. RGC = Red-Giant Clump

27. PN = Planetary Nebula

28. SN = SuperNova

29. SNR = SuperNova Remnant

As discussed in Chap. 3, the LMC, the SMC and the Galaxy form an interacting system. At times the interactions have had severe effects. Features observed in the LMC and the SMC, including radial velocities of individual objects, are therefore not necessarily determined solely by their rotation and motions.

When the first HI 21 cm-line radial velocities were measured, it was concluded that both Clouds were rotating systems, flattened and tilted and with extensive spiral structures (Kerr and de Vaucouleurs 1955a,b). The rotational motions were at first looked for using the optical centres, 5h24m, −69°.8 for the LMC and 0h51m,−73°. 1 for the SMC. In order to obtain symmetrical rotation curves it was, however, necessary to introduce radio centres of rotation: in the LMC, at 5h20m, −68°.8, appreciably displaced from the optical centre, and in the SMC 1h10m, − 73°.25. The centre of rotation in the LMC has since been a source of much discussion (see Sect. 3.4.4). In the SMC the problems are of another nature.

The structure and kinematics of the LMC

Images of the LMC in most wavelength regions are dominated by radiation from its Extreme Population I constituent (stellar associations, supergiants, etc.) or the connected gas (HII regions, HI complexes, molecular clouds) and dust which display the regions of recent star formation as an asymmetric pattern, not completely at random but with some structure.

Cepheids, RR Lyrae stars, Mira variables, clusters and OB stars have dominated in the attempts to determine accurate distances to the Magellanic Clouds in recent years. The subject is of vital importance not only for Magellanic Cloud research but also for the determination of extragalactic distances (see Feast 1988, van den Bergh 1989, de Vaucouleurs 1993). With the accurate photometry possible nowadays the uncertainties in the distance determinations arise mainly through their dependence on the metallicities and ages of the objects involved and on the interstellar absorption in the line of sight, be it in the Galaxy or in the Clouds themselves.

The reddening, foreground and internal

The interstellar reddening affects virtually all studies of the Clouds. It is frequently difficult to determine its magnitude from the material available and mean values found in the literature are then, by necessity, applied. This may occasionally lead to erroneous results, in particular as the objects used to determine the mean may not be representative of the whole galaxy. In the following, some investigations will be referred to either as giving general ideas about the mean reddening or as examples of typical deviations.

The galactic foreground colour excess towards the Magellanic Clouds has been investigated by Schwering and Israel (1991) on a scale of 48 arcmin from galactic HI maps, using EB-v = 0.17 10−21 N(HI) H cm−2.

Knowledge about the chemical abundances of objects of various ages in the Magellanic Clouds is essential for our understanding of their evolution in the past. The abundances of their lighter elements have been derived from spectra of HII regions, and those of the heavier elements from spectra of supergiants. An extensive study was carried out by Pagel et al. (1978). A summary of the composition of the HII regions was presented by Dufour (1984). Little has changed since then. The summary is still often used as a reference for comparison with stellar abundances, and it is therefore reproduced in Table 11.1.

A problem in interpreting the many results derived for the metallicity of cluster and field stars is the model dependence. The early works pointed towards an underabundance in the cluster stars relative to the field stars, but, as will be seen below, there is a clear tendency towards more and more agreement in metallicity between cluster and field stars. Differences have also frequently been found between the chemical abundances of the ISM and the stars in the Clouds, though warnings against overinterpreting the observational results have been given (Pagel 1993). The uncertainties in the derived data, for the stars as well as for the ISM, are still considerable.

For the study of the stellar abundances in the Magellanic Clouds only the most luminous and most extreme supergiants (A-type, Mv ≤ −9) were available for high-dispersion studies up to about 1975.

