Galaxy structure is one of the fundamental ways in which galaxy properties are described and by which galaxy evolution is inferred. There is a long history of the development of this idea, which began with the earliest observations of galaxies, and continues up to the modern day as one of the major ways we study galaxies. This review gives a detailed description of the progress made up to late-2013 in using galaxy structure to understand galaxy formation and evolution. It is meant to be used as a primer for obtaining basic information from galaxy structures, including how they are measured and applied through cosmic time.
The introduction to this review first gives an outline of the basic events in the history of galaxy morphology and structure analyses, while the second part of the introduction describes how galaxy structure fits into the general picture of galaxy formation. I also give a detailed description of the goals of this review at the end of the introduction.
1.1. Historical Background
Galaxy morphology has a long history, one that even predates the time we knew galaxies were extragalactic. When objects which today we call galaxies were first observed what clearly distinguished them from stars was their resolved structure. Since this time, structure and morphology has remained one of the most common ways galaxies are described and studied. Initially this involved visual impressions of galaxy forms. This has now been expanded to include quantitative methods to measure galaxy structures all the way back to the earliest galaxies we can currently see.
The first published descriptions of galaxy structure and morphology predates the telescopic era. For example, the Andromeda nebula was described as a 'small cloud' by the Persian astronomer Abd al-Rahman al-Sufi in the 10th century, (Kepple & Sanner 1998). The study of galaxies remained descriptive until the late 20th century, although more and more detail was resolved as technology improved. As a result, for about 150 years the science of galaxies was necessarily restricted to cataloging and general descriptions of structure, with notable achievements by Messier and William and John Herschel who located galaxies or `nebula' by their resolve structure as seen by eye. Even before photography revolutionized the study of galaxies some observers such as William Parsons, the 3rd Earl of Rosse, noted that the nebulae have a spiral morphology and first used this term to describe galaxies, most notably and famously in the case of M51.
It was however the advent of photography that astronomers could in earnest begin to study the morphologies and structures of external galaxies. The most notable early schemes were developed by Wolf (1908), and Lundmark (1926), among others. This ultimately led to what is today called the Hubble classification which was published in essentially its modern form in Hubble (1926), with the final 'Hubble Tuning Fork' established in Hubble (1936) and Sandage (1961). The basic Hubble sequence (Figure 1) consists of two main types of galaxies, ellipticals and spirals, with a further division of spirals into those with bars and those without bars. Hubble, and the astronomers who followed him, could classify most nearby bright galaxies in terms of this system.
Figure 1. A modern form of the Hubble sequence showing the sequence of ellipticals and S0s, and the `tuning fork' in spirals. The elliptical sequence is determined by the overall shape of the galaxy, while spiral classifications are divided into different types (a-c) depending upon how wound-up spiral arms are, how large the bulge relative to the disk is, and how smooth the spiral arms in the spirals arm. The tuning-fork is the differential between spirals with and without bars. Also shown is the extension of this sequence to dwarf spheroidal galaxies and irregular galaxies, both of which are lower mass systems (Kormendy & Bender 2012).
The development of morphological classification methods continued into the 20th century, with newer methodologies based solely on visual impressions. For example, de Vaucouleurs (1959) developed a revised version of the Hubble sequence which included criteria such as as bars, rings and other internal features that were prominent on photographic plates of galaxies. Likewise, van den Bergh (1960, 1976), and later Elmegreen & Elmegreen (1987) developed a system to classify galaxies based on the form of spiral arms, and the apparent clumpiness of the light in these arms.
While it is important to classify galaxies visually, and all systems have some use, as all features should be explained by physics, it is not obvious which structural features of galaxies are fundamental to their formation history. Ultimately morphology and structure needs to prove to be useful for understanding galaxies, as there is now extensive use of photometric and spectroscopic methods permitting measurements of perhaps more fundamental measures of stellar populations and dust/gas properties in galaxies. Along these lines, at roughly the same time as progressively complicated classification systems were developed, astronomers such as Holmberg (1958) established that physical properties of nearby galaxies correlate with morphology in a broad context. Holmberg (1958) found that ellipticals are typically massive and red, and show little star formation, while spirals are less massive, bluer and have evidence for ongoing star formation. This quantitatively expands into other physical parameters as well (e.g., Roberts 1963; Roberts & Haynes 1994; Conselice 2006a; Allen et al. 2006). It is also well known that this segregation of morphology in the local universe provides an important clue for understanding the physics of galaxy formation, especially as local environment is found to strongly correlate with a galaxy's morphology (e.g., Dressler 1984; see Section 4.7).
A revolution in morphological and structurally studies came about with the advent of photometric photometry, and especially the later use of Charged Coupled Devices (CCD), which made detailed quantitative measurements of light distributions in galaxies possible. The first major contribution from this type of work was by de Vaucouleurs (1948) who used photometry to show that the light profiles of what we would identify today as massive ellipticals all follow roughly the same fundamental light distribution, known as the de Vaucouleurs profile.
This was later expanded by others, most notably Sérsic (1963), who demonstrated that a more general form of light distributions matched galaxy light profiles with disks having exponential light profiles, while the light distribution within massive ellipticals generally following the de Vaucouleurs profile. This has led to a huge industry in measuring the light profiles of galaxies in the nearby and distant universe which continues today (Section 2.2).
