🌠Astrophysics I Unit 11 – Galaxies and Galaxy Clusters

What Are Galaxies?

• Galaxies are vast cosmic systems consisting of stars, planets, gas, dust, and dark matter held together by gravity
• Contain anywhere from a few billion to trillions of stars, with the Milky Way estimated to have between 100-400 billion stars
• Span sizes ranging from a few thousand to hundreds of thousands of light-years in diameter
  • The Milky Way is approximately 100,000 light-years across
• Host a variety of celestial objects, including stars, planets, nebulae, star clusters, and supermassive black holes at their centers
• Serve as the primary building blocks of the observable universe, with billions of galaxies estimated to exist
• Grouped into larger structures called galaxy clusters and superclusters, which are connected by filaments of galaxies and dark matter
• Provide a cosmic laboratory for studying the formation and evolution of stars, planets, and the universe as a whole

Galaxy Formation and Evolution

• Galaxies formed from the gravitational collapse of slightly denser regions in the early universe, known as primordial fluctuations
• The first galaxies began to form a few hundred million years after the Big Bang, as gas cooled and condensed into protogalaxies
• Early galaxies were smaller and more irregular than present-day galaxies, undergoing frequent mergers and interactions
• Gas within galaxies undergoes gravitational collapse to form stars, with the first generation of stars being composed primarily of hydrogen and helium
• Stellar evolution enriches galaxies with heavier elements through supernovae and stellar winds, enabling the formation of planets and the development of complex chemistry
• Galaxies grow and evolve through a combination of internal processes (star formation, stellar evolution) and external influences (mergers, interactions with other galaxies)
  • Major mergers can dramatically alter a galaxy's morphology and trigger intense starbursts
• Feedback from supernovae and active galactic nuclei (AGN) can regulate star formation and shape the interstellar medium within galaxies
• The evolution of galaxies is influenced by their environment, with galaxies in dense clusters experiencing different evolutionary paths than isolated galaxies

Types of Galaxies

• Galaxies are classified based on their morphology, or shape, with the primary categories being elliptical, spiral, and irregular galaxies
• Elliptical galaxies have smooth, ellipsoidal shapes and are composed mainly of older, redder stars
  • They range from spherical (E0) to highly elongated (E7) and have little to no ongoing star formation
• Spiral galaxies, like the Milky Way, have distinct spiral arms extending from a central bulge and are actively forming stars
  • Classified as barred (SB) or unbarred (SA) depending on the presence of a central bar-like structure
• Lenticular galaxies (S0) have characteristics of both elliptical and spiral galaxies, with a central bulge and disk but no spiral arms
• Irregular galaxies lack a well-defined structure and are often smaller than other galaxy types
  • They may have undergone recent mergers or gravitational interactions that disrupt their morphology
• Dwarf galaxies are small galaxies with lower masses and luminosities compared to their larger counterparts
  • Include dwarf ellipticals, dwarf spheroidals, and dwarf irregulars
• The Hubble sequence, or "tuning fork" diagram, organizes galaxies based on their morphology and serves as a tool for understanding galaxy evolution

Structure of the Milky Way

• The Milky Way is a barred spiral galaxy with a diameter of approximately 100,000 light-years
• Consists of four main components: the disk, bulge, halo, and central supermassive black hole
• The disk is the flat, rotating component that contains the spiral arms, young stars, and the majority of the galaxy's gas and dust
  • Divided into the thin disk (~300 light-years thick) and the thick disk (~1,000 light-years thick)
• The bulge is the central, spheroidal component composed primarily of older stars and has a diameter of about 10,000 light-years
• The halo is a roughly spherical component that surrounds the disk and bulge, containing ancient stars, globular clusters, and a significant portion of the galaxy's dark matter
  • Extends up to 100,000 light-years from the galactic center
• The central supermassive black hole, known as Sagittarius A*, has a mass of approximately 4 million solar masses and serves as the gravitational anchor of the galaxy
• The Sun is located in the Orion Arm, a minor spiral arm in the disk, about 27,000 light-years from the galactic center
• The Milky Way is part of the Local Group, a galaxy group that includes the Andromeda Galaxy (M31) and several dozen smaller galaxies

