Glossary

11.1 Galaxy classification and properties

2 min read•july 25, 2024

Galaxies come in diverse shapes and sizes, from elegant spirals to smooth ellipticals. We classify them using Hubble's , which considers features like spiral arms and size. This helps us understand their structure and evolution.

Beyond appearance, galaxies differ in properties like stellar populations, gas content, and star formation rates. We use spectroscopy to analyze their composition and motion, while functions help us study galaxy populations and evolution across cosmic time.

Galaxy Classification and Properties

Galaxy classification scheme

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  • Hubble's tuning fork diagram organizes galaxies into ellipticals (E), spirals (S), barred spirals (SB), lenticulars (S0), and irregulars (Irr) based on visual appearance
  • Morphological features considered include presence of spiral arms, bulge-to- ratio, and presence of a bar
  • Elliptical galaxies classified E0 to E7 based on ellipticity (roundness to flattened shape)
  • Spiral galaxies classified Sa, Sb, Sc based on tightness of spiral arms and bulge size (tightly wound to loosely wound)
  • Limitations include not accounting for galaxy evolution and difficulty classifying distant or faint galaxies

Properties of galaxy types

  • Elliptical galaxies exhibit smooth, featureless appearance with little gas or dust, contain older stellar populations, and supported by random stellar motions
  • Spiral galaxies feature disk component with spiral arms, central bulge, younger stellar populations in disk, ongoing star formation in spiral arms (HII regions), and rotation-supported structure
  • Irregular galaxies lack definite shape or structure, often result from galaxy interactions (mergers), typically smaller and less massive, and display active star formation regions

Spectroscopy for galaxy analysis

  • Emission and absorption lines indicate presence of specific elements (hydrogen, helium, oxygen)
  • Doppler shift measures galaxy's radial velocity (approaching or receding)
  • Spectral energy distribution reveals galaxy's overall composition and age (young vs old stars)
  • measurements determine galaxy distances and calculate Hubble constant
  • Metallicity indicators trace chemical evolution of galaxies (iron, oxygen abundance)
  • indicators include H-alpha emission line strength
  • Active galactic nuclei identified by broad emission lines (quasars, Seyfert galaxies)

Galaxy luminosity functions

  • Number of galaxies per unit volume as a function of luminosity
  • Schechter function models luminosity function: ϕ(L)=ϕ∗(L/L∗)αe−L/L∗\phi(L) = \phi^* (L/L^*)^\alpha e^{-L/L^*}
  • Parameters include ϕ∗\phi^* (normalization factor), L∗L^* (characteristic luminosity), and α\alpha (faint-end slope)
  • Applications study galaxy evolution across cosmic time, constrain galaxy formation models, and estimate total luminosity density of the universe
  • Variations exist with galaxy type and environment (ellipticals vs spirals, clusters vs field)

Key Terms to Review (21)