During the 1970s and 1980s the study of galaxy structure expanded to include the decomposition of galaxy light into bulge and disk profiles (e.g., Kormendy 1977) as well as features such as bars, rings and lenses (e.g., Kormendy 1979; de Vaucouleurs et al. 1993). The three dimensional structure of disk galaxies was investigated (e.g., van der Kruit & Searle 1982), as well as detailed studies of bulges and disk in spiral systems (e.g., de Jong 1996; Peletier & Balcells 1996). We also now know there is a great diversity in elliptical galaxy internal structures (e.g., Kormendy et al. 2009).
Similar investigations demonstrated that secular evolution within disks can provide an explanation for how bars, rings and lenses can form (e.g., Kormendy 1979; Combes & Sanders 1981). These effects, not driven by hierarchical galaxy formation, are also likely responsible for the formation of pseudo-bulges and may drive the formation of central massive black holes (e.g., Kormendy & Kennicutt 2004; Sellwood 2013).
While there is a large amount of work done on the structures and morphologies of galaxies in the nearby universe (e.g., see Kormendy et al. 2009; Buta 2013), it is difficult to investigate more than the very basics of structure and morphology when studying distant galaxies. This is due to the fact that current technology does not allow us to resolve these distant galaxies in the same detail as we can for closer systems. As such, this review will concentrate on the features and properties of galaxy structure which we can measure in distant galaxies, and how this reveals how galaxy evolution and formation occurs.
The result of this is that one of the areas where galaxy structure and morphology has made its biggest impact is its ability to measure fundamental properties of distant galaxies that we can compare with nearby galaxies to determine evolution. There are extensive methods for studying galaxy evolution which galaxy structure analyses are becoming an essential aspect of, and providing unique information on, the history and physics of galaxy assembly, which I detail in this review.
1.2. Galaxy Structure within the Context of Galaxy Formation
We know that there is significant evolution in galaxies over time as the stellar mass density of galaxies evolves rapidly at 1 < z < 3, with about half of all stellar mass formed by z = 1 (e.g., Bundy et al. 2005; Mortlock et al. 2011). We also know that there is a vast diversity of star formation histories for individual galaxies, and that the integrated star formation rate density in the universe's history peaks at z ~ 2.5, and declines at higher and lower redshifts (e.g., Shapley 2011; Madau & Dickinson 2014, this volume). However, it is not clear from these observations what are/were the driving forces creating galaxies.
Theory offers several approaches for understanding how galaxies form which detailed studies are starting to probe. We now believe that galaxy formation can happen in a number of ways. This includes: in-situ star formation in a collapsed galaxy, major and minor mergers, and gas accretion from the intergalactic medium. Galaxy structure and morphology are perhaps the best ways to trace these processes, as I discuss in this review.
Another major question I address in this review is how do the structures and morphologies of galaxies change through cosmic time. Major issues that this topic allows us to address include: the formation history of the Hubble Sequence; whether galaxies form 'in-side-out' or 'out-side-in'?; how long does a galaxy retain its morphology?; is morphology a invariant quantity in a galaxy over a long cosmic time-span?, and furthermore what relative role does star formation and merging play in galaxy formation?
Galaxy structure and morphology has made a significant impact on these questions largely due to the Hubble Space Telescope (HST) and its various 'Deep Field' campaigns starting in the mid-1990s, finding thousands of galaxies at redshifts z > 1 within these images. This is complemented by extensive imaging and spectroscopy for nearby galaxies carried out by surveys such as the Sloan Digital Sky Survey (SDSS) and the Millennium Galaxy Catalog (e.g., Shen et al. 2003; De Propris et al. 2007). Combining these surveys makes it possible to study in detail the structures of distant galaxies, and to compare these with structures at different redshifts. This has led to a renaissance in the analysis of galaxy structure, including parametric fitting using Sérsic profiles, and the development of non-parametric measurements of galaxy structure that have allowed us to use galaxy morphology/structure as a tool for deciphering how galaxy assembly occurs over cosmic time.
We are in fact now able to resolve galaxies back to redshifts z = 8 with imaging from space, and recently as well with adaptive optics from the ground (e.g., Conselice & Arnold 2009; Carrasco et al. 2010; Akiyama et al. 2008). This reveals that galaxy structure is significantly different in the early universe from what it is today, and that there is a progression from the highest redshifts, where galaxies are small, peculiar, and undergoing high star formation rates to the relative quiescent galaxies that we find in the nearby universe. How this change occurs, and what it implies for galaxy evolution, is another focus of this review.
Another ultimate goal is to describe the methods for measuring galaxy structure and morphology for nearby galaxies up to the most distant ones we can see. I also discuss how galaxy structure correlates with physical properties of galaxies, such as their star formation rate, merging and their overall scale. I then provide a description of the observed structural evolution of galaxies, and a discussion of what this implies for the driving mechanisms behind galaxy formation using the calibrated methods.
The amount of information we have about the structures and properties of galaxies declines as one goes to higher redshift systems, and issues that arise due to observational bias must be dealt with. I therefore also discuss systematics that can be addressed through imaging simulations to determine the real evolution of the morphologies and structures of galaxies. I finish this review with a discussion of future uses of galaxy structure/morphology, including the potential with the advent of JWST and Euclid.
This review is structured as follows. In Section 2, I describe the analysis methods used for measuring the morphologies and structures of galaxies. In Section 3 I describe how structures and morphologies reveal fundamental galaxy properties and evolutionary processes, while Section 4 describes the observed evolution of the structures of galaxies through cosmic time. I finish this review with a description of how galaxy structure and evolution is becoming an important aspect for understanding the underlying theory of galaxy formation and cosmology in Section 5 and give a summary and future outlook in Section 6.