Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.stecf.org/coordination/esa_eso/galpops/report.pdf Äàòà èçìåíåíèÿ: Thu Sep 4 17:58:31 2008 Äàòà èíäåêñèðîâàíèÿ: Mon Oct 1 23:59:08 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: carina



Rep ort by the ESA-ESO Working Group on Galactic p opulations, chemistry and dynamics

Abstract
Between the early 40s, when Baade showed the first evidence for the existence of two distinct stellar populations, and today, with our Galaxy surprising us with new substructures discovered almost on a monthly basis, it is clear that a remarkable progress has been achieved in our understanding of the Galaxy, of its structure and stellar populations, and of its chemical and dynamical signatures. Yet, some questions have remained open and have proven to be very challenging. The main task of this Working Group has been to review the state-of-theart knowledge of the Milky Way galaxy, to identify the future challenges, and to propose which tools (in terms of facilities, infrastructures, instruments, science policies) would be needed to successfully tackle and solve the remaining open questions. Considering the leadership position that Europe has reached in the field of Galactic astronomy (thanks to the Hipparcos mission and the Very Large Telescope) and looking at the (near-)future ma jor initiatives it has undertaken (VISTA and VST survey telescopes, Gaia mission), this work clearly has been very timely. It is of uttermost importance for European astronomy to keep and further consolidate its leading position. This Working Group has made recommendations that would allow dissecting our backyard laboratory, the Galaxy, even further. ESO survey telescopes about to become operational and the upcoming ESA Gaia mission are a guarantee for opening new horizons and making new discoveries. We, the astronomers, with the support of our funding agencies, are ready to fully commit to the best exploitation of the treasure that is ahead of us. The main recommendations this Working Group has made to ESA and ESO are to guarantee the expected tremendous capabilities of these new facilities, to vigourously organise their synergies and to jointly give ways to European astronomers to be leaders in the exploitation of their output data.


Background
Following an agreement to cooperate on science planning issues, the executives of the European Southern Observatory (ESO) and the European Space Agency (ESA) Science Programme and representatives of their science advisory structures have met to share information and to identify potential synergies within their future pro jects. The agreement arose from their joint founding membership of EIROforum (www.eiroforum.org) and a recognition that, as pan-European organisations, they serve essentially the same scientific community. At a meeting at ESO in Garching during September 2003, it was agreed to establish a number of working groups that would be tasked to explore these synergies in important areas of mutual interest and to make recommendations to both organisations. The chair and co-chair of each group were to be chosen by the executives but thereafter, the groups would be free to select their membership and to act independently of the sponsoring organisations. The following membership and terms of reference for the working group on "Galactic populations, chemistry and dynamics" was agreed on:

Memb ership
Catherine Turon (Chair) Francesca Primas (Co-Chair) James Binney Cristina Chiappini Janet Drew Amina Helmi Annie Robin Sean Ryan catherine.turon@obspm.fr fprimas@eso.org binney@thphys.ox.ac.uk cristina.chiappini@obs.unige.ch j.drew@herts.ac.uk ahelmi@astro.rug.nl annie@obs-besancon.fr s.g.ryan@herts.ac.uk

Additional contributors: Yveline Lebreton, Fran¸ coise Combes

Organisation and co ordination: Wolfram Freudling Thanks and acknowledgments Thanks are also due to Ted Dame, Mike Irwin, Sergio Molinari, Toby Moore, Stuart Sale, Eline Tolstoy, and Sandro D'Odorico who provided figures or advanced information on their work. ii


