|Annu. Rev. Astron. Astrophys. 2002. 40:487-537
Copyright © 2002 by Annual Reviews. All rights reserved
Like most spiral galaxies, our Galaxy has several recognizable structural components that probably appeared at different stages in the galaxy formation process. These components will retain different kinds of signatures of their formation. We will describe these components in the context of other disk galaxies, and use images of other galaxies in Figure 3 to illustrate the components.
Figure 3. (a) Sketch of Milky Way showing the stellar disk (light blue), thick disk (dark blue), stellar bulge (yellow), stellar halo (mustard yellow), dark halo (black) and globular cluster system (filled circles). The radius of the stellar disk is roughly 15 kpc. The baryon and dark halos extend to a radius of at least 100 kpc. (b) Infrared image of the Milky Way taken by the DIRBE instrument on board the Cosmic Background Explorer (COBE) Satellite. [We acknowledge the NASA Goddard Space Flight Center and the COBE Science Working Group for this image.] (c) M104, a normal disk galaxy with a large stellar bulge (from AAO). (d) Hubble Heritage image of the compact group Hickson 87; one galaxy has a peanut-shaped stellar bulge due to dynamical interaction with other group members. (e) Image of the SO galaxy NGC 4762 (Digital Sky Survey) shows its thin disk and stellar bulge. (f) A deeper image of NGC 4762 (DSS) shows its more extended thick disk. The base of the arrows in (e) and (f) shows the height above the plane at which the thick disk becomes brighter than the thin disk (Tsikoudi 1980). (g) Image of the pure disk galaxy IC 5249 (DSS) shows its thin disk and no stellar bulge. (h) A deeper image of IC 5249 (DSS) shows no visible thick disk, although a very faint thick disk has been detected in deep surface photometry.
3.1. The Bulge
First, compare images of M104 and IC 5249 (Figures 3c,g): These are extreme examples of galaxies with a large bulge and with no bulge. Large bulges like that of M104 are structurally and chemically rather similar to elliptical galaxies: Their surface brightness distribution follows an r1/4 law (e.g., Pritchet & van den Bergh 1994) and they show similar relations of [Fe/H] and [Mg/Fe] with absolute magnitude (e.g., Jablonka et al. 1996). These properties lead to the view that the large bulges formed rapidly. The smaller bulges are often boxy in shape, with a more exponential surface brightness distribution (e.g., Courteau et al. 1996). The current belief is that they may have arisen from the stellar disk through bending mode instabilities.
Spiral bulges are usually assumed to be old, but this is poorly known, even for the Galaxy. The presence of bulge RR Lyrae stars indicates that at least some fraction of the galactic bulge is old (Rich 2001). Furthermore, the color-magnitude diagrams for galactic bulge stars show that the bulge is predominantly old. McWilliam & Rich (1994) measured [Fe/H] abundances for red giant stars in the bulge of the Milky Way. They found that, while there is a wide spread, the abundances ([Fe/H] ≈ -0.25) are closer to the older stars of the metal rich disk than to the very old metal poor stars in the halo and in globular clusters, in agreement with the abundances of planetary nebulae in the Galactic bulge (e.g., Exter et al. 2001).
The COBE image of the Milky Way (Figure 3b) shows a modest somewhat boxy bulge, typical of an Sb to Sc spiral. Figure 3d shows a more extreme example of a boxy/peanut bulge. If such bulges do arise via instabilities of the stellar disk, then much of the information that we seek about the state of the early galaxy would have been lost in the processes of disk formation and subsequent bulge formation. Although most of the more luminous disk galaxies have bulges, many of the fainter disk galaxies do not. Bulge formation is not an essential element of the formation processes of disk galaxies.
3.2. The Disk
Now look at the disks of these galaxies. The exponential thin disk, with a vertical scale height of about 300 pc, is the most conspicuous component in edge-on disk galaxies like NGC 4762 and IC 5249 (Figures 3e,g). The thin disk is believed to be the end product of the quiescent dissipation of most of the baryons and contains almost all of the baryonic angular momentum. For the galactic disk, which is clearly seen in the COBE image in Figure 3b, we know from radioactive dating, white dwarf cooling and isochrone estimates for individual evolved stars and open clusters that the oldest disk stars have ages in the range 10 to 12 Ga (see “Stellar Age Dating” above).
The disk is the defining stellar component of disk galaxies, and understanding its formation is in our view the most important goal of galaxy formation theory. Although much of the information about the pre-disk state of the baryons has been lost in the dissipative process, some tracers remain, and we will discuss them in the next section.
Many disk galaxies show a second fainter disk component with a larger scale height (typically about 1 kpc); this is known as the thick disk. Deep surface photometry of IC 5249 shows only a very faint thick disk enveloping the bright thin disk (Abe et al. 1999): Compare Figures 3g,h. In the edge-on S0 galaxy NGC 4762, we see a much brighter thick disk around its very bright thin disk (Tsikoudi 1980): The thick disk is easily seen by comparing Figures 3e, f. The Milky Way has a significant thick disk (Gilmore & Reid 1983): Its scale height (∼1 kpc) is about three times larger than the scale height of the thin disk, its surface brightness is about 10% of the thin disk's surface brightness, its stellar population appears to be older than about 12 Ga, and its stars are significantly more metal poor than the stars of the thin disk. The galactic thick disk is currently believed to arise from heating of the early stellar disk by accretion events or minor mergers (see “Disk Heating by Accretion” below).
