10.2 Galaxy surveys
12 min read•august 20, 2024
Galaxy surveys are powerful tools for understanding the Universe's structure and evolution. They employ various techniques to observe and characterize galaxies across different wavelengths, providing crucial data on their properties, distribution, and evolution over cosmic time.
These surveys reveal key galaxy properties like redshifts, luminosities, and morphologies. By mapping large-scale structures and studying , they offer insights into dark matter, dark energy, and the fundamental properties of our cosmos.
Techniques for galaxy surveys
- Galaxy surveys are essential tools for understanding the properties, distribution, and evolution of galaxies across the Universe
- Surveys employ various techniques to observe and characterize galaxies at different wavelengths and depths
- The choice of survey technique depends on the scientific goals, available instrumentation, and observational constraints
Photometric vs spectroscopic surveys
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- Photometric surveys measure the brightness of galaxies in multiple wavelength bands using broadband filters
- Provide information on galaxy colors, approximate redshifts, and crude estimates of stellar populations
- Enable the detection of a large number of galaxies over wide areas of the sky
- Examples include the (SDSS) and the Dark Energy Survey (DES)
- Spectroscopic surveys obtain detailed spectra of galaxies using spectrographs
- Reveal the precise redshifts, chemical compositions, star formation histories, and kinematics of galaxies
- Require longer integration times and are limited to smaller galaxy samples compared to photometric surveys
- Examples include the SDSS spectroscopic survey and the DEEP2 Galaxy Redshift Survey
Advantages of multi-wavelength surveys
- Multi-wavelength surveys observe galaxies across different regions of the electromagnetic spectrum (optical, infrared, radio, X-ray)
- Provide a comprehensive view of galaxy properties and the physical processes driving their evolution
- Optical and near-infrared observations trace stellar populations and star formation
- Mid- and far-infrared data reveal dust content and obscured star formation
- Radio observations probe atomic and molecular gas, as well as (AGN)
- X-ray emission is associated with hot gas, AGN, and high-energy phenomena
- Enable the study of the interplay between different components of galaxies (stars, gas, dust, AGN) and their environment
Challenges in deep sky surveys
- Deep sky surveys aim to detect faint and distant galaxies, pushing the limits of observational capabilities
- Require long integration times and large telescopes with sensitive detectors to collect sufficient light from faint sources
- Suffer from increased contamination by foreground stars and galaxies, making it difficult to identify and characterize distant galaxies
- Redshift determination becomes challenging for faint galaxies due to the lack of strong spectral features and the shifting of features to longer wavelengths
- Require careful data reduction, calibration, and analysis techniques to extract reliable information from low signal-to-noise observations
Key galaxy properties from surveys
- Galaxy surveys provide a wealth of information on the key properties of galaxies, enabling astronomers to study their formation, evolution, and distribution in the Universe
- These properties include redshifts, distances, luminosities, masses, colors, and morphologies, which are essential for understanding the physical processes shaping galaxies and their environments
Redshift measurements and distance estimates
- Redshift is a crucial property measured from galaxy spectra, indicating the shift of spectral features due to the expansion of the Universe
- Cosmological redshift is caused by the stretching of light wavelengths as galaxies recede from us
- Measured by identifying known spectral features (emission or absorption lines) and comparing their observed wavelengths to their rest-frame values
- Redshifts enable the estimation of galaxy distances using the , which relates redshift to distance in the expanding Universe
- Accurate distance measurements are essential for constructing 3D maps of the galaxy distribution and studying the large-scale structure of the Universe
- Redshifts also provide a proxy for cosmic time, allowing astronomers to study galaxy evolution across different epochs
Luminosity and mass distributions
- Galaxy surveys measure the apparent brightnesses of galaxies, which can be converted to intrinsic luminosities using distance estimates
- Luminosity is a fundamental property related to the total energy output of a galaxy, primarily from its stellar population
- Luminosity distributions provide insights into the range of galaxy sizes and the relative abundances of bright and faint galaxies
- Stellar mass is another key property estimated from galaxy colors and luminosities using stellar population synthesis models
- Stellar mass is a measure of the total mass locked up in stars within a galaxy
- Mass distributions reveal the hierarchical nature of galaxy formation and the efficiency of star formation in different environments
Color-magnitude diagrams
