1.5 Galaxy morphology and environment
9 min read•august 20, 2024
Galaxy morphology explores the diverse shapes and structures of galaxies. From spirals to ellipticals, these forms reveal a galaxy's history, star formation, and dynamics. Understanding morphology is key to unraveling galaxy evolution and cosmic structure formation.
Environmental factors play a crucial role in shaping galaxy morphology. Clusters, interactions, and mergers can transform galaxies over time. Studying these effects helps astronomers piece together the complex story of how galaxies form and evolve throughout the universe.
Types of galaxy morphologies
- Galaxies exhibit a wide range of morphologies, or shapes and structures, that can be broadly classified into several distinct categories
- The morphology of a galaxy is determined by its star formation history, stellar populations, gas and dust content, and dynamical properties
- Understanding the different types of galaxy morphologies is crucial for studying galaxy formation and evolution processes
Spiral galaxies
Top images from around the web for Spiral galaxies
Group 10 Galaxies And Black Holes - Space View original
Is this image relevant?
The Architecture of the Galaxy · Astronomy View original
Is this image relevant?
File:Andromeda galaxy.jpg - Wikipedia View original
Is this image relevant?
Group 10 Galaxies And Black Holes - Space View original
Is this image relevant?
The Architecture of the Galaxy · Astronomy View original
Is this image relevant?
1 of 3
Top images from around the web for Spiral galaxies
Group 10 Galaxies And Black Holes - Space View original
Is this image relevant?
The Architecture of the Galaxy · Astronomy View original
Is this image relevant?
File:Andromeda galaxy.jpg - Wikipedia View original
Is this image relevant?
Group 10 Galaxies And Black Holes - Space View original
Is this image relevant?
The Architecture of the Galaxy · Astronomy View original
Is this image relevant?
1 of 3
- Characterized by a flat, rotating containing that extend from a central
- Spiral arms are regions of active star formation, often traced by bright young blue stars and glowing HII regions (ionized hydrogen gas clouds)
- Spiral galaxies are typically rich in gas and dust, which fuels ongoing star formation
- Examples include the Milky Way and the Andromeda Galaxy (M31)
Barred spiral galaxies
- A subclass of spiral galaxies that feature a prominent elongated structure, or "bar," crossing the center of the disk
- The bar is composed of older stars and acts to funnel gas into the central regions, triggering star formation and bulge growth
- Roughly two-thirds of all spiral galaxies are barred, suggesting bars are a common evolutionary phase
- Examples include the Milky Way and NGC 1300
Elliptical galaxies
- Smooth, ellipsoidal systems lacking spiral arms or other complex structures
- Composed mainly of old, red stars with little ongoing star formation and minimal gas and dust
- Range from spherical (E0) to highly elongated (E7) shapes, with increasing ellipticity
- Believed to form through mergers of smaller galaxies or the exhaustion of gas in spirals
- Examples include M87 and Centaurus A
Irregular galaxies
- Galaxies that lack a well-defined symmetrical structure and do not fit into the other morphological categories
- Often smaller galaxies with chaotic appearances, possibly due to gravitational interactions or mergers
- Can be gas-rich with pockets of active star formation, or gas-poor with minimal star formation
- Examples include the Large and Small Magellanic Clouds
Lenticular galaxies
- Transitional class between spiral and elliptical galaxies, designated as S0
- Feature a prominent bulge and a disk, but lack spiral arms or extensive star formation
- May represent an evolutionary stage as spirals exhaust their gas and cease star formation
- Contain mainly older, redder stellar populations similar to ellipticals
- Example: NGC 3115
Hubble sequence of galaxies
- The , also known as the Hubble tuning fork diagram, is a morphological classification scheme for galaxies proposed by Edwin Hubble in 1926
- Galaxies are arranged along a sequence based on their appearance, with ellipticals on the left, spirals on the right, and lenticulars at the intersection
- The Hubble sequence represents a continuous spectrum of galaxy morphologies and is still widely used today, with some modifications and extensions
Early vs late type galaxies
- Galaxies on the left side of the Hubble sequence (ellipticals and lenticulars) are referred to as "early-type" galaxies
- Early-type galaxies are typically older, redder, and have little ongoing star formation
- Galaxies on the right side of the sequence (spirals and irregulars) are called "late-type" galaxies
- Late-type galaxies are generally younger, bluer, and have more active star formation
- The terms "early" and "late" do not imply an evolutionary sequence, but rather reflect historical naming conventions
Spiral arms and bulges
- Spiral galaxies are characterized by their distinctive spiral arms, which are regions of enhanced star formation and
- Spiral arms are thought to be caused by density waves that compress gas and trigger star formation as they rotate through the disk
- The central bulge of a is a spheroidal component composed mainly of older, redder stars