Active Galactic Nucleus: An active galactic nucleus (AGN) is a highly energetic region at the center of some galaxies, powered by supermassive black holes that actively accrete matter. This process releases immense amounts of energy across the electromagnetic spectrum, often outshining the entire galaxy and giving rise to various phenomena such as jets and radiation. AGNs are classified into different types based on their emission characteristics and can provide insights into galaxy formation and evolution.
Barred spiral: A barred spiral is a type of galaxy characterized by a central bar-shaped structure made up of stars, which extends outward to spiral arms. This distinctive shape differentiates barred spirals from regular spiral galaxies, often leading to different star formation rates and patterns. The presence of the bar can influence the dynamics of stars and gas within the galaxy, affecting the overall structure and evolution of the galaxy.
Bulge: The bulge refers to the rounded, central region of a galaxy, typically containing a higher concentration of stars and stellar populations. This feature is particularly prominent in spiral galaxies and is often surrounded by a flat disk of stars and interstellar matter. The bulge is significant because it plays a crucial role in the dynamics, formation, and evolution of the galaxy, influencing both its structure and the orbits of stars within it.
Cosmic web: The cosmic web is the large-scale structure of the universe, characterized by a vast network of interconnected filaments of dark matter and galaxies. This structure forms a complex pattern of voids, filaments, and clusters, shaping how matter is distributed throughout the universe. Understanding the cosmic web is crucial for studying galaxy formation, the behavior of dark matter, and the overall evolution of cosmic structures.
Disk: In astronomy, a disk refers to a flattened, rotating structure that contains gas, dust, and stars, commonly found in galaxies. This shape arises due to the conservation of angular momentum during the formation of galaxies and plays a crucial role in the distribution of stellar populations and interstellar materials within them.
Edwin Hubble: Edwin Hubble was an American astronomer who played a crucial role in establishing the field of extragalactic astronomy and is best known for Hubble's Law, which demonstrates the expansion of the universe. His work fundamentally transformed our understanding of the cosmos, linking the historical development of astronomy with concepts such as stellar populations and galaxy classification, and providing insights into active galactic nuclei through his observational techniques.
Elliptical galaxy: An elliptical galaxy is a type of galaxy characterized by its smooth, featureless light distribution and an ellipsoidal shape. These galaxies vary in size and can be found in a range of luminosities, typically containing older stars with little to no ongoing star formation. Their structure and composition highlight significant aspects of galaxy classification and the properties of various galactic types.
Galactic collision: A galactic collision occurs when two or more galaxies pass close enough to each other that their gravitational fields interact, leading to significant structural changes in their shapes and star formations. These collisions can create new stars and sometimes even lead to the formation of a larger galaxy. The properties and classifications of galaxies help us understand the different outcomes of such collisions.
Galaxy cluster: A galaxy cluster is a large-scale structure in the universe that consists of hundreds to thousands of galaxies bound together by gravity. These clusters serve as some of the largest known gravitationally-bound structures and can contain significant amounts of dark matter and hot gas, influencing the formation and evolution of galaxies within them. They are essential in understanding the distribution of matter in the universe and the overall cosmic structure.
Galaxy merger: A galaxy merger is an event where two or more galaxies collide and gravitationally interact, often resulting in the formation of a larger galaxy. This process can trigger significant changes in the galaxies' structure, star formation rates, and the dynamics of their supermassive black holes, influencing their evolution over time.
Halo: In astronomy, a halo refers to a spherical region surrounding a galaxy, particularly prominent in spiral galaxies, which contains a mix of stars, gas, and dark matter. The halo plays a crucial role in the formation and evolution of galaxies, as it acts as a reservoir for gas that can fuel star formation and influences the overall structure of the galaxy.
Hubble Sequence: The Hubble Sequence is a classification scheme for galaxies developed by Edwin Hubble in the 1920s, categorizing them based on their morphology. It includes elliptical galaxies, spiral galaxies, and irregular galaxies, helping to describe their shapes and structures, and indicating their evolutionary stages. This sequence has been foundational in understanding the properties and behaviors of different types of galaxies.
Irregular galaxy: An irregular galaxy is a type of galaxy that does not have a distinct regular shape, unlike spiral or elliptical galaxies. These galaxies often exhibit chaotic structures, lack a central bulge, and can be rich in gas and dust, which allows for active star formation. Their irregularities can result from gravitational interactions with other galaxies, leading to their unique and varied appearances.
Luminosity: Luminosity is the total amount of energy emitted by a star or celestial object per unit time, typically measured in watts. It provides crucial insight into a star's life cycle, size, and energy output, and helps in understanding its position on various astronomical diagrams and classifications.
Morphology: Morphology refers to the study of the structure and form of galaxies, including their shapes, sizes, and physical characteristics. Understanding the morphology of galaxies allows astronomers to classify them into different categories, revealing important information about their formation and evolution over time. This classification often connects morphological features with other properties such as star formation rates and the presence of supermassive black holes.
Redshift: Redshift refers to the phenomenon where light from an object is shifted to longer wavelengths, making it appear more red than it actually is. This effect occurs when an object moves away from the observer, and it plays a crucial role in understanding the universe's expansion and the motion of celestial bodies.
Spiral galaxy: A spiral galaxy is a type of galaxy characterized by its distinct spiral arms that wind outward from its central bulge, which typically contains a supermassive black hole. These galaxies are often rich in gas and dust, leading to ongoing star formation and a variety of stellar populations. Their structure and formation help astronomers understand the dynamics and evolution of galaxies in the universe.
Star formation rate: The star formation rate (SFR) measures the amount of mass converted into stars within a specific region of space, typically expressed in solar masses per year. This rate is crucial for understanding the evolution and dynamics of galaxies, as it influences the stellar population, chemical enrichment, and the overall lifecycle of galactic systems. The SFR is used to classify galaxies and analyze their properties, revealing how different environments affect star formation activity.
Tidal forces: Tidal forces are the gravitational effects exerted by one celestial body on another, causing distortions and variations in shape, particularly noticeable in oceans and other large bodies of water. These forces arise from the gravitational pull of a larger body, like a planet or moon, acting differently on different parts of a smaller body due to distance variations, leading to phenomena such as tides. Understanding tidal forces helps explain the dynamics of galaxy interactions and influences on galactic structure.
Tuning Fork Diagram: A tuning fork diagram is a graphical representation used to classify galaxies based on their morphology and other key properties. This diagram helps illustrate the relationship between different types of galaxies, primarily focusing on the progression from elliptical to spiral forms, resembling the shape of a tuning fork. By categorizing galaxies this way, astronomers can better understand their formation, evolution, and the role they play in the broader structure of the universe.
Vera Rubin: Vera Rubin was an American astronomer known for her pioneering work on the rotation curves of galaxies, which provided critical evidence for the existence of dark matter. Her research transformed our understanding of the Milky Way's structure and dynamics and influenced how galaxies are classified and studied, as well as how we perceive galaxy clusters and large-scale cosmic structures.
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Glossary
Glossary
11.2 Galaxy formation and evolution