- plot galaxy colors (e.g., B-V, U-B) against their absolute magnitudes, revealing distinct galaxy populations
- The "red sequence" contains mostly older, quiescent galaxies with little ongoing star formation
- The "blue cloud" consists of actively star-forming galaxies with younger stellar populations
- The "green valley" represents a transition region between the red sequence and blue cloud, potentially containing galaxies undergoing quenching of star formation
- Color- diagrams provide insights into the star formation histories and evolution of galaxies, as well as the impact of environmental factors on galaxy properties
Morphological classifications
- Galaxy surveys enable the classification of galaxies based on their morphologies, which reflect their structural and dynamical properties
- The Hubble sequence is a widely used morphological classification scheme, dividing galaxies into three main types:
- : smooth, ellipsoidal shape with little ongoing star formation
- : disk-like structure with spiral arms and active star formation
- Irregular galaxies: chaotic appearance without a well-defined structure, often associated with recent interactions or mergers
- Morphological classifications can be performed visually by experts or through automated algorithms applied to survey images
- Studying the distribution and evolution of provides insights into the physical processes (mergers, interactions, secular evolution) shaping galaxies over cosmic time
Major galaxy survey projects
- Several groundbreaking galaxy survey projects have revolutionized our understanding of the Universe by providing extensive datasets for studying galaxy properties, distribution, and evolution
- These surveys cover different regions of the sky, wavelength ranges, and depths, each contributing unique insights into the galaxy population and the large-scale structure of the Universe
Sloan Digital Sky Survey (SDSS)
- One of the most influential and comprehensive galaxy surveys, covering over one-third of the celestial sphere
- Utilizes a dedicated 2.5-meter telescope at Apache Point Observatory in New Mexico, equipped with a 120-megapixel camera and a pair of spectrographs
- Provides photometric and spectroscopic data for millions of galaxies, enabling studies of galaxy properties, clustering, and evolution
- Has undergone multiple phases (SDSS-I, II, III, IV) with increasing sky coverage, depth, and scientific scope
- Key scientific results include the discovery of , the mapping of large-scale structure, and the characterization of galaxy populations
Hubble Deep Field and Ultra Deep Field
- Groundbreaking deep imaging surveys conducted by the Hubble Space Telescope, revealing the distant Universe in unprecedented detail
- The Hubble Deep Field (HDF) was observed in 1995, covering a small patch of sky in the constellation Ursa Major
- Stared at a single point for over 100 hours, detecting galaxies up to 12 billion light-years away
- Provided the first glimpse into the early stages of galaxy formation and evolution
- The Hubble Ultra Deep Field (HUDF) was observed in 2003-2004, surpassing the depth of the HDF
- Covered a smaller area but reached even fainter galaxies, some as distant as 13 billion light-years
- Revealed a wealth of intricate galaxy morphologies and the prevalence of small, irregular galaxies in the early Universe
- These deep fields have become iconic images and have driven numerous follow-up studies and theoretical investigations into galaxy formation and evolution
Galaxy and Mass Assembly (GAMA) survey
- A spectroscopic survey designed to study the distribution and evolution of galaxies in the nearby Universe (z < 0.5)
- Covers approximately 286 square degrees of the sky, providing spectra for over 300,000 galaxies
- Combines data from several ground-based and space-based facilities, including the Anglo-Australian Telescope, the Visible and Infrared Survey Telescope for Astronomy (VISTA), and the Herschel Space Observatory
- Aims to understand the interplay between galaxy properties (mass, size, morphology, star formation) and their environment, from isolated galaxies to dense clusters
- Provides insights into the role of dark matter in galaxy formation and evolution, as well as the impact of feedback processes on galaxy growth
Future large-scale survey missions
- Upcoming galaxy surveys will push the boundaries of our understanding of the Universe by covering larger areas, reaching deeper magnitudes, and utilizing cutting-edge instrumentation
- The Vera C. Rubin Observatory (formerly LSST) will conduct a 10-year survey of the southern sky, imaging billions of galaxies and measuring their positions, shapes, and colors
- Will enable unprecedented studies of dark matter, dark energy, and the evolution of the Universe
- The Euclid mission, led by the European Space Agency, will map the 3D distribution of galaxies over a large portion of the sky, probing the nature of dark energy and the growth of cosmic structure
- The Nancy Grace Roman Space Telescope (formerly WFIRST) will conduct wide-field infrared surveys, studying the expansion history of the Universe and the growth of galaxies and clusters