Terms of Reference
(1) To outline the current state of knowledge of the field. (This is not intended as a free-standing review but more as an introduction to set the scene.) (2) To review the observational and experimental methods used or envisaged for the characterisation of the Galactic population and dynamics. (3) To perform a worldwide survey of the relevant programmes and associated instruments that are operational, planned or proposed, both on the ground and in space. (4) For each of these, to summarise the scope and specific goals of the observation/experiment and to point out the limitations and possible extensions; (5) Within the context of this global effort, examine the role of ESO and ESA facilities; analyse their expected scientific returns; identify areas of potential overlap and thus assess the extent to which the facilities complement or compete; identify open areas that merit attention by one or both organisations and suggest ways in which they could be addressed. (6) Make an independent assessment of the scientific cases for large facilities planned or proposed. (7) Propose sets of recommendations on how ESO and ESA, both separately and together, can optimise the exploitation of current and planned missions, and how the agencies can collaborate in the planning of future missions. (8) The chair of the working group is appointed by ESO and ESA. The chair will select a co-chair. Other working group membership will be established by the chair and co-chair. The resulting views and recommendations made in the final report will be the responsibility of the group alone. A final report should be submitted to ESO and ESA by mid-2008.

Catherine Cesarsky(ESO)

Alvaro Gim´ enez (ESA)

Decemb er 2006

iii


Contents
Memb ership Terms of Reference 1 Executive summary 2 Intro duction 3 Outline of the current state of knowledge of the field 3.1 The main structures of the Galaxy . . . . . . . . . . . . . . . . . . 3.1.1 3.1.2 3.1.3 3.1.4 3.1.5 3.1.6 3.1.7 3.1.8 3 .2 The halo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The bulge . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii iii 1 3 7 7 8 9

The disc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Fossil populations . . . . . . . . . . . . . . . . . . . . . . . . 15 Gas and dust content . . . . . . . . . . . . . . . . . . . . . . 16 OB associations and open clusters . . . . . . . . . . . . . . . 18 The globular clusters . . . . . . . . . . . . . . . . . . . . . . 19 The satellites . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Present day star-formation census . . . . . . . . . . . . . . . . . . . 22 3.2.1 3.2.2 3.2.3 3.2.4 3.2.5 3.2.6 The present day initial mass function . . . . . . . . . . . . . 23 Environmental factors in star formation . . . . . . . . . . . 24

Star formation as a structure tracer . . . . . . . . . . . . . . 26 Star clusters and the cluster mass function . . . . . . . . . . 30 Star formation profile along the Galactic plane . . . . . . . . 31 Interactions with the intergalactic medium (IGM) . . . . . . 34

3.3

Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 iv


3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.3.6 3.3.7 3.3.8 3.4

The central few parsecs . . . . . . . . . . . . . . . . . . . . . 38 The bar/bulge . . . . . . . . . . . . . . . . . . . . . . . . . . 39 The rotation curve . . . . . . . . . . . . . . . . . . . . . . . 42 The solar neighbourhood . . . . . . . . . . . . . . . . . . . . 45 Spiral structure . . . . . . . . . . . . . . . . . . . . . . . . . 46 Tidal shredding . . . . . . . . . . . . . . . . . . . . . . . . . 48 The warp . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Dark matter . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Composition, kinematics and ages of stellar populations . . . . . . . 55 3.4.1 3.4.2 3.4.3 3.4.4 3.4.5 3.4.6 Age determinations . . . . . . . . . . . . . . . . . . . . . . . 55 Stellar yields . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Timescales . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Primordial nucleosynthesis . . . . . . . . . . . . . . . . . . . 62 Ages, kinematics, and chemical abundances . . . . . . . . . 67 Comparative studies in the Local Group . . . . . . . . . . . 70

3.5

Galaxy assembly and evolution . . . . . . . . . . . . . . . . . . . . 74 3.5.1 3.5.2 3.5.3 3.5.4 History of the halo . . . . . . . . . . . . . . . . . . . . . . . 75 History of the bulge . . . . . . . . . . . . . . . . . . . . . . . 76 History of the disc . . . . . . . . . . . . . . . . . . . . . . . 78 History of the globular-cluster system . . . . . . . . . . . . . 82 84