- Bulges are thought to grow through mergers, gas accretion, and secular evolution processes
- The relative size of the bulge compared to the disk varies among spiral galaxies and is used as a classification criterion
Barred vs unbarred spirals
- Spiral galaxies are further classified as barred or unbarred, depending on the presence of a central elongated structure called a bar
- Bars are composed of older stars and are thought to form through instabilities in the disk
- Barred spirals are designated as SB, while unbarred spirals are designated as SA
- The presence of a bar can affect gas dynamics, star formation, and bulge growth in a galaxy
Ellipticals and spheroids
- Elliptical galaxies are smooth, ellipsoidal systems with little internal structure
- They are classified based on their ellipticity, from E0 (spherical) to E7 (highly elongated)
- Ellipticals are composed mainly of older, redder stars and have little ongoing star formation
- Dwarf elliptical and spheroidal galaxies are smaller, low-mass systems that share similar properties with larger ellipticals
- The formation and evolution of elliptical galaxies are thought to involve mergers, gas exhaustion, and feedback processes
Impact of environment on morphology
- The morphology of a galaxy is not only determined by its internal properties but also influenced by its surrounding environment
- Galaxies in dense environments, such as clusters and groups, often exhibit different morphologies compared to isolated galaxies
- can lead to the transformation of galaxy morphologies over time
Galaxy clusters and morphology
- Galaxy clusters are the largest gravitationally bound structures in the universe, containing hundreds to thousands of galaxies
- Galaxies in clusters tend to have a higher fraction of elliptical and lenticular morphologies compared to field galaxies
- The dense environment of clusters can strip gas from galaxies, quench star formation, and lead to morphological changes
- Examples of morphology-rich clusters include the Coma Cluster and the Virgo Cluster
Morphology-density relation
- There is a strong correlation between galaxy morphology and the local density of the environment, known as the morphology-density relation
- The fraction of elliptical and lenticular galaxies increases with increasing environmental density, while the fraction of spiral and irregular galaxies decreases
- This relation suggests that environmental processes play a significant role in shaping galaxy morphologies
- The morphology-density relation holds across a wide range of environments, from rich clusters to loose groups
Tidal interactions and mergers
- Galaxies can experience tidal interactions and mergers with other galaxies, which can drastically alter their morphologies
- Tidal interactions occur when galaxies pass close to each other, causing gravitational distortions and triggering star formation
- Mergers involve the collision and coalescence of two or more galaxies, resulting in the formation of a single larger galaxy
- Mergers can lead to the formation of elliptical galaxies, as well as trigger intense starbursts and AGN activity
- Examples of merging galaxies include the Antennae Galaxies and the Mice Galaxies
Ram pressure stripping effects
- Ram pressure stripping is a process by which the hot intracluster medium (ICM) in a strips away the cold gas from a galaxy as it moves through the cluster
- The removal of cold gas quenches star formation in the galaxy and can lead to morphological changes over time
- Spiral galaxies experiencing ram pressure stripping often show asymmetric or truncated gas disks, with star formation confined to the central regions
- Examples of galaxies undergoing ram pressure stripping include NGC 4522 in the Virgo Cluster
Evolution of galaxy morphology
- Galaxy morphologies are not static but evolve over cosmic time due to a variety of physical processes
- The observed morphologies of galaxies at different redshifts provide insights into the growth and assembly of galaxies throughout the history of the universe
- Understanding the mechanisms driving morphological evolution is a key goal of galaxy formation and evolution studies
Secular evolution processes
- Secular evolution refers to the slow, internal processes that can change a galaxy's morphology over long timescales
- Examples of secular processes include gas accretion, bar formation, and bulge growth through disk instabilities
- Secular evolution can lead to the transformation of spiral galaxies into lenticular or elliptical galaxies as they exhaust their gas and cease star formation
- Secular processes are thought to be dominant in low-density environments where interactions are less frequent
Morphological transformation mechanisms
- Various mechanisms can drive the transformation of galaxy morphologies, often in combination with each other
- Mergers and tidal interactions can disrupt galaxy structures, trigger starbursts, and lead to the formation of elliptical galaxies
- Ram pressure stripping and tidal stripping can remove gas and stars from galaxies, leading to morphological changes and quenching of star formation
- Feedback processes, such as supernovae and AGN activity, can expel gas from galaxies and regulate star formation, affecting their morphological evolution
Redshift dependence of morphology
