The structures and morphologies of galaxies change with time. Determining the history and cause of this galaxy structure-redshift relationship, including the origin of modern galaxy morphologies (i.e., ellipticals, disks) is perhaps the missing, and until now overlooked, link in understanding galaxy formation. While we currently have a good understanding of global galaxy formation and evolution, such as the star formation and mass assembly history (e.g., Madau et al. 1996; Dickinson et al. 2003), we are only beginning to understand how galaxy formation occurs as opposed to simply when. During the last few years it has become clear with the advent of wide-field imaging surveys from the ground, and from space using the Hubble Space Telescope, that galaxy structure evolves (e.g., Driver et al. 1995; Glazebrook et al. 1995; Abraham et al. 1996; Brichmann & Ellis 2000; Conselice et al. 2004b). There is a clear galaxy structure (or morphology)-redshift relationship such that galaxies in the more distant universe are peculiar while those in the local universe are more regular or normal 1 (van den Bergh et al. 2001). Determining the physics behind the morphology-redshift relationship is critical for any ultimate understanding of galaxies and the physical causes of galaxy structure and its evolution.
The morphology-redshift relationship can furthermore potentially be used as a key test of galaxy formation models. Theories of galaxy formation can be divided into two main ideas - the monolithic collapse of material early in the universe to form stars and galaxies within a very short time (e.g., Larson 1975; Tinsley & Gunn 1976) and the hierarchical formation scenario (e.g., White & Rees 1978; Blumenthal et al. 1984; White & Frenk 1992; Cole et al. 2000). Observationally, we know that galaxies do not appear to form rapidly in the early universe, but have an extended star formation history that does not decline significantly until the universe is about half its current age. Likewise, about half of all stellar mass in the universe formed between z ~ 1 (8 Gyrs ago) and today (Dickinson et al. 2003). The fact that star formation occurs over time, and not quickly at very high redshift, largely rules out rapid collapses as the primary method for forming all galaxies. High redshift galaxies also tend to be small with likely small stellar masses (Papovich et al. 2001; Ferguson et al. 2004). Therefore a large fraction of all galaxies must have formed gradually throughout time. Understanding this process, that is what is causing mass to build up in galaxies, requires studying their internal properties.
There are several ways to measure the physical processes responsible for forming galaxies which can potentially explain the observed morphology-redshift relationship (Section 3). One method, and by far the most common, is to study global galaxy properties, such as the evolution of stellar mass and star formation, and to compare these with predictive models (e.g., Somerville et al. 2001). Other methods, which are now just being explored, involve probing the internal features of high-z galaxies either through spectroscopy or high resolution imaging. While integral field spectroscopy for high-z galaxies is still in its infancy, understanding the internal structural features of high redshift galaxies in now in a golden age, utilizing new techniques (e.g., Conselice et al. 2000a; Peng et al. 2002) with high resolution Hubble Space Telescope imaging (e.g., Giavalisco et al. 2004; Rix et al. 2004).
The idea that the structures of galaxies hold clues towards understanding their current and past formation histories is a new, and perhaps still controversial, idea. There is however increasing amounts of evidence that suggests galaxy structure reveals fundamental past and present properties of galaxies (see Conselice 2003 and references within and Section 2). Utilizing these tools, we can begin to determine the origin of the galaxy morphology-redshift relationship. Understanding this relationship will in turn help us determine physical formation mechanisms, and the history of galaxy assembly. Furthermore, it is now possible to compare observations of the galaxy morphology-redshift relationship with theoretical models based on cosmological and dark matter ideas, connecting the universe as a whole to its constitute galaxies. I argue in this review that the galaxy morphology-redshift relationship is a pillar for understanding galaxy formation. It may also hold clues for understanding the relationship between the baryonic content of galaxies and their dark matter halos, the evolution of galaxies and their black holes, and the relationship between cosmological parameters and the evolution of galaxy structure. In summary, I will address in detail the following issues:
(i) What is the galaxy structure-redshift relationship and how does it evolve?
(ii) What is the physical causes for the formation and evolution of the galaxy structure-redshift relationship?
(iii) What does the evolution of the galaxy-structure redshift relationship tell us about galaxy formation?
I do not address some morphological evolution problems, such as the formation and evolution of bars, rings or other internal structures, as these are dealt with in other contributions (e.g., Jogee, Sheth). Throughout I will assume a cosmology with H0 = 70 km s-1 Mpc-1 and relative densities of = 0.7 and m = 0.3.
1 Throughout this review, I refer to 'regular' or 'normal' galaxies to denote systems that are on the present day Hubble sequence; namely ellipticals and spirals. Peculiar galaxies and/or mergers are not normal galaxies according to this criteria. This differs from the usual meaning of normal which refers to galaxies without the presence of an active galactic nuclei (AGN). Back.