The Missing Satellites Problem (MSP) broadly refers to the overabundance of predicted Cold Dark Matter (CDM) sub-halos compared to satellite galaxies known to exist in the Local Group. The most popular interpretation of the MSP is that the smallest dark matter halos in the universe are extremely inefficient at forming stars. The question from that standpoint is to identify the feedback source that makes small halos dark and to identify any obvious mass scale where the truncation in the efficiency of galaxy formation occurs.

Among the most exciting developments in near-field cosmology in recent years is the discovery of a new population satellite galaxies orbiting the Milky Way and M31. Wide field, resolved star surveys have more than doubled the dwarf satellite count in less than a decade, revealing a population of ultra faint galaxies that are less luminous that some star clusters. For the first time, there are empirical reasons to believe that there really are more than 100 missing satellite galaxies in the Local Group, lurking just beyond our ability to detect them, or simply inhabiting a region of the sky that has yet to be surveyed.

Remarkably, both kinematic studies and completeness-correction studies seem to point to a characteristic potential well depth for satellite sub-halos that is quite close to the mass scale where photoionization and atomic cooling should limit galaxy formation. Among the more pressing problems associated with this interpretation is to understand the selection biases that limit our ability to detect the lowest mass galaxies.

1.1 Introduction

What does our Galaxy look like? We can compare the COBE image of our Galaxy, taken in the near-IR, with the visible image of the edge on spiral NGC 891. Our Galaxy would probably look much like NGC 891 if it were observed in visible light from far away (see Figure 1.1). The Milky Way is very clearly a disk galaxy: its disk is the primary component and is supported almost entirely by its rapid rotation. We also see a small central bulge which contributes about 20% of the total light. Some galaxies have much larger bulges. The small bulge of the Milky Way is a pointer to the events that occurred as it formed and evolved. We would like to understand how our Galaxy came to look like this.

Figure 1.2 shows schematically the five main components of the stellar galaxy. The thin disk and bulge are the main visible components. The thin disk is enveloped in a thicker thick disk which contributes only about 10% of the light of the disk. These thick disks are very common and their formation appears to be part of the formation process of disk galaxies. The stellar halo provides only about 1—2% of the total light but is very important for understanding how the Galaxy was assembled. The stars of the halo are metal-poor, mostly with abundances of [Fe/H] < —1.