- The morphological mix of galaxies changes with redshift, reflecting the evolution of galaxy populations over cosmic time
- At higher redshifts (earlier times), a larger fraction of galaxies exhibit irregular or peculiar morphologies, indicative of more frequent interactions and mergers
- The fraction of spiral and lenticular galaxies increases towards lower redshifts (later times), as galaxies settle into more regular structures
- Elliptical galaxies are observed at all redshifts, but their properties and formation histories may vary with cosmic time
Morphology in the early universe
- Observations of galaxies in the early universe (high redshifts) provide crucial tests for galaxy formation theories
- At redshifts z > 2, many galaxies appear clumpy and irregular, with high star formation rates and turbulent gas motions
- The first massive elliptical galaxies are observed at z ~ 2-3, likely formed through rapid mergers and gas accretion
- The morphological diversity of early galaxies suggests that multiple formation pathways and processes were at play in the young universe
Observational techniques for morphology
- Studying galaxy morphology requires a combination of observational techniques and data analysis methods
- Advances in imaging technology, from ground-based telescopes to space observatories, have greatly expanded our ability to resolve and classify galaxy morphologies across cosmic time
- Quantitative methods and machine learning algorithms are increasingly used to complement visual classification and provide more objective morphological measurements
Visual classification methods
- Visual classification of galaxy morphologies has been the traditional approach, dating back to the Hubble scheme
- Astronomers examine images of galaxies and assign them to morphological categories based on their appearance and structural features
- Visual classification is often done using a decision tree or a sequence of criteria, such as the presence of spiral arms, bars, or bulges
- Citizen science projects, like Galaxy Zoo, have engaged the public in visual classification and produced large morphological catalogs
Automated classification algorithms
- Automated classification algorithms use computational methods to classify galaxy morphologies based on their image properties
- These algorithms can process large datasets quickly and provide objective, reproducible classifications
- Machine learning techniques, such as convolutional neural networks (CNNs), have been successfully applied to galaxy morphology classification
- Automated methods can also identify rare or unusual morphologies that may be missed by visual classification
Quantitative morphological parameters
- Quantitative parameters are used to describe and measure specific aspects of galaxy morphology
- Examples include the Sérsic index (a measure of the light profile), the bulge-to-total ratio (the relative strength of the bulge), and the concentration-asymmetry-smoothness (CAS) parameters
- These parameters can be derived from galaxy images using various software packages and algorithms
- Quantitative parameters allow for more objective comparisons of galaxy morphologies and can reveal trends and correlations with other galaxy properties
Multi-wavelength morphological studies
- Galaxy morphology can vary depending on the observed wavelength, as different components (e.g., stars, gas, dust) emit at different wavelengths
- Multi-wavelength studies provide a more comprehensive view of galaxy structure and the distribution of various components
- Optical and near-infrared imaging trace the stellar content, while ultraviolet imaging reveals young, star-forming regions
- Mid-infrared and radio observations can probe dust obscured regions and cold gas, respectively
- Comparing morphologies across wavelengths can provide insights into the interplay between stars, gas, and dust in galaxies and their evolution over time
Key Terms to Review (28)
Barred spiral galaxy: A barred spiral galaxy is a type of spiral galaxy characterized by a central bar-shaped structure composed of stars, extending from the core and influencing the motion of stars and gas within the galaxy. This distinct feature enhances the organization of star formation and leads to unique structures in the galaxy's spiral arms. Barred spiral galaxies are considered to be an important stage in galactic evolution, offering insight into how galaxies develop over time.
Bulge: In the context of galaxies, a bulge refers to the dense, central region found in many galaxies, typically characterized by a high concentration of stars, gas, and dust. Bulges can vary in size and composition, playing a crucial role in the overall structure and dynamics of a galaxy, influencing star formation and the evolutionary processes of the galaxy as a whole.
Density: Density is a measure of mass per unit volume, often expressed in terms like grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³). In the context of astronomical structures, density plays a critical role in determining the formation, evolution, and classification of galaxies, as well as the behavior and life cycle of molecular clouds. Higher density regions often indicate areas where gravitational forces can lead to star formation or influence galactic morphology.
