Annu. Rev. Astron. Astrophys. 1997. 35: 637-675 Copyright © 1997 by Annual Reviews. All rights reserved

5. FORMATION SCENARIOS

5.1. Are Bulges Related to Their Haloes?

Analyses of globular cluster systems in external galaxies conclude that they are more metal-poor in the mean than the underlying stellar light, at all radii in all galaxies (Harris 1991). It is worth noting that the Milky Way is sometimes considered an anomaly here, in that the metallicity distribution function for the (metal-poor, also known as halo) globular cluster system is not very different from that of field halo stars, with differences restricted to the wings of the distributions (e.g. Ryan & Norris 1991). It is important to note, however, that this comparison is done in the Milky Way at equivalent halo surface brightness levels well below those achievable in external galaxies. The higher surface brightness part of the Milky Way, that part which is appropriate to compare to similar studies in other galaxies, is the inner bulge. As discussed above, the metallicity there is well above that of the globular clusters. The Milky Way is typical. More importantly, this (single) test suggests the possibility that all spiral galaxies that have globular cluster systems have a corresponding field halo, which in turn is systematically more metal-poor and extended than is the more metal-rich observable bulge.

If this is true, the Local Group galaxies are typical, and the concept of "stellar halo" must be distinguished from that of "stellar bulge." In addition, although haloes seem ubiquitous, they are always of low luminosity and seem generally more extended than bulges. Bulges are not ubiquitous, as they are only found in earlier type galaxies, and cover a very wide range of luminosities. This is, in fact, clearly seen in the Hubble classification criteria from Sa to Sc types.

What is the evolutionary relationship, if any, between bulges and haloes? The Milky Way is an ideal case to study this because it has both bulge and halo. We noted above that the bulge is more metal-rich and possibly younger than the halo, contrary to the argument of Lee (1992). What of its dynamics?

In the Milky Way, the bulge stars do show significant net rotation (e.g. Ibata & Gilmore 1995b, Minniti et al 1995), but the very concentrated spatial distribution of these stars leads to low angular momentum orbits. Indeed, the angular momentum (per unit mass) distribution of the bulge is very similar to that of the stellar halo and very different from that of the disk (Wyse & Gilmore 1992, Ibata & Gilmore 1995b); see Figure 7. As discussed below, this is suggestive of the Eggen et al (1962) scenario, with the bulge as the central region of the halo but formed with significantly more dissipation. Furthermore, the available estimates of the masses of the stellar halo and bulge give a ratio of ~ 1:10, which is (coincidentally?) about the ratio predicted by models in which the bulge is built up by gas loss from star-forming regions in the halo (e.g. Carney et al 1990, Wyse 1995). The real test of this model is determination of the rate of formation and chemical enrichment of the stars in each of the halo and bulge. This is feasible and only requires good data on element ratios (e.g. Wyse & Gilmore 1992).

Figure 7. Cumulative distribution functions of specific angular momentum for the four major Galactic stellar populations. The solid curve is the distribution for the bulge, from Ibata & Gilmore (1995b). The other curves are taken from Wyse & Gilmore (1992)): The dashed-dotted curve represents the halo, the dotted curve represents the thick disk, and the dashed curve represents the thin disk. It is clear that the halo and bulge are more like each other than they are like the disk components.