- These future surveys will provide a wealth of data for advancing our understanding of galaxy formation, evolution, and the fundamental properties of the Universe
Cosmological implications of galaxy surveys
- Galaxy surveys not only reveal the properties and evolution of individual galaxies but also provide crucial insights into the large-scale structure and fundamental properties of the Universe
- By mapping the 3D distribution of galaxies and studying their statistical properties, galaxy surveys enable tests of cosmological models and constrain key parameters governing the expansion and growth of structure in the Universe
Mapping large-scale structure
- Galaxy surveys reveal the intricate web-like structure of the Universe, consisting of galaxies, galaxy clusters, and vast cosmic voids
- The large-scale structure is characterized by filaments and sheets of galaxies surrounding underdense voids, forming a complex network that traces the underlying
- Studying the topology and evolution of the large-scale structure provides insights into the gravitational instability process that amplified primordial density fluctuations into the observed cosmic web
- Comparing the observed large-scale structure with cosmological simulations allows astronomers to test theories of structure formation and constrain cosmological parameters
Constraints on dark matter and dark energy
- Galaxy surveys provide indirect evidence for the existence and properties of dark matter and dark energy, the two dominant components of the Universe
- Dark matter is inferred from its gravitational influence on galaxy motions and the growth of structure
- Galaxy clustering measurements can constrain the density and clustering properties of dark matter
- The observed shapes and abundances of galaxy clusters are sensitive to the nature of dark matter (e.g., cold vs. warm dark matter)
- Dark energy is probed through its effect on the expansion history of the Universe and the growth of structure
- Measuring the distances to galaxies as a function of redshift (using standard candles like Type Ia supernovae) reveals the accelerating expansion caused by dark energy
- The evolution of galaxy clustering over cosmic time is influenced by the competing effects of dark matter gravity and dark energy's accelerated expansion
Galaxy clustering and baryon acoustic oscillations
- Galaxy surveys enable precise measurements of galaxy clustering, which encodes information about the matter distribution and cosmological parameters
- The two-point correlation function and its Fourier transform, the , are commonly used statistical tools to quantify galaxy clustering
- These measures describe the excess probability of finding galaxy pairs at different separations compared to a random distribution
- The amplitude and shape of the clustering measures are sensitive to the matter density, dark energy, and the nature of gravity
- Baryon acoustic oscillations (BAO) are a distinctive feature imprinted in the galaxy distribution, arising from sound waves in the early Universe
- BAO appear as a peak in the correlation function or a series of wiggles in the power spectrum at a characteristic scale (~150 Mpc)
- This scale serves as a "standard ruler" for measuring cosmic distances and constraining the expansion history of the Universe
- Measuring the BAO scale at different redshifts provides a powerful probe of dark energy and the curvature of the Universe
Evolution of galaxy populations over cosmic time
- Galaxy surveys spanning a wide range of redshifts enable the study of galaxy evolution over billions of years of cosmic history
- By comparing the properties (luminosities, colors, morphologies, star formation rates) of galaxies at different epochs, astronomers can trace the growth and transformation of galaxies
- Key evolutionary trends include:
- The decline of star formation activity and the build-up of massive, quiescent galaxies since z ~ 2
- The emergence of the Hubble sequence and the increasing prevalence of disk galaxies at later times
- The role of mergers and interactions in shaping galaxy properties and triggering starburst and AGN activity
- Studying the environmental dependence of galaxy evolution (e.g., comparing galaxies in clusters vs. the field) provides insights into the physical processes driving galaxy transformation
- Confronting the observed evolutionary trends with theoretical models and simulations helps to constrain the physics of galaxy formation, including feedback processes, gas accretion, and the impact of dark matter halos
Statistical analysis of survey data
- Galaxy surveys generate vast amounts of data, requiring sophisticated statistical techniques to extract meaningful information and test theoretical models
- Statistical analysis is essential for quantifying the properties of galaxy populations, characterizing their distribution in space and time, and comparing observations with simulations and theoretical predictions
Completeness and selection effects
- Galaxy surveys are subject to various selection effects and biases that must be accounted for in statistical analyses
- Magnitude-limited surveys preferentially detect brighter galaxies, leading to an incomplete sampling of the faint end of the galaxy population