Disk: In the context of galaxies, a disk refers to a flattened, rotating structure that contains stars, gas, and dust. This component is typically found in spiral and some irregular galaxies, where it forms the main region of star formation and contains the galaxy's spiral arms. The disk is essential for understanding galaxy morphology and dynamics, as well as the overall structure of galaxies in the universe.
Dynamical mass: Dynamical mass refers to the mass of an astronomical object derived from its gravitational effects on nearby objects rather than directly measuring the object's matter content. This concept is crucial for understanding the mass distribution and dynamics of galaxies, as it helps explain how gravitational forces influence the motion of stars and gas within different morphological structures of galaxies.
Early-type galaxy: An early-type galaxy is a classification of galaxies that includes elliptical and lenticular galaxies, typically characterized by their smooth, featureless light profiles and lack of significant amounts of gas and dust. These galaxies are often older and more evolved compared to late-type galaxies, showing little star formation activity and primarily consisting of older stellar populations.
Elliptical galaxy: An elliptical galaxy is a type of galaxy characterized by its smooth, featureless light profile and an ellipsoidal shape, often containing older stars with little to no ongoing star formation. These galaxies range from nearly spherical to elongated structures and are usually found in larger galaxy clusters. Their structure and composition connect them to various cosmic phenomena and the broader structure of the universe.
Environmental Effects: Environmental effects refer to the impact that the surrounding cosmic environment has on the formation, structure, and evolution of galaxies. These effects can shape galaxy morphology, influencing whether a galaxy develops features such as spiral arms or elliptical shapes, and can also affect star formation rates and the presence of active galactic nuclei. Understanding these influences helps astronomers decipher the complex interactions between galaxies and their environments.
Galactic Collision: A galactic collision is a significant astronomical event where two or more galaxies interact gravitationally, leading to dramatic structural changes in their shapes, star formation rates, and overall dynamics. This phenomenon can lead to the merging of galaxies and the formation of new stellar systems, while also influencing the environment of each galaxy involved. Such collisions can occur over millions of years, altering the course of galactic evolution and the characteristics of the involved galaxies.
Galaxy cluster: A galaxy cluster is a large grouping of galaxies that are bound together by gravity. These clusters can contain anywhere from a few dozen to thousands of galaxies, along with hot gas and dark matter, making them the largest known structures in the universe. They play a significant role in understanding galaxy morphology and the overall cosmic web due to their interactions and the influence they exert on their environment.
Galaxy group: A galaxy group is a collection of galaxies that are gravitationally bound to one another, typically consisting of a few to several dozen galaxies. These groups are the smallest structures in the larger cosmic web and play a crucial role in understanding galaxy morphology and the environments in which galaxies exist. Galaxy groups can vary significantly in terms of their composition, luminosity, and dynamics, influencing the way we study galaxy formation and evolution.
Gravitational Influence: Gravitational influence refers to the effect that the gravitational pull of a massive object, like a galaxy or cluster of galaxies, has on surrounding objects, such as stars, gas, and other galaxies. This force can shape the structure and behavior of galaxies, affecting their morphology and how they interact with their environment. Gravitational influence plays a crucial role in the dynamics of galaxy formation, evolution, and the large-scale structure of the universe.
Halo: In astronomy, a halo refers to the diffuse, extended region of stars, gas, and dark matter surrounding a galaxy, particularly prominent in spiral galaxies. This area is crucial for understanding galaxy formation and evolution, as it contains ancient stars and the materials from which galaxies formed. The halo plays an essential role in how galaxies interact with their environment and helps to inform the classification of galaxies based on their structure and morphology.
Hierarchical Structure Formation: Hierarchical structure formation is a cosmological model that describes how the universe evolves from small, simple structures to larger, more complex ones, often involving the merging of smaller entities to form bigger systems. This process plays a vital role in shaping the formation and distribution of galaxies, leading to the diversity of galaxy types and their environments. As smaller structures collapse under gravity, they create gravitational wells that attract more mass, eventually resulting in the formation of galaxy clusters and large-scale structures in the universe.
Hubble Sequence: The Hubble Sequence is a morphological classification scheme for galaxies, introduced by Edwin Hubble in the early 20th century. It categorizes galaxies based on their shapes and structures into three main types: elliptical, spiral, and irregular. This sequence helps in understanding the evolutionary pathways and relationships between different galaxy types, revealing how their morphology relates to factors like environment and dynamics.