For a complete picture of the history of star formation in the Magellanic Clouds, knowledge about the age distribution as well as the surface distribution of the intermediate-age and the oldest stars is needed. The evolved red giants are the most luminous in these generations and have therefore attracted most interest and contributed much to our understanding of the stellar evolution. It is now possible to reach stars as faint as V = 24 mag, so that main-sequence field stars much older than 3–4 Gyr may be observed in the Clouds. Their importance for solving star formation problems will increase as observations of still fainter stars have become available (using the HST). A brief summary of what is known today is therefore appropriate.

Main-sequence stars

A large number of colour–magnitude diagrams reaching main-sequence stars below Mv = 2 mag have been presented for regions in many parts of the LMC. The bulk of stars appear to have formed about 3–4 Gyr ago (Butcher 1977, Stryker 1984, Hardy et al. 1984, Hodge 1987). Some doubts have been expressed about the reliability of the photographic photometry on which these results were based, as well as on the age calibrators used, but the overall results have been confirmed (Vallenari et al. 1994c). The main-sequence differential luminosity functions for seven LMC fields are compared in Fig. 7.1.

30 Doradus is the most luminous HII region in the Local Group of Galaxies and is outstanding in many ways. Its structure is fascinatingly complex, with a large number of bright arcs and apparently dark areas in between, and it is also eloquently called the Tarantula Nebula. Its precise boundary has been difficult to establish. In the Henize (1956) catalogue a large nebulosity, N135, ∼ 2 deg, encompasses 30 Doradus, SGS LMC 2 and SGS LMC 3, and would, seen at a greater distance, be identified as a single supergiant HII region (Fig. 10.1). Its central portion, N157≡DEM263 (Davies et al. 1976, DEM), contains in its NW part N157A, 30 Doradus ≡NGC 2070, and N157B ≡NGC 2060. Henize estimated the diameter of N157A to be ∼ 30′ often values as low as 15′ may be seen for 30 Doradus.

30 Doradus provides a most spectacular O association for which detailed information is now available (Fig. 10.2). Its brightest object, R136 ≡HD 38268, now known to be a cluster of clusters in itself, was for a while considered to be a supermassive star by many astronomers. Several of the brighter members in the immediate surroundings of the dense cluster core belong to the rare O3If*/WN-A class (Melnick 1985, Walborn 1986) and are among the most massive stars known.

The interaction in a galaxy between the stars and the interstellar medium (ISM) through mass loss, accretion, and the formation of new objects is exceedingly important for its evolution. The composition of its ISM tells us about its past history and, correctly understood, also about its future. The ISM represents the youngest generation and has therefore, and as a consequence of experience from our Galaxy, been considered to be basic in studies of the kinematics of external galaxies. It has played, and plays, a fundamental role in all discussions of the Magellanic Clouds. Historically, the Lick Expedition radial velocities of a few emission nebulae in the LMC and one in the SMC (see Chap. 1) served chiefly to prove that the Clouds were outside the main body of the Galaxy. An indication of a gradient of velocities in the LMC pointed towards rotation or translation. The existence of this gradient was put beyond doubt in the 1950s, when 21 cm HI line observations began. Since then, other components of the ISM have been observed, making important contributions regarding the composition and kinematics of the Clouds.

The neutral hydrogen

The continuous radio emission from the Magellanic Clouds was first detected by Mills in 1953. Radio isophotes at 3.5 m were obtained in 1954 (Mills 1954). The line radiation from neutral hydrogen in the Clouds was detected in 1953 and a preliminary survey of its distribution was presented (Kerr et al. 1954).

Luminous stars in the Magellanic Clouds are very suitable for the study of evolutionary processes because their distances and, hence, their luminosities can be determined with a reasonably good precision, permitting a direct comparison with theoretical tracks in the HR diagram. The evolutionary paths of these massive stars depend critically upon the mass-loss rates at different stages of the stellar development. The mass lost may determine whether the stars evolve back to the blue or end their lives as red supergiants (e.g. Chiosi and Maeder 1986, Wood and Faulkner 1987). There are at least some stars in the LMC and the SMC showing that a post-red supergiant evolution occurs, resulting in anomalous He-burning A-type supergiants (Humphreys et al. 1991). Other stars, more massive, may return to the blue stage as WR stars (Sect. 5.2.3 or, like SN 1987A, as B supergiants (Sect. 9.4).