Dark Matter in Galaxies

• Dark matter is a hypothetical form of matter that does not interact with electromagnetic radiation but has gravitational effects on visible matter
• Accounts for approximately 85% of the matter in the universe and plays a crucial role in the formation, structure, and evolution of galaxies
• Its presence is inferred from observations of galaxy rotation curves, which show that galaxies rotate faster than expected based on their visible matter alone
  • This suggests the existence of a dark matter halo that extends beyond the visible components of galaxies
• Dark matter halos provide the gravitational scaffolding for galaxy formation and help explain the observed large-scale structure of the universe
• The distribution of dark matter within galaxies influences their morphology and the motion of stars and gas
  • In spiral galaxies, dark matter is thought to be concentrated in a roughly spherical halo surrounding the disk
• Dark matter is also essential for explaining the gravitational lensing effects observed around galaxies and galaxy clusters
• While the nature of dark matter remains unknown, leading candidates include weakly interacting massive particles (WIMPs) and axions
• Ongoing research aims to detect dark matter directly, study its properties, and understand its role in the evolution of galaxies and the universe

Galaxy Clusters and Superclusters

• Galaxy clusters are gravitationally bound systems containing hundreds to thousands of galaxies, hot X-ray emitting gas, and large amounts of dark matter
• Clusters are the largest gravitationally bound structures in the universe, with masses ranging from $10^{14}$ to $10^{15}$ solar masses and diameters of several megaparsecs
• The distribution of galaxies within clusters is not uniform, with a higher concentration of galaxies near the center and a decreasing density towards the outskirts
• The intracluster medium (ICM) is a hot ($10^7$ to $10^8$ K), tenuous plasma that fills the space between galaxies in a cluster
  • The ICM emits X-rays due to thermal bremsstrahlung and serves as a tracer of the cluster's gravitational potential well
• Dark matter accounts for about 80-90% of the total mass in galaxy clusters, with the ICM and galaxies making up the remainder
• Superclusters are the largest known structures in the universe, consisting of groups of galaxy clusters and the filaments that connect them
  • Examples include the Virgo Supercluster, which contains the Local Group, and the Laniakea Supercluster
• Studying galaxy clusters provides insights into cosmology, dark matter, and the large-scale structure of the universe
  • The abundance and distribution of clusters can be used to constrain cosmological parameters and test models of structure formation

Observing Distant Galaxies

• Observing distant galaxies allows astronomers to study the evolution of galaxies and the universe over cosmic time
• The light from distant galaxies is redshifted due to the expansion of the universe, with the redshift increasing with distance
  • Measuring the redshift of a galaxy provides an estimate of its distance and lookback time
• Hubble's law describes the relationship between a galaxy's distance and its recessional velocity, with the Hubble constant ($H_0$) parameterizing the current expansion rate of the universe
• Deep sky surveys, such as the Hubble Ultra-Deep Field, have revealed galaxies as far back as 400 million years after the Big Bang
  • These early galaxies are smaller, less massive, and more irregular than present-day galaxies
• Studying the properties of distant galaxies, such as their morphology, star formation rates, and chemical composition, helps constrain models of galaxy formation and evolution
• Gravitational lensing by intervening galaxy clusters can magnify the light from distant galaxies, enabling detailed studies of their properties
• The James Webb Space Telescope (JWST) and future ground-based telescopes will provide unprecedented views of the early universe and the formation of the first galaxies

Current Research and Open Questions

• Understanding the nature of dark matter and its role in galaxy formation and evolution remains a major challenge in astrophysics
  • Efforts to directly detect dark matter particles and map its distribution within galaxies are ongoing
• The formation and growth of supermassive black holes in the early universe and their co-evolution with host galaxies is an active area of research
• Studying the properties of the intergalactic medium (IGM) and its role in the cycle of gas accretion and outflows in galaxies is crucial for understanding galaxy evolution
• The role of mergers and interactions in shaping galaxy morphology and driving star formation is an ongoing topic of investigation
  • Simulations and observations aim to quantify the impact of mergers on galaxy evolution
• Constraining the value of the Hubble constant and resolving the discrepancy between measurements based on the cosmic microwave background and local distance indicators is a key goal in cosmology
• The search for the earliest galaxies and the sources responsible for the reionization of the universe is a frontier in observational cosmology
  • Upcoming telescopes, such as the JWST, will probe this critical epoch in the history of the universe
• Developing a comprehensive theory of galaxy formation and evolution that incorporates the complex interplay of dark matter, gas dynamics, star formation, and feedback processes remains an ongoing challenge in astrophysics