- Completeness corrections are applied to account for the missing galaxies and to reconstruct the true luminosity and mass distributions
- Other selection effects include:
- Malmquist bias: the tendency to detect intrinsically brighter galaxies at larger distances
- Eddington bias: the scattering of galaxies across luminosity or mass bins due to observational uncertainties
- Surface brightness selection: the difficulty in detecting low surface brightness galaxies, which may be missed in surveys
- Careful modeling and correction for selection effects are crucial for deriving unbiased statistical properties of galaxy populations
Luminosity and mass functions
- The and the stellar mass function are fundamental statistical descriptors of galaxy populations
- The luminosity function describes the number density of galaxies as a function of their intrinsic luminosity
- Typically modeled by a Schechter function, which combines a power-law behavior at the faint end with an exponential cutoff at the bright end
- The shape and evolution of the luminosity function provide insights into the processes governing galaxy formation and evolution
- The stellar mass function describes the number density of galaxies as a function of their stellar mass
- Derived from the luminosity function using mass-to-light ratios estimated from galaxy colors or spectral energy distributions
- Provides a more direct link to the underlying physics of galaxy formation, as stellar mass is a key driver of galaxy properties and evolution
- Comparing observed luminosity and mass functions with theoretical predictions helps to constrain models of galaxy formation and the role of feedback processes
Correlation functions and power spectra
- Correlation functions and power spectra are essential tools for quantifying the clustering of galaxies and the large-scale structure of the Universe
- The two-point correlation function measures the excess probability of finding galaxy pairs at different separations compared to a random distribution
- Can be computed in real space (spatial correlation function) or in redshift space (redshift-space correlation function)
- The redshift-space correlation function is affected by peculiar velocities, leading to distortions (e.g., fingers-of-god) that can be used to probe the growth of structure
- The power spectrum is the Fourier transform of the correlation function and describes the distribution of clustering power on different scales
- Provides a complementary view of galaxy clustering, with the advantage of separating different physical processes (e.g., BAO, redshift-space distortions) in Fourier space
- Can be directly compared with theoretical predictions from cosmological perturbation theory and simulations
- Higher-order statistics, such as the three-point correlation function and the bispectrum, provide additional information on the non-Gaussian nature of galaxy clustering and the growth of structure
Machine learning approaches to galaxy classification
- Machine learning techniques are increasingly being applied to galaxy survey data to automate and optimize various tasks, such as galaxy classification, redshift estimation, and anomaly detection
- Supervised learning methods, such as decision trees, support vector machines, and convolutional neural networks, can be trained on labeled galaxy samples to classify galaxies based on their morphologies, colors, or spectral features
- These methods can efficiently process large datasets and provide consistent and reproducible classifications
- However, they require reliable training data and may be limited by the quality and representativeness of the labeled samples
- Unsupervised learning methods, such as clustering algorithms and dimensionality reduction techniques, can identify patterns and structures in galaxy data without prior labeling
- These methods can discover new classes of galaxies or reveal hidden relationships between galaxy properties
- They are particularly useful for exploring large, high-dimensional datasets and identifying outliers or rare objects
- Deep learning techniques, such as convolutional neural networks, have shown promising results in galaxy classification and redshift estimation
- These methods can automatically learn relevant features from galaxy images or spectra, potentially surpassing traditional feature engineering approaches
- However, deep learning models require large training datasets and can be computationally expensive to train and optimize
- Machine learning approaches complement traditional statistical analysis and can help to maximize the scientific return of galaxy surveys by enabling the efficient processing and interpretation of large, complex datasets
Key Terms to Review (24)
Active Galactic Nuclei: Active Galactic Nuclei (AGN) are extremely bright regions found at the centers of some galaxies, powered by supermassive black holes that are actively accreting material. These regions can outshine entire galaxies due to the tremendous energy produced as matter falls into the black hole, often resulting in various forms of radiation across the electromagnetic spectrum. The presence of AGN indicates dynamic processes related to black hole growth, galaxy evolution, and interactions with surrounding matter.