4 The Galaxy and the Lo cal Group as templates 4.1 4.2 4.3

To constrain stellar evolution models and their uncertainties . . . . 84 To constrain population synthesis models . . . . . . . . . . . . . . . 92 To constrain galaxy formation models . . . . . . . . . . . . . . . . . 94 v


4.4 4.5

To constrain dynamical models and theories of the nature of dark matter and gravity . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 To interpret observations of galaxies beyond the Local Group . . . . 98 101

5 Top questions, and prop osed solutions 5.1

Global questions and proposed solutions . . . . . . . . . . . . . . . 101 5.1.1 5.1.2 5.1.3 5.1.4 5.1.5 5.1.6 5.1.7 5.1.8 Which stars form and have been formed where? . . . . . . . 101 What is the mass distribution throughout the Galaxy? . . . 102 What is the spiral structure of our Galaxy? . . . . . . . . . 103 How is mass cycled through the Galaxy? . . . . . . . . . . . 104 How universal is the initial mass function? . . . . . . . . . 106

What is the impact of metal-free stars on Galaxy evolution? 107 What is the merging history of the Galaxy? . . . . . . . . . 108 Is the Galaxy consistent with CDM? . . . . . . . . . . . . 109

5.2

The central pc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 5.2.1 5.2.2 5.2.3 Open questions . . . . . . . . . . . . . . . . . . . . . . . . . 111 Tools and tracers . . . . . . . . . . . . . . . . . . . . . . . . 111 Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

5.3

The bar/bulge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 5.3.1 5.3.2 5.3.3 Open questions . . . . . . . . . . . . . . . . . . . . . . . . . 112 Tools and tracers . . . . . . . . . . . . . . . . . . . . . . . . 113 Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

5.4

The thin disc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 5.4.1 5.4.2 5.4.3 Open questions . . . . . . . . . . . . . . . . . . . . . . . . . 115 Tools and tracers . . . . . . . . . . . . . . . . . . . . . . . . 116 Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 vi


5.5

The thick disc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 5.5.1 5.5.2 5.5.3 Open questions . . . . . . . . . . . . . . . . . . . . . . . . . 118 Tools and tracers . . . . . . . . . . . . . . . . . . . . . . . . 119 Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

5.6

The stellar halo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 5.6.1 5.6.2 5.6.3 Open questions . . . . . . . . . . . . . . . . . . . . . . . . . 120 Tools and tracers . . . . . . . . . . . . . . . . . . . . . . . . 121 Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

5.7

The globular cluster system . . . . . . . . . . . . . . . . . . . . . . 122 5.7.1 5.7.2 5.7.3 Open questions . . . . . . . . . . . . . . . . . . . . . . . . . 122 Tools and tracers . . . . . . . . . . . . . . . . . . . . . . . . 123 Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

5.8

The dark matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 5.8.1 5.8.2 5.8.3 Open questions . . . . . . . . . . . . . . . . . . . . . . . . . 124 Tools and tracers . . . . . . . . . . . . . . . . . . . . . . . . 125 Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

6 Relevant programmes and asso ciated instruments, ground-based and space 127 6.1 6.2 6.3 6.4 Astrometric surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Spectroscopic surveys . . . . . . . . . . . . . . . . . . . . . . . . . . 132 Photometric surveys . . . . . . . . . . . . . . . . . . . . . . . . . . 136 Other ground-based facilities . . . . . . . . . . . . . . . . . . . . . . 143 147 152 172 vii

