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The Baryon Halo of the Milky Way: a Fossil Record of Its Formation Joss Bland-Hawthorn1 and Ken Freeman2

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The Baryon Halo of the Milky Way: a Fossil Record of Its Formation Joss Bland-Hawthorn1 and Ken Freeman2 T HE M ILKY W AY 26. A. Udalski et al., Acta Astron. 43, 289 (1993). Proceedings of the Third Stromlo Symposium, B. Gib- 37. R. Williams et al., Astron J. 112, 1335 (1996). 27. C. Alard, S. Mao, J. Guibert, Astron. Astrophys. 300, son, T. Axelrod, M. Putnam, Eds. (Astronomical Soci- 38. R. Peccei and H. Quinn, Phys. Rev. Lett. 38, 1440 L17 (1996). ety of the Pacific, San Francisco, CA, 1999), pp. (1977). 28. C. Alcock et al., Astrophys. J. 499, L9 (1998). 503Ð514]. Theoretical investigations of what could 39. P. Sikivie Phys. Rev. Lett. 51, 1415 (1983). 29. A. Becker et al., in preparation; C. Alcock et al.,in be learned from this proposed survey are described 40. C. Hagmann et al., Phys. Rev. Lett. 80, 2043 (1998). preparation. by A. Gould [Astrophys. J. 517, 719 (1999)] and by N. 41. I thank my colleagues on the MACHO Project for 30. K. Sahu, Nature 370, 275 (1994). Evans and E. Kerins (preprint available at http:// their help and advice. Everything I know in this 31. D. Zaritsky and D. Lin, Astron J. 114, 2545 (1998). xxx.lanl.gov/abs/astro-ph/9909254). field has been learned through working with them. 32. H.-S. Zhao, in Proceedings of the Third Stromlo Sym- 35. B. Hansen, Nature 394, 860 (1998); Astrophys. J. 520, I also thank M. Merrifield, B. Sadoulet, and K. van posium, B. Gibson, T. Axelrod, M. Putnam, Eds. (As- 680 (1999). tronomical Society of the Pacific, San Francisco, CA, 36. R. Ibata et al., Astrophys. J. 524, 95 (1999). R. A. Bibber for supplying figures. Work performed at 1999), pp. 125Ð129. Me«ndez and D. Minniti (preprint available at http:// the Lawrence Livermore National Laboratory is 33. N. Evans et al., Astrophys. J. 501, 45 (1998). xxx.lanl.gov/abs/astro-ph/9908330) have shown that supported by the U.S. Department of Energy under 34. A specific proposal is described by C. Stubbs [in there are 10 similar objects in the Southern HDF. contract W7405-ENG-48. REVIEW The Baryon Halo of the Milky Way: A Fossil Record of Its Formation Joss Bland-Hawthorn1 and Ken Freeman2 Astronomers believe that the baryon (stellar) halo of the Milky Way re- duction, radiative cooling and heating, and tains a fossil imprint of how it was formed. But a vast literature shows that prescriptions for star formation. The models the struggle to interpret the observations within a consistent framework predict that lower mass clumps are denser, continues. The evidence indicates that the halo has built up through a which is borne out by theory (6) and obser- process of accretion and merging over billions of years, which is still going vation (7). Moreover, the outer parts of gal- on at a low level. Future satellite missions to derive three-dimensional axies are expected to be accreting low mass 7 8 space motions and heavy element (metal) abundances for a billion stars (10 to 10 MJ) objects even to the present will disentangle the existing web and elucidate how galaxies like our own day (8, 9). came into existence. The orbital time scales of stars in the outer parts of galaxies are several billion years and In recent years, we have passed an interesting are gas-rich (mostly disk spiral and irregular it is here we would expect to find surviving landmark. With the most powerful tele- galaxies) and 20% are gas-poor [including remnants of accretion. Observational studies scopes, we can now reach as many galaxies the elliptical, earliest-type (S0) and dwarf of the Galactic halo attempt to find stars of a as there are stars in our Galaxy: about 100 spheroidal galaxies]. In the special environ- given type within a localized region of six- billion sources. The oldest stars in our Galaxy ment of dense galaxy clusters, only about dimensional phase space where each star has are of an age similar to the light travel (look 40% of the galaxies are gas-rich. But in the a velocity (vx,vy,vz) and a location (x,y,z) back) time of the most distant galaxies in the early universe, the Hubble Deep Field has within the Galaxy. Most stellar types can Hubble Deep Field (1). For these galaxies, shown us that galaxies are mostly irregular. exist over a range of metal abundances. The the cosmological redshift (2) measured from Broadly speaking, disk spirals and small heavy element abundance can provide infor- galaxy spectra presently takes us to within spheroids are supported by rotation, whereas mation on when in the Galaxy’s history the 5% of the origin of cosmic time—the Big large spheroids are supported by random stel- star was formed (3). The published literature