Hubble's Classification: Hubble's Classification is a system developed by Edwin Hubble in the 1920s that categorizes galaxies based on their morphology and structural features. This classification helps in understanding the diversity of galaxy shapes, sizes, and environments, linking the physical properties of galaxies to their formation and evolutionary processes. By organizing galaxies into distinct categories, Hubble's system aids astronomers in studying the relationship between galaxy morphology and their surrounding environments.
Irregular galaxy: An irregular galaxy is a type of galaxy that lacks a distinct shape or structure, distinguishing it from more organized types like spiral and elliptical galaxies. These galaxies often appear chaotic and have an uneven distribution of stars, gas, and dust, leading to their non-uniform appearance. Irregular galaxies are typically smaller and can be found in various environments, often interacting with other galaxies which can influence their shape and star formation activity.
Late-type galaxy: A late-type galaxy is a classification of galaxies that typically have a high proportion of gas and dust, as well as ongoing star formation. These galaxies often exhibit a spiral or irregular morphology, characterized by their well-defined arms and bright, young stars. Late-type galaxies are often found in less dense environments compared to their early-type counterparts, making them significant in understanding galaxy formation and evolution.
Lenticular Galaxy: A lenticular galaxy is a type of galaxy that features a central bulge surrounded by a flat, disc-like structure with a noticeable lack of spiral arms, resembling a combination of features found in both spiral and elliptical galaxies. These galaxies are often found in dense environments and can provide insight into the evolutionary processes between different galaxy types, particularly through the Hubble sequence, where they are categorized as S0 galaxies.
Merger: A merger is the process where two or more galaxies combine to form a single, larger galaxy. This phenomenon plays a crucial role in galaxy evolution, as mergers can trigger star formation, alter galaxy morphology, and influence the overall dynamics of galaxies within their environment.
Monolithic collapse: Monolithic collapse refers to a model of galaxy formation where a single massive gas cloud collapses under its own gravity to form a galaxy. This process implies that the entire structure forms rapidly and cohesively, leading to a more uniform distribution of stars and materials. This model contrasts with other theories, emphasizing how the environment and initial conditions play crucial roles in determining the resulting galaxy morphology.
Photometry: Photometry is the science of measuring the intensity of light and its properties, especially as it relates to celestial objects. This measurement plays a vital role in understanding the brightness and luminosity of stars, galaxies, and other astronomical phenomena, allowing astronomers to categorize objects, analyze their composition, and understand their distances and environments.
Rotational Curve: A rotational curve is a plot that represents the rotational velocity of stars or gas in a galaxy as a function of their distance from the galaxy's center. This curve is crucial in understanding the distribution of mass within galaxies and provides insights into their structure and dynamics, particularly in distinguishing between normal matter and dark matter.
Spectroscopy: Spectroscopy is the study of the interaction between light and matter, particularly focusing on how light is absorbed, emitted, or scattered by atoms and molecules. This technique allows astronomers to analyze the composition, temperature, density, and motion of celestial objects, providing crucial insights into their physical properties and behaviors.
Spiral Arms: Spiral arms are regions of enhanced density in spiral galaxies, characterized by the presence of young stars, gas, and dust, which create a visually striking pattern as they extend outward from the galactic center. These structures play a critical role in the ongoing star formation process within the galaxy and are often associated with the spiral density wave theory, which explains how they maintain their distinct form over time. They also contribute to the overall morphology of galaxies and help astronomers classify different types of galaxies.
Spiral galaxy: A spiral galaxy is a type of galaxy characterized by its distinct spiral arms that wind outward from a central bulge, typically containing a mix of young and old stars, gas, and dust. These galaxies are often rich in star formation, particularly in the arms, and are one of the most common galaxy types observed in the universe. Their structure and formation provide insights into the evolutionary processes of galaxies and their environments.
Tidal Forces: Tidal forces are the gravitational interactions between celestial bodies that result in the distortion of their shapes, leading to phenomena such as tides on planets and moons. These forces are crucial in understanding how galaxies interact, affecting their morphology and environment, as well as playing a significant role in the dynamics of tidal interactions and galaxy mergers.
Void: A void refers to vast, relatively empty regions of space that contain few or no galaxies, typically found in the large-scale structure of the universe. These expansive areas are crucial in understanding the distribution of galaxies and their environments, highlighting how matter is organized in the cosmic web, where galaxies are often found along filaments and in clusters while voids remain largely unpopulated.