7 Recommendations References List of abbreviations


1

Executive summary

ESA and ESO initiated a series of Working Groups to explore synergies between space and ground-based instrumentation in astronomy. The first three working groups dealt with Extra-Solar Planets (Perryman et al., 2005), the Herschel-ALMA Synergies (Wilson & Elbaz, 2006) and Fundamental Cosmology (Peacock et al., 2006). This, the fourth Working Group concentrated on Galactic stellar populations, Galactic structure, dynamics, chemistry and history. This work was especially timely given the planning currently being undertaken for Gaia by ESA and the impending first-light of dedicated survey telescopes (VISTA and VST) in the optical and near-infrared by ESO. Moreover, in the future E-ELT era, new utilisations of 4- and 8-meter telescopes and instrumentation can be envisaged. In this context, a few remarks should be highlighted: · the volume and quality of data that Gaia will provide will revolutionise the study of the Galaxy even more than Hipparcos revolutionised the study of the solar neighbourhood · there is a need for very large statistically-significant samples in order to undertake many of the dynamical, kinematic and compositional studies of the Galaxy · it will be important to develop the capabilities to cover what Gaia will not, such as high-resolution spectroscopy follow-up for a large number of targets selected from Gaia data, medium-resolution spectroscopy for a large number of selected faint stars for which no spectroscopic data will be obtained from Gaia, or achieving wide wavelength coverage in photometry and spectroscopy · advances in infrared (IR) astronomy will allow us to tap the benefits of infrared wavelengths for astrometric, spectroscopic and photometric observations of the obscured Galactic bulge and Galactic central region · stellar population science needs access to the visible (including blue) spectrum of stars in the Galaxy, and not to just the IR which is favoured by much cosmological work. · the cost of these forthcoming data justify, and indeed demand, significant improvements to underlying theory, modelling and analysis techniques Europe is now poised to take a leading position in Galactic research thanks to an ambitious space mission and to innovative ground-based telescopes. The 1


ma jor recommendations resulting from this Working Group are for ESA to pay a particular attention to guaranteeing the expected tremendous capabilities of Gaia; for ESO to consider the construction of highly multiplexed spectrographs for follow-up and complementary observations of selected Gaia targets; and for ESA and ESO to jointly organise the exploitation of synergies between Gaia and ground-based observations, and consider ways to give European astronomers a lead in the exploitation of the Gaia catalogue. Detailed recommendations are given in Section 7.

2


2

Intro duction

The blackout of Los Angeles from 1942 enabled Walter Baade (1944) to resolve the brightest stars in M31 and two of its companions. He found them to be red giants, similar to those seen in globular clusters, rather than the blue giants that are the most luminous stars near the Sun. Baade concluded galaxies comprise two stellar populations, Population I typified by the Solar neighbourhood, and Population II typified by globular clusters. He assumed that blue giants are young because they are associated with dust and gas, so he inferred that Population I is still being formed, while Population II was formed in the past. In the years between Baade's seminal paper and the Vatican conference in 1958 the concept of a stellar population struck deep roots. Spectroscopic observations and the emerging theory of stellar evolution showed that Population I contains young, metal-rich stars, whereas Population II stars are all old and metal-poor. Population I stars are all on nearly circular orbits, whereas Population II stars are generally on highly inclined and/or eccentric orbits. Moreover, within Population I the ages, metallicities and random velocities of stars are correlated in the sense that younger, more metal-rich stars have orbits that are more nearly circular than those of older, more metal-poor stars. Thus by 1958 Baade's original simple dichotomy had blossomed into a concept that connects stellar evolution and the progressive enrichment of the interstellar medium (ISM) with heavy elements, to the pattern of star formation in the Galaxy and the Galaxy's dynamical evolution. Much of contemporary astrophysics is still concerned with using this connection to infer the history of galaxies, especially our own. At this point it is useful to define two terms. A simple stel lar population is a group of stars that are formed at a single time from gas of given chemical composition. Globular clusters are the ob jects that most nearly realise the definition of single stellar population. Galaxies always have more complex stellar populations. For example, the Galactic bulge contains stars that have a measurable spread in age, even though they are all old, and cover a significant range in metallicity. We can consider a compound population such as that of the bulge to be a superposition of a large number of simple stellar populations. The population of the solar neighbourhood is a compound population that differs from that of the bulge in that it contains very young stars as well as old ones. An obvious question is whether the compound population of the solar neighbourhood can be represented as a sum of simple stellar populations, one for each time in the history of the disc. Remarkably this is not possible because the stars of a given age have a significant spread in metallicity. 3


