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Do We Live in an Elliptical Galaxy? Ferent Relationships Between Intrinsic from M _NA~ru~~~v~o~L.~~~21~1~uL_v_•-~-----------------------NNfEW~S~A~N~D)VV~IEVW~S;-----------------------------------~2~17 7 weigh between 1 kg and 31 tons • K. Through the application of metal­ developed by fitting together their inter­ Esbensen and colleagues8•9 at the Technical lurgical, geochemical and analytical tech­ pretation of star count data (which does University of Denmark found surprisingly niques, and the concerted efforts of not include the recent work of Gilmore and large differences in composition among meteorite hunters and curators across the Reid) with data on stars near the Sun which these samples. For example, they found world, we are beginning to learn much show the high-velocity kinematics required twofold variations in the concentrations of about the nature and formation of metallic for them to spend most of their lives out­ iridium and gold, and up to 10 per cent cores of asteroids. Such studies, when side the disc, and the kinematic data on the change in iridium concentration over 0.9 m augmented by field studies of asteroids, overall rotation of the Galaxy. Adding a in a 20-ton specimen. Such variations are should lead to a greater understanding of concentrated mass component at the centre not compatible with radial plane-front the more inaccessible planetary cores. 0 of the Galaxy, the relative masses they solidification of a core 10-50 km in radius. deduce for disc/spheroid/centre/dark Esbensen and colleagues favour dendritic halo are 5.6:0.27: 1.1: 56. In other words, a Edward R.D. Scott is in the Institute of growth or growth from blocks that were Meteoritics and Department of Geology, dominant dark halo, but a visible spheroid detached from the outer edge of the core University ofNew Mexico, Albuquerque, New which is rather negligible compared with and accumulated nearer the centre. Mexico 87131. the disc. Gilmore and Reid can say nothing about the dark halo (except that its mass is not in the form of nuclear-energy­ Astronomy generating stars), but the reason for the disagreement on the importance of the visi­ ble halo is not obvious. It may lie in the dif­ Do we live in an elliptical galaxy? ferent relationships between intrinsic from M. G. Edmunds brightness and numbers assumed for the spheroid stars by the two groups, but the IT is perhaps surprising, at a time when a stellar types and distances from infrared true picture may only surface in later great deal of astronomical research is photometry, to push the determination of debate. directed to probing the most distant parts the relative numbers of stars of different in­ Further interesting evidence of the rela­ of the observable Universe, that fun­ trinsic brightness to about 100 times fainter tionship between spiral and elliptical galax­ damental details of the structure of our than previous reliable determinations. ies has come from the comparison of the own Galaxy should remain uncertain. We Then, by counting the relative numbers of kinematics of stars in the spheroidal com­ know quite a lot about its most con­ stars of different observed brightness and ponents of spirals with the kinematics of spicuous part - the thin disc of stars, gas colour towards the South Galactic Pole stars in ellipticals. A few years ago much and dust in which active star formation is (that is, up through and out of the plane of excitement was generated by the realization occurring, and which contains our own the Galaxy), they determine the distribu­ that bright massive elliptical galaxies were Sun- but the faint 'spheroid' of old stars tion in space of the stars2 . They identify not not flattened into their elliptical shape by which surrounds the disc is less well only the well known thin galactic disc, but rotational forces. The result was a flurry of understood. Two recent, and conflicting, also what they claim is a 'thick' disc exten­ theoretical work on the dynamics of such studies of the number of stars in this ding above the plane some four times fur­ systems, and it was subsequently observa­ spheroid highlight the uncertainty; and ther than the thin disc. This thick disc may tionally demonstrated that, in contrast, the together with new information on the represent a somewhat flattened spheroid spheroidal components of spiral galaxies dynamical properties of the spheroids of distribution, although (as I shall argue in did rotate fast enough to provide their other galaxies, they show that we can no detail elsewhere) the resultant mass of such observed flattenings. This argued for a fun­ longer ignore the possibility that the a spheroid could be large- indeed it could damental difference between ellipticals and spheroid may be a major (or even domi­ be between one-third and three times the spheroidal components. But the spheroids nant) component of the Galaxy. mass of the disc. Such a thick disc or of spirals are typically considerably less Since the ages and distribution of stars in spheroid (and the two models cannot be luminous than the large elliptical galaxies the spheroid may be very like that of an distinguished on the basis of the present that had been observed, and a recent paper elliptical galaxy, it is possible to speculate star counts) would then have to be accepted by Davies et a/. 5 has shown that ellipticals that our Galaxy is simply an elliptical which as a critical component in the formation of comparable luminosity to the spheroids happens to have been born with, or ac­ and evolution of the Galaxy. of spirals do rotate in the same way. Thus it quired- perhaps very early on - a disc of Examples of dominating spheroids are is the bright massive ellipticals which are in gas in which classic spiral structure subse­ known in other spiral galaxies. A recent some way unusual, and the spheroids of quently developed. series ofpapers by P. C. van der Kruit and spirals appear very similar to the ellipticals The difficulty of judging the importance I. Searle shows a range from fairly negligi­ of comparable brightness. of the spheroidal component comes from ble spheroids right up to cases (see, for ex­ The true nature of the dark halo matter its intrinsic faintness. It contains few (if ample, ref.3) in which the visible spheroid in our own and other galaxies remains an any) of the short-lived, bright and recently has at least as much mass as the disc, and embarrassing problem, but the recent formed massive stars that delineate spiral probably more. It has become customary to research indicates that the study of what we structure in the disc. Furthermore, distinguish between 'spheroid', which can see - the spheroid stars in our Galaxy although the local density of spheroid mass means the visible stars which are not part of - may be more rewarding than has often near the Sun is very much less than that of the disc, and the 'halo' which means the been realized. The spheroid may be the disc stars, the total numbers of stars could almost spherical distribution of dark (or closest elliptical galaxy that we can study. 0 be comparable when summed over the almost non-luminous) matter which is in­ much larger volume occupied by the voked to explain the behaviour of rotation M.G. Edmunds is a/the Department ofApplied Mathematics and Astronomy, University Col­ spheroid. velocity as a function of radius in our own lege, CardiffCFJ /XL. Stars can now be counted on and other spiral galaxies. While the rota­ photographic plates by measuring 1. Gilmore, G. & Reid, N. Mon. Not. R. astr. Soc. tion curves seem to have forced us to accept 201, 73 (1982). machines with computer-controlled scan­ a dominance of the halo mass, the con­ 2. Gilmore, G. & Reid, N. Mon. Not. R. astr. SO<". ning light beams, with a vast increase in tribution of spheroids has, until recently, 202. 1025 (1983) . 3. van der Kruit, P.C. & Searle, I. Astr. Astrophys. speed and objectivity over earlier manual remained uncertain. 110.79 (1982). and ocular methods. G. Gilmore and N. A somewhat different picture from that 4. Bahcall, J.N .. Schmidt, M. & Soneira, R.M. Astrophys. J. 1 26!1. 730 (1983). Reid have used these new techniques, of Gilmore and Reid has been painted by 5. Davies, R.L.. Efstathiou, G., Fall, S.M., lllingworth. G. 4 linked with fairly extensive calibration of J. N. Bah call et a!. • Their model is & Schechter, P. L. Asrrophys. J. 266. 41 ( 1983). 0028.0836/83/290217-01$01.00 © 1983 Macmillan Journals Ltd .
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