The thick disk may be one of the most significant components for studying signatures of galaxy formation because it presents a snap frozen relic of the state of the (heated) early disk. Although some apparently pure-disk galaxies like IC 5249 do have faint thick disks, others do not (Fry et al. 1999): These pure-disk galaxies show no visible components other than the thin disk. As for the bulge, formation of a thick disk is not an essential element of the galaxy formation process. In some galaxies the dissipative formation of the disk is clearly a very quiescent process.
3.3. The Stellar Halo
There are two further components that are not readily seen in other galaxies and are shown schematically in Figure 3a. The first is the metal-poor stellar halo, well-known in the Galaxy as the population containing the metal-poor globular clusters and field stars. Its mass is only about 1% of the total stellar mass (about 109 M⊙: e.g., Morrison 1993). The surface brightness of the galactic halo, if observed in other galaxies, would be too low to detect from its diffuse light. It can be seen in other galaxies of the Local Group in which it is possible to detect the individual evolved halo stars. The metal-poor halo of the Galaxy is very interesting for galaxy formation studies because it is so old: Most of its stars are probably older than 12 Ga and were probably among the first galactic objects to form. The galactic halo has a power law density distribution ρ ∝ r-3.5 although this appears to depend on the stellar population (Vivas et al. 2001, Chiba & Beers 2000). Unlike the disk and bulge, the angular momentum of the halo is close to zero (e.g., Freeman 1987), and it is supported almost entirely by its velocity dispersion; Some of its stars are very energetic, reaching out to at least 100 kpc from the galactic center (e.g., Carney et al. 1990).
The current view is that the galactic halo formed at least partly through the accretion of small metal-poor satellite galaxies that underwent some independent chemical evolution before being accreted by the Galaxy (Searle & Zinn 1978, Freeman 1987). Although we do still see such accretion events taking place now, in the apparent tidal disruption of the Sgr dwarf (Ibata et al. 1995), most of them must have occurred long ago. Accretion of satellites would dynamically heat the thin disk, so the presence of a dominant thin disk in the Galaxy means that most of this halo-building accretion probably predated the epoch of thick disk formation ∼ 12 Ga ago. We can expect to see dynamically unmixed residues or fossils of at least some of these accretion events (e.g., Helmi & White 1999).
Of all the galactic components, the stellar halo offers the best opportunity for probing the details of its formation. There is a real possibility to identify groups of halo stars that originate from common progenitor satellites (Eggen 1977, Helmi & White 1999, Harding et al. 2001, Majewski et al. 2000). However, if the accretion picture is correct, then the halo is just the stellar debris of small objects accreted by the Galaxy early in its life. Although it may be possible to unravel this debris and associate individual halo stars with particular progenitors, this may tell us more about the early chemical evolution of dwarf galaxies than about the basic issues of galaxy formation. We would argue that the thin disk and thick disk of our Galaxy retain the most information about how the Galaxy formed. On the other hand, we note that current hierarchical CDM simulations predict many more satellites than are currently observed. It would therefore be very interesting to determine the number of satellites that have already been accreted to form the galactic stellar halo.
We should keep in mind that the stellar halo accounts for only a tiny fraction of the galactic baryons and is dynamically distinct from the rest of the stellar baryons. We should also note that the stellar halo of the Galaxy may not be typical: The halos of disk galaxies are quite diverse. The halo of M31, for example, follows the r1/4 law (Pritchet & van den Bergh 1994) and is much more metal rich in the mean than the halo of our Galaxy (Durrell et al. 2001) although it does have stars that are very metal weak. It should probably be regarded more as the outer parts of a large bulge than as a distinct halo component. For some other disk galaxies, like the LMC, a metal-poor population is clearly present but may lie in the disk rather than in a spheroidal halo.
3.4. The Dark Halo
The second inconspicuous component is the dark halo, which is detected only by its gravitational field. The dark halo contributes at least 90% of the total galactic mass and its ρ ∼ r-2 density distribution extends to at least 100 kpc (e.g., Kochanek 1996). It is believed to be spheroidal rather than disklike (Crézé et al. 1998, Ibata et al. 2001b; see Pfenniger et al. 1994 for a contrary view). In the current picture of galaxy formation, the dark halo plays a very significant role. The disk is believed to form dissipatively within the potential of the virialized spheroidal halo, which itself formed through the fairly rapid aggregation of smaller bodies.
CDM simulations suggest that the halo may still be strongly substructured (see “Signatures of the CDM Hierarchy” below). If this is correct, then the lumpy halo would continue to influence the evolution of the galactic disk, and the residual substructure of the halo is a fossil of its formation. If the dark matter is grainy, it may be possible to study the dynamics of this substructure through pixel lensing of the light of background galaxies (Widrow & Dubinski 1998, Lewis et al. 2000). Another possibility is to look for the signatures of substructure around external galaxies in gravitationally lensed images of background quasars (Metcalf 2001, Chiba 2001). The dispersal of tidal tails from globular clusters appears to be sensitive to halo substructure (Ibata et al. 2001b), although this is not the case for dwarf galaxies (Johnston et al. 2001).
Within the limitations mentioned above, each of these distinct components of the Galaxy preserves signatures of its past and so gives insights into the galaxy formation process. We now discuss these signatures.