Within any annulus around the Galactic centre, interstellar gas is believed to be well mixed and to have a well-defined metallicity, which increases over time, most rapidly at small galactocentric radii. Therefore the stars that form in an annulus within a small time interval constitute a simple stellar population. The spread in metallicity of the stars near the Sun that have a given age implies that the simple stellar populations that form in different annuli mix over time. This mixing is probably driven by spiral structure and the Galactic bar, so by decomposing the compound stellar population of the disc into simple stellar populations and taking account of the uncertainties affecting metallicities and stellar ages, it should be possible to learn about the Galaxy's dynamical and chemical history. So far the word metal licity is used as if it were a one-dimensional variable. In fact, while stars that are deficient relative to the Sun in, say, iron by a factor 100 will be strongly deficient in sodium or magnesium by a substantial factor, that factor may differ appreciably from 100, and we can learn a lot from what that factor actually is. The basic application of this idea is to measure [/Fe]1 , which is the abundance relative to iron of the nuclides (16 O, 20 Ne, 24 Mg, 28 Si, 32 S, 36 Ar and 40 Ca). The nuclides are synthesised alongside Fe in stars more massive than 8 M < that explode as core-col lapse supernovae 30 Myr after the star's birth, scattering a mixture of nuclides and Fe into the interstellar medium (ISM). Consequently, any episode of star formation that lasts longer than 30 Myr will contain stars with both nuclides and Fe synthesised in massive stars that formed earlier in the episode. Some fraction of stars with masses < 8 M explode as type Ia supernovae 1 Gyr after the star's birth, and these supernovae scatter mainly Fe into the ISM. So if a star-formation episode lasts longer than 1 Gyr, it can lead to the formation of stars enhanced in Fe relative to the nuclides, or low [/Fe]. In general, Population II stars have higher [/Fe] than Population I stars, indicating that the ob jects to which they belong formed on timescales shorter than 1 Gyr. In the case of globular clusters this deduction can be verified by showing that the cluster's stars are distributed in the colour-magnitude diagram as we would expect if they were all formed at the same time. This time-scale of 1 Gyr is well established in the solar vicinity, but might be much smaller in other environments and other types of galaxies. The gas clouds that give rise to low-mass star clusters and associations may be enriched by only a handful of supernovae, with the result that many of the cluster's stars have abundance patterns that reflect peculiarities of the enriching supernovae. Thus if one looks in great detail at the abundances of stars in a given cluster, one may see a pattern that is unique to the cluster, in the same way that
where the bracket notation [X/Y] refers to the logarithmic ratio of the abundances of elements X and Y in the star minus the same quantity in the Sun
1