The youngest population in the Magellanic Clouds (age ≤ 100 Myr) contains the most recently formed stars and the remaining or returned gas and dust. Best studied among the stars are the OB stars and a minor number of supergiants and hypergiants of types B–G, suitable for abundance analyses. Massive stars are believed to be born primarily in clusters and in associations. There is, however, a fair number of massive stars so far from any cluster or OB association that they can hardly have moved from those centres of star formation to their present locations during their short lifetimes.

Problems of the Magellanic Clouds, W. Buscombe, S.C.B. Gascoigne, G. de Vaucouleurs, 1955, Austr. J. Sci. Suppl. 17, No. 3.

Recent Developments in Studies of the Magellanic Clouds, A.D. Thackeray, 1963, Advances in Astron. Astrophys. 2, 264.

The Galaxy and the Magellanic Clouds, IAU/URSI Symp. No. 20,1964, eds. F.J. Kerr, A.W. Rodgers (Austr. Acad. Sci., Canberra).

The Magellanic Clouds, Symposium at Mt Stromlo Observatory, 1965, eds. J.V. Hindman, B.E. Westerlund, E.R. Wilkie.

Magellanic Clouds, B.J. Bok, 1966, Ann. Rev. Astron. Astrophys. 4, 95.

The Stellar Content of the Magellanic Clouds, B.E. Westerlund, 1970, Vistas in Astronomy, 12, 335.

The Magellanic Clouds, European Southern Observatory Symposium in Santiago de Chile, March 1969, 1971, ed. A. Muller (D. Reidel, Dordrecht).

Die Magellanschen Wolken, Th. Schmidt, 1972, Mitteilungen der Astron. Geseltschaft Nr. 31.

Structure and Dynamics of Barred Spiral Galaxies, in particular of the Magellanic Type, G. de Vaucouleurs and K.C. Freeman, 1973, Vistas in Astr. 14, 163.

Galaxien vom Magellanischen Typ, J.V. Feitzinger, 1980, Space Sci. Rev. 27, 35.

Structure and Evolution of the Magellanic Clouds, IAU Symp. No. 108, 1984, eds. S. van den Bergh, K.S. de Boer (D. Reidel, Dordrecht).

The Magellanic Clouds: their evolution, structure and composition, B.E. Westerlund, 1990, A&AR 2, 29.

The Magellanic Clouds, IAU Symp. No. 148, 1991, eds. R. Haynes, D. Milne (Kluwer, Dordrecht).

Recent Development of Magellanic Cloud Research, A European Colloquium, 1989, eds. K.S. de Boer, F. Spite, G. Stasińska.

Broad emission lines are one of the dominant features of many AGN spectra. The broad-line region (BLR) plays a particularly important role in our understanding of AGNs by virtue of its proximity to the central source. The BLR potentially can provide a useful probe of the central source for at least two reasons. First, the bulk motions in the BLR are almost certainly determined by the central source, with gravity and radiation pressure competing. Second, the BLR reprocesses energy emitted by the continuum source at ionizing ultraviolet energies that cannot be observed directly, and thus the emission lines provide indirect information about this important part of the continuum.

Broad-Line Spectra

The broad-line spectra of AGNs show a great deal of diversity in terms of relative line strengths and profiles. The spectra shown in Figs. 1.1, 1.2, and 2.2 should be understood to be only more or less typical, as are the line strengths and equivalent widths of the stronger lines given in Table 1.1.

Widths of the broad lines show considerable differences from object to object. It is assumed that the lines are Doppler-broadened (§5.2), so line widths are almost always measured in velocity units, usually in terms of either (a) the full width at half maximum intensity (FWHM) or (b) the full width at zero intensity (FWZI); while the latter better reflects the true range of line-of-sight velocities, it is subject to uncertainties arising from ambiguities as to where the line actually reaches the continuum level and, in cases of especially high velocities, blending of the wings of different lines.