Bang. For the stars, their upper atmospheres lar motions and have little or no rotation. To on the baryon halo is very extensive and, for provide fossil evidence of the available met- confuse matters, some spheroidal galaxies the most part, in a state of flux. However, als at the time of formation, and astronomers have a disk component, and most disk galax- most astronomers agree that tantalizing clues use a variety of techniques for dating a star ies like the Milky Way have spheroidal com- are beginning to emerge of how the Galaxy from its spectrum (3). The old Galactic stars ponents. While the various galaxy types pose materialized out of the hot, dense, early uni- and the distant galaxies provide a record of a challenge to any formation theory, the rel- verse. In this review, we focus on the fossil conditions at early times in cosmic history, ative importance of the disk and the spheroid evidence from the baryon halo of the Milky and both harbor clues to the sequence of accounts for much of the variety in galaxy Way (near-field cosmology) with occasional events which led to the formation of galaxies morphologies (4). reference to the high redshift universe (far- like the Milky Way. But the oldest stars, like When the early universe was cool enough field cosmology). the most distant galaxies, are exceedingly to form atoms, dark matter and baryons were faint and lie at the limit of modern observing thought to have co-existed in small clumps The Milky Way techniques. (5). As time progressed, gravity caused the Our Galaxy, the Milky Way, can be divided Galaxies as we see them now, at low clumps to cluster together. This picture forms into three parts: a baryon halo (which in- redshift, can be divided into two classes: 80% the basis for the hierarchical cold dark matter cludes the stellar halo and globular clusters), (CDM) model, which places galaxy forma- a baryon disk with the associated stellar tion within a cosmological context. Sophisti- bulge, and an unseen dark (non-baryonic) 1 Anglo-Australian Observatory, PO Box 296, Epping, cated N-body CDM simulations of the halo, which accounts for about 95% of the NSW 2121, Australia. 2Mount Stromlo and Siding Spring Observatories, Private Bag, Weston Creek ACT growth of structures in the early Universe mass of the Galaxy (10) (Fig. 1). The disk 2611, Australia. have been successful at reproducing some and the dark halo are addressed in other To whom correspondence should be addressed: E- observational properties of galaxies (5). Cur- review articles in this special issue (11). mail: [email protected] rent models include gas pressure, metal pro- While the gravitational influence of dark ha- www.sciencemag.org SCIENCE VOL 287 7 JANUARY 2000 79 T HE M ILKY W AY los is easily observed through the rotation of globular clusters dispersed throughout the in- distinct from the faint stellar halo and the the gas in the outer parts of galaxies, its ner and outer halo (Fig. 1). Only a few glob- thick disk, as demonstrated by the 1400 RR character is one of the fundamental, unan- ular clusters are visible to the naked eye, Lyrae variable stars identified by the DUO swered questions of our day. although more than a third of the 150 Galac- microlensing survey (25). While most RR The thin, baryon disk accounts for about tic globular clusters are visible with a pair of Lyrae are associated with the thick disk and 90% of the visible light in the Milky Way high-quality binoculars. Globular clusters are the halo, about 7% are concentrated in the (5% by mass) as we observe from the thin dense swarms of about 100,000 stars and bulge. band of light which arcs across the night sky. constitute the most ancient stellar systems in The characteristic age of spiral bulges is Almost all of the stars seen by the human eye, the Galaxy. Most are so old [13 Ϯ 2.5 billion traditionally assumed to be old but is in fact and in most astronomical photographs, are years ago (Ga)] that they challenge age esti- poorly known, even for the Galaxy (26). The from the thin disk and the bulge. Since the mates of the universe derived from Hubble’s existence of bulge RR Lyrae stars indicates 1980s, we have come to recognize a faint, constant (18). that some fraction of the Galactic bulge is thick disk of old stars extending to 1 to 2 While the baryon halo accounts for only a old. Furthermore, the color-magnitude se- kiloparsecs (kpc) beyond the thin disk (12). small fraction (2%) of the light, and an even quence derived from the Hubble Space Tele- The stellar disk is in rapid, differential smaller fraction (Ͻ0.2%) of the total mass, it scope (HST) (27) shows that the bulge is rotation and contains stars with a very wide plays a key role in unraveling the sequence of predominantly old.
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