4


the members of a particular tribe may reveal a characteristic genetic sequence. Moreover, clusters and associations are sub ject to disruption, and stars that were once in a cluster may now be widely distributed within the Galaxy. By looking for abundance patterns ­ genetic fingerprints ­ in field stars, it may be possible to identify members of a cluster that was disrupted long-ago. Thus depending on the criteria used to define a stellar population, it can range in scale from the members of a disrupted star cluster to Baade's original Population I, which is made up of stars formed over 10 Gyr from material that has a wide range of abundances of Fe and the nuclides. However, in every case the members of a stellar population have a common history and similar dynamics; moreover, with appropriate observational material they can usually be identified from their spectra with little regard to their phase-space positions. Galaxy models of the sophistication that will be required in the middle of the next decade to make sense of the Gaia catalogue, will probably interpret the Galaxy as made up of a series of related populations, each with its own distribution function (DF) f (x, v , t) that gives the probability density with which the population's stars are distributed in phase space at time t. As the Galaxy ages, diffusive phenomena will cause f to become less sharply peaked in phase space, and therefore the DFs of different populations to overlap more and more. Notwithstanding the overlapping of their DFs, the populations will retain their integrity because their member stars will be identifiable by their chemical imprints. Observational techniques which have traditionally been restricted to targets in the Galaxy are becoming feasible for increasingly distant ob jects. In particular, we are beginning to be able to study the stellar populations of Local Group galaxies in sufficient detail to be able to decipher their hosts' star-formation histories. This work gives insight into the histories of galaxies that cover a wide range of morphological types, from gas-rich star-forming galaxies such as the Large Magellanic Cloud and IC 1613, to red and dead galaxies such as M32. Moreover, a large fraction of the current star-formation in the Universe takes place in groups similar to the Local Group, and in fact in galaxies similar to our own, so understanding the Local Group is the key to understanding a significant fraction of the Universe. The smaller galaxies of the Local Group show abundant evidence of interactions with the Group's two massive members, M31 and the Galaxy, so studies of Local Group stellar populations should reveal the impact that interactions have on star-formation rates and the build up of heavy elements. A cosmological framework is essential for any discussion of galaxy formation, and the Lambda cold dark matter (CDM) theory provides the standard framework. In this theory baryons comprise 17 ± 1% of the matter in the Universe (Spergel et al., 2007), the rest being made up of matter that does not experience 5


strong or electromagnetic interaction but at redshift 3100 begins to clump and eventually forms the dark halos within which visible galaxies form after redshift 10. Vacuum energy (`dark energy'), currently dominates the mean cosmic energy density: it contributes 76.3 ± 3.4% of the energy density with 24% of the energy density contributed by matter (Spergel et al., 2007); baryons account for only 4.6% of the energy density. On account of the repulsive gravitational force that vacuum energy generates, the expansion of the universe has been accelerating since a redshift 0.5. Events similar to those we hope to uncover through studies of Local Group stellar populations, can be observed as they happen at redshifts 0.5­3. These facts make it important to address the question `what does the Local Group look like to observers who see it at redshifts 0.5, 2 or 3?' This is a thoroughly nontrivial question, but one that could be answered given a sufficient understanding of the Local Group's stellar populations. Answering this question would enable our understanding of high-redshift observations to take a big step forward, for we would then know which ob jects were evolving towards analogues of the Local Group, and which will merge into rich clusters of galaxies. So the stellar populations of the Local Group are templates from which a much broader understanding of the Universe can be fashioned. In this Report, we examine the issues raised above as follows. In Section 3 we examine the current state of our knowledge of the structure of the Galaxy, and our knowledge of the processes which have shaped and which continue to shape its stellar populations and gas. At the end of Section 3 we sketch the current picture of how the Galaxy was assembled from its building blocks. In Section 4 we describe how the Galaxy can be used as a laboratory in which to study the processes that shape galaxies, and to constrain theoretical models of galaxy formation and evolution. In the course of Sections 3 and 4, we identify a number of limits on our current knowledge, and hint at future work that would overcome these. These issues are brought into sharp focus in Sections 5 and 7, where we identify the top remaining questions, and suggest how possible solutions might be provided by investment in new facilities, planned and yet to be planned. In Section 6 we review ground- and space-based facilities that have played and/or will play a ma jor role in achieving our scientific goals. The ma jor recommendations of the Working Group are drawn together in Section 7. Since the original motivation for this Report was a desire on the part of ESO and ESA to consider pro jects that would complement the Gaia mission, the panel's expertise lays primarily in stellar and dynamical astronomy and this fact may have led to a relative neglect of such important areas as high-energy astrophysics and studies of the interstellar medium.