Baryon Acoustic Oscillations: Baryon acoustic oscillations refer to the periodic fluctuations in the density of visible baryonic matter (normal matter) in the universe, which were produced by sound waves in the early universe. These oscillations are critical as they provide evidence of the distribution of matter and energy in the cosmos, influencing structures like galaxy clusters, superclusters, and voids.
Color-magnitude diagrams: Color-magnitude diagrams (CMDs) are graphical tools used in astronomy to plot stars based on their color (which indicates temperature) against their brightness (magnitude). These diagrams help astronomers understand the distribution and characteristics of stars in a galaxy, revealing important information about stellar populations, their evolutionary stages, and the overall structure of galaxies.
Cosmic Microwave Background: The cosmic microwave background (CMB) is the afterglow radiation from the Big Bang, permeating the universe and providing a snapshot of the early universe when it was just about 380,000 years old. This faint glow, detected in the microwave part of the electromagnetic spectrum, is crucial for understanding the formation and evolution of structures in the universe, linking various aspects of cosmology and astrophysics.
Dark matter distribution: Dark matter distribution refers to the way dark matter is spread out throughout galaxies and the universe, influencing gravitational forces and the overall structure of cosmic formations. Understanding how dark matter is distributed helps astronomers explain the motions of galaxies and clusters, as well as how these celestial bodies interact with visible matter and radiation. This concept is essential in studying the dynamics of galaxies and the large-scale structure of the universe.
Distance modulus: Distance modulus is a mathematical expression that relates the apparent magnitude and absolute magnitude of a celestial object, allowing astronomers to determine its distance from Earth. It provides a way to measure distances to galaxies and other astronomical objects by using their brightness, which is crucial in galaxy surveys to understand the structure and distribution of the universe.
Edwin Hubble: Edwin Hubble was an American astronomer who played a pivotal role in establishing the field of extragalactic astronomy and is best known for Hubble's law, which describes the expansion of the universe. His work not only led to the classification of galaxies but also revolutionized our understanding of the cosmos, connecting various concepts like the cosmic web and the cosmological principle.
Elliptical Galaxies: Elliptical galaxies are a type of galaxy characterized by their smooth, featureless light profiles and an elliptical shape. They generally contain older stars, little to no gas or dust, and exhibit minimal star formation compared to spiral galaxies. Understanding their formation and evolution provides insights into the processes that govern galaxy development and structure.
Galaxy clustering: Galaxy clustering refers to the large-scale grouping of galaxies in the universe, where galaxies are not evenly distributed but tend to be found in regions of higher density. These clusters can contain dozens to thousands of galaxies bound together by gravity, forming a significant part of the cosmic web. Understanding galaxy clustering helps scientists study the large-scale structure of the universe, including the nature of dark matter and the influence of cosmic expansion.
Galaxy morphologies: Galaxy morphologies refer to the classification of galaxies based on their shapes and structural features, typically categorized into distinct types such as spiral, elliptical, and irregular. Understanding galaxy morphologies is essential for studying galaxy formation and evolution, as different shapes can indicate varying histories and physical processes that have affected a galaxy over time.
Hierarchical formation: Hierarchical formation refers to the process in which structures, such as galaxies, develop through a series of merging events, creating larger systems from smaller components. This concept explains how smaller galaxies and star systems come together over time to form larger and more complex galaxies, revealing insights into the evolution of cosmic structures. It plays a crucial role in understanding the formation and evolution of different galaxy types, the interactions between galaxies, and the large-scale structure of the universe.
Hubble Space Telescope Survey: The Hubble Space Telescope Survey refers to a series of observational projects conducted using the Hubble Space Telescope (HST) to gather detailed images and data of celestial objects, including galaxies, nebulae, and other astronomical phenomena. These surveys have significantly advanced our understanding of the universe by providing high-resolution images and precise measurements, helping astronomers to study galaxy formation, evolution, and distribution in the cosmos.
