For a PDF version of the article, click here.


Christopher J. Conselice

Centre for Astronomy and Particle Theory
School of Physics and Astronomy
University of Nottingham, United Kingdom

Abstract: I present a comprehensive review of the evolution of galaxy structure in the universe from the first galaxies we can currently observe at z ~ 6 down to galaxies we see in the local universe. I further address how these changes reveal galaxy formation processes that only galaxy structural analyses can provide. This review is pedagogical and begins with a detailed discussion of the major methods in which galaxies are studied morphologically and structurally. This includes the well-established visual method for morphology; Sérsic fitting to measure galaxy sizes and surface brightness profile shapes; non-parametric structural methods including the concentration (C), asymmetry (A), clumpiness (S) (CAS) method, the Gini/M20 parameters, as well as newer structural indices. Included is a discussion of how these structural indices measure fundamental properties of galaxies such as their scale, star formation rate, and ongoing merger activity. Extensive observational results are shown demonstrating how broad galaxy morphologies and structures change with time up to z ~ 3, from small, compact and peculiar systems in the distant universe to the formation of the Hubble sequence dominated by spirals and ellipticals we find today. This review further addresses how structural methods accurately identify galaxies in mergers, and allow measurements of the merger history out to z ~ 3. The properties and evolution of internal structures of galaxies are depicted, such as bulges, disks, bars, and at z > 1 large star forming clumps. The structure and morphologies of host galaxies of active galactic nuclei and starbursts/sub-mm galaxies are described, along with how morphological galaxy quenching occurs. Furthermore, the role of environment in producing structure in galaxies over cosmic time is treated. Alongside the evolution of general structure, I also delineate how galaxy sizes change with time, with measured sizes up to a factor of 2-5 smaller at high redshift at a given stellar mass. This review concludes with a discussion of how the evolving trends in sizes, structures, and morphologies reveal the formation mechanisms behind galaxies which provides a new and unique way to test theories of galaxy formation.

Key words : galaxy evolution, galaxy morphology, galaxy structure

Table of Contents