6


3
3.1

Outline of the current state of knowledge of the field
The main structures of the Galaxy

Our Galaxy is a late-type spiral, as is directly evident from the high concentration of stars, gas and dust into a narrow strip strung across the night sky. Just as directly, we know from the changing surface brightness of this flattened structure with longitude, that the Sun is at a significant distance from the centre of the Galaxy. Our best estimate of that distance has been lowered gradually over the time-scale of a generation of astronomers: the most recent value for 7.62 ± 0.32 kpc (Eisenhauer et al., 2005), to be compared with the figure of 10 kpc accepted 40 years ago. The Galaxy is conventionally decomposed into: (i) a bar/bulge that has a luminosity 1 ± 0.3 â 1010 L and extends out to 3 kpc (Launhardt et al., 2002); (ii) a nearly spherical halo that extends from with the bulge out to of order 100 kpc and is studded with globular clusters; (iii) a disc that defines the Galactic plane < and is probably confined to radii R 15 kpc. The disc is often decomposed into two components, a thin disc, in which the density falls with distance |z | from the plane exponentially with scale height 300 pc and a thick disc, which is characterised by a vertical scale height 900 pc (Cabrera-Lavers et al., 2005; Juri´ et al., 2008). c The thin disc is patterned into spiral arms but even now, the final word on the number and positioning of the arms present has not been spoken. Beyond the Sun the thin disc is warped. The baryonic mass of the Galaxy is thought to be less than 1011 M , with 3/4 of it in the Galactic disc, and nearly all the rest in the bulge. Sgr A*, the black hole at the centre of the Galaxy, is estimated to have a mass of (3.6 ± 0.3) â 106 M (Ghez et al., 2005, adopting the Eisenhauer et al. (2005) distance). The ma jor mass component in the Galaxy is believed to be the dark-matter halo. This is constrained by measurements of the space motions of Galactic satellites and remote globular clusters, and is thought to be 1 - 3 â 1012 M (Wilkinson & Evans, 1999; Sakamoto et al., 2003; Battaglia et al., 2005). The mass within 50 kpc, i.e. the distance to the Large Magellanic Cloud (LMC), is only about a quarter of the total (Sakamoto et al., 2003, 5 ­ 5.5 â1011 M ). The dark-matter radial profile is much shallower (roughly R-2 ) than that of the luminous Galactic disc component for which is modelled by an exponentially decreasing surface density, with scale length 2 - 3 kpc (e.g. Drimmel & Spergel, 2001; Robin et al., 2003; Juri´ et al., 2008). c 7


3.1.1

The halo

The halo comprises old stars that have heavy-element abundances less than a tenth of the solar value. Its radial density profile is close to a power law r-2.8 and it is slightly flattened at the Galactic poles to axis ratio 0.8 (Juri´ et al., c 2008). Despite being flattened, near the Sun its net rotation is consistent with zero (Figure 1); further out it may rotate slightly in the opposite sense to the disc (Carollo et al., 2007).

Figure 1: Top row Fig 11 of Ivezic et al. (2008). The rotational velocities of 18 000 stars with [Fe/H] > -0.9 (left) or [Fe/H] < -1.1 (right) plotted against height |z |. Symbols indicate medians and dashed lines 2 boundaries at fixed |z |. Stars plotted in the left panel are associated with the disc and those in the right panel with the halo (see Figure 3).

Bell et al. (2008) find that a smooth model of the halo's luminosity density can account for only 60% of the total luminosity: the halo is rich in substructure. Some substructures are certainly the debris of ob jects that have been tidally shredded by the Galaxy (Section 3.3.6 below), as is evidenced by the tidal streams of the Sgr dwarf spheroidal galaxy, which wraps more than once around the Galaxy (Chou et al., 2007) and of the Magellanic Clouds, which arches right over the southern hemisphere (Putman et al., 2003). The globular clusters Pal 5 and NGC 5466 are known to have significant tidal tails (Odenkirchen et al., 2002; Belokurov et al., 2006a) and an exceptionally long and narrow tail suffers from disputed parenthood (Belokurov et al., 2007b; Jin & Lynden-Bell, 2007; Sales et al., 2008). The ma jority of halo stars are on plunging orbits, so near the Sun we see stars that spend most of their time at large galactocen