Hubble-Lemaître Law: The Hubble-Lemaître Law states that the velocity at which a galaxy is receding from an observer is directly proportional to its distance from that observer. This fundamental relationship supports the idea of an expanding universe and forms the cornerstone of modern cosmology, highlighting the connection between distance and redshift observed in galaxy surveys.
Luminosity Function: The luminosity function is a statistical distribution that describes the number density of celestial objects, such as stars or galaxies, as a function of their luminosity. It provides insights into the overall population of these objects, revealing how their brightness varies and helping astronomers understand formation processes and evolutionary stages across different environments.
Magnitude: Magnitude refers to the measure of brightness of celestial objects, allowing astronomers to quantify how bright a star or galaxy appears from Earth. It is an essential concept in astronomy, as it helps in comparing the brightness of different objects in the sky and understanding their distances and properties. The scale is logarithmic, meaning that a difference of 5 magnitudes corresponds to a brightness factor of 100.
Merger theory: Merger theory is a concept in astrophysics that explains the formation and evolution of galaxies through the collision and merging of smaller galaxies. This process plays a crucial role in shaping the structure and characteristics of galaxies, particularly elliptical galaxies, where the mergers lead to a more massive, smooth, and featureless appearance. Additionally, it provides insight into the formation of supermassive black holes, the distribution and evolution of galactic populations in surveys, and helps reconstruct the history of our own galaxy through galactic archaeology.
Photometric Analysis: Photometric analysis is a technique used to measure the intensity of light and its distribution across different wavelengths. This method is crucial for understanding the brightness and color of celestial objects, allowing astronomers to infer important characteristics such as distance, composition, and the presence of other astronomical phenomena.
Power Spectrum: The power spectrum is a representation that shows how the power or variance of a signal is distributed across different frequencies. In the context of cosmology, it helps scientists understand fluctuations in temperature and density, particularly through the cosmic microwave background (CMB) anisotropies and the distribution of galaxies in surveys. By analyzing these patterns, researchers can infer important information about the early universe and the large-scale structure of the cosmos.
Redshift Measurement: Redshift measurement refers to the change in the wavelength of light from an object, typically a galaxy or other celestial body, as it moves away from the observer. This shift towards longer wavelengths occurs due to the expansion of the universe, allowing astronomers to determine the velocity at which an object is receding and, consequently, its distance from Earth. Understanding redshift measurements is critical for galaxy surveys, as it helps in mapping the large-scale structure of the universe and provides insights into its expansion history.
Sloan Digital Sky Survey: The Sloan Digital Sky Survey (SDSS) is an astronomical survey that has mapped millions of celestial objects, including stars, galaxies, and quasars, using advanced imaging and spectroscopic techniques. It has provided crucial data for understanding the structure and evolution of the universe, significantly contributing to the study of quasars, galaxy formation, large-scale structures, and the history of our galaxy.
Spectrograph: A spectrograph is an instrument that disperses light into its component colors to analyze the spectrum of light emitted or absorbed by objects in the universe. This tool allows astronomers to study the properties of galaxies, stars, and other celestial bodies by examining their light spectra, providing insights into their composition, temperature, motion, and distance from Earth.
Spiral Galaxies: Spiral galaxies are a type of galaxy characterized by their distinct spiral arms that radiate from a central bulge, containing stars, gas, and dust. These galaxies are significant because their structure and dynamics provide insights into stellar formation, galactic evolution, and the gravitational forces at play within the universe.
Telescope array: A telescope array is a collection of multiple telescopes that work together to observe astronomical objects, improving the overall resolution and sensitivity of the observations. By combining data from various telescopes spread over large distances, astronomers can achieve higher imaging quality and gather more information than could be obtained from a single telescope. This technique allows for the study of galaxies and other cosmic phenomena in greater detail, making it a vital tool in modern astronomy.
Vesto Melvin Slipher: Vesto Melvin Slipher was an American astronomer known for his pioneering work in the study of galaxies, particularly for providing some of the first evidence for the expansion of the universe through redshift measurements. His observations of the light spectra from distant galaxies revealed a systematic shift toward longer wavelengths, indicating that these galaxies were moving away from Earth, a fundamental observation that contributed to our understanding of cosmic expansion and helped lay the groundwork for later developments in cosmology.