To be published in Planets, Stars, and Stellar Systems, Vol. 6, Series Editor T. D. Oswalt, Volume editor W. C. Keel, Springer Reference, 2011
astro-ph/1102.0550

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Abstract: Galaxy morphology is a product of how galaxies formed, how they interacted with their environment, how they were influenced by internal perturbations, AGN, and dark matter, and of their varied star formation histories. This article reviews the phenomenology of galaxy morphology and classification with a view to delineating as many types as possible and how they relate to physical interpretations. The old classification systems are refined, and new types introduced, as the explosion in available morphological data has modified our views on the structure and evolution of galaxies.

Subject headings: galaxies: spiral; galaxies: elliptical; galaxies: S0s; galaxies: structure; galaxies: high redshift; galaxies: classification; galaxies: peculiar; galaxies: dwarfs; galaxies: clusters; galaxies: active; galaxies: isolated; galaxies: Galaxy Zoo project

Index terms: Hubble-Sandage-de Vaucouleurs classification; spiral galaxies; elliptical galaxies; S0 galaxies; irregular galaxies; resonance ring galaxies; outer rings; inner rings; nuclear rings; secondary bars; nuclear bars; pseudorings; grand design spirals; flocculent spirals; merger morphologies; tidal tails; collisional ring galaxies; polar ring galaxies; barred galaxies; lens galaxies; spiral arm classes; counter-winding spiral galaxies; anemic spiral galaxies; spiral arm multiplicity; dust lanes; pseudobulges; classical bulges; dust-lane elliptical galaxies; shell/ripple galaxies; ultra-luminous infrared galaxies; bar ansae; brightest cluster members; low surface brightness galaxies; dwarf galaxies; dwarf spiral galaxies; dwarf spheroidal galaxies; Galaxy Zoo project; isolated galaxies; blue compact dwarf galaxies; star-forming galaxy morphologies; warped disks; stellar mass morphology; luminosity classes; boxy ellipticals; disky ellipticals; late-type galaxies; early-type galaxies; Magellanic barred galaxies; color galaxy morphology; atomic and molecular gas galaxy morphology

Table of Contents

• INTRODUCTION
• OVERVIEW
• GALAXY CLASSIFICATION
• A CONTINUUM OF GALACTIC FORMS
• GALAXY TYPES: STAGE, FAMILY, AND VARIETY
• Elliptical and Spheroidal Galaxies
• S0 and Spiral Galaxies
• Irregular Galaxies
• OTHER DIMENSIONS TO GALAXY MORPHOLOGY
• Outer Rings and Pseudorings
• Inner and Outer Lenses
• Nuclear Rings and Bars
• Spiral Arm Morphologies
• Luminosity Effects
• THE MORPHOLOGY OF GALACTIC BARS AND OVALS
• DUST MORPHOLOGIES
• THE MORPHOLOGIES OF GALACTIC BULGES
• EFFECTS OF INTERACTIONS AND MERGERS
• Normal versus Catastrophic Rings
• Environmental Effects on Star-Forming Disks
• Interacting and Peculiar Galaxies
• Tidal Tails, Arms, and Bridges
• Dust-Lane Ellipticals
• Shell/Ripple Galaxies
• Ultra-Luminous Infrared Galaxies
• Warps
• The Morphology of Active Galaxies
• The Morphology of Brightest Cluster Members
• STAR FORMATION MORPHOLOGIES
• H{alpha} Imaging
• Ultraviolet Imaging
• Atomic and Molecular Gas Morphology
• INFRARED OBSERVATIONS: GALACTIC STELLAR MASS MORPHOLOGY
• INTERMEDIATE AND HIGH REDSHIFT GALAXY MORPHOLOGY
• GIANT LOW SURFACE BRIGHTNESS GALAXIES
• GALAXY MORPHOLOGY IN COLOR
• Normal Galaxies
• Dwarf Galaxies
• dE, dS0, BCD, and cE Galaxies
• Local Group Dwarf Spheroidals and Irregulars
• Dwarf Spirals
• Galaxy Zoo Project
• Isolated Galaxies
• Deep Field Color Imaging
• LARGE-SCALE AUTOMATED GALAXY CLASSIFICATION
• THE STATUS AND FUTURE OF MORPHOLOGICAL STUDIES
• REFERENCES