🚀Astrophysics II Unit 8 – Active Galactic Nuclei & Black Holes

Key Concepts

• Active Galactic Nuclei (AGN) are extremely luminous cores of galaxies powered by accretion onto supermassive black holes (SMBHs)
• AGN emit radiation across the electromagnetic spectrum from radio to gamma-rays
• Unified model of AGN explains the different observed types as a result of viewing angle and obscuration
• Supermassive black holes have masses ranging from millions to billions of solar masses ($M_{\odot}$)
• Accretion disks around SMBHs are the primary source of AGN luminosity
• Jets, highly collimated outflows of relativistic particles, are observed in some AGN (radio-loud AGN)
• AGN feedback, the interaction between the AGN and its host galaxy, can significantly impact galaxy evolution

Historical Background

• In 1943, Carl Seyfert discovered highly luminous nuclei in some galaxies, later known as Seyfert galaxies
• Maarten Schmidt identified the first quasar (quasi-stellar radio source) 3C 273 in 1963
• Advances in radio astronomy in the 1950s and 1960s led to the discovery of radio galaxies and quasars
• X-ray observations in the 1970s revealed many AGN emit strongly in X-rays
• Unified models of AGN developed in the 1990s to explain the diverse appearance of AGN
  • Orientation and obscuration are key factors in the observed differences

Types of AGN

• Seyfert galaxies are spiral galaxies with bright nuclei and strong emission lines
  • Seyfert 1 galaxies show broad and narrow emission lines
  • Seyfert 2 galaxies show only narrow emission lines
• Quasars are the most luminous AGN, outshining their host galaxies
  • Radio-loud quasars have strong radio emission and jets
  • Radio-quiet quasars lack strong radio emission
• Blazars are AGN with jets pointed directly towards the observer
  • BL Lac objects show weak or no emission lines
  • Flat-spectrum radio quasars (FSRQs) show strong emission lines
• Radio galaxies are elliptical galaxies with extended radio emission
  • Fanaroff-Riley I (FR I) radio galaxies have bright cores and fainter lobes
  • Fanaroff-Riley II (FR II) radio galaxies have bright hotspots in their lobes
• Low-ionization nuclear emission-line regions (LINERs) are low-luminosity AGN with weak emission lines

Black Hole Basics

• Black holes are regions of spacetime where gravity is so strong that nothing, not even light, can escape
• The boundary of a black hole is called the event horizon, the point of no return
• Schwarzschild radius ($R_s$) is the radius of the event horizon for a non-rotating black hole: $R_s = \frac{2GM}{c^2}$
• Supermassive black holes are believed to reside at the centers of most galaxies
• Black holes are characterized by three properties: mass, spin, and charge (usually assumed to be neutral)
• Accretion of matter onto black holes is the most efficient energy production mechanism known
• Eddington luminosity is the maximum luminosity an object can achieve when radiation pressure balances gravitational force

AGN Structure and Components

• Supermassive black hole is the central engine of an AGN
• Accretion disk surrounds the SMBH and is the primary source of luminosity
  • Accretion disk emits thermal radiation from infrared to X-rays
• Broad-line region (BLR) is a region of high-velocity gas clouds close to the SMBH
  • BLR produces broad emission lines due to Doppler broadening
• Narrow-line region (NLR) is a region of lower-velocity gas clouds farther from the SMBH
  • NLR produces narrow emission lines
• Dusty torus is a donut-shaped structure of gas and dust that can obscure the central region
  • Torus is responsible for the differences between Type 1 and Type 2 AGN
• Jets are highly collimated outflows of relativistic particles perpendicular to the accretion disk
  • Jets can extend hundreds of kiloparsecs from the nucleus
• Lobes are extended regions of radio emission fed by the jets
  • Hotspots are bright regions at the ends of the lobes where the jet interacts with the intergalactic medium

Observational Techniques

• Multi-wavelength observations are essential to study AGN due to their emission across the electromagnetic spectrum
• Radio observations (e.g., Very Large Array, Very Long Baseline Interferometry) reveal jets, lobes, and radio core
• Infrared observations (e.g., Spitzer Space Telescope, James Webb Space Telescope) probe the dusty torus and host galaxy
• Optical and UV observations (e.g., Hubble Space Telescope, Sloan Digital Sky Survey) study emission lines and continuum
• X-ray observations (e.g., Chandra X-ray Observatory, XMM-Newton) probe the hot accretion disk and corona
• Gamma-ray observations (e.g., Fermi Gamma-ray Space Telescope) detect high-energy emission from relativistic jets
• Polarization measurements can help distinguish between different emission mechanisms and geometries
• Variability studies across multiple wavelengths provide insights into the size and structure of the emitting regions

Energy Production and Emission Mechanisms

• Accretion onto supermassive black holes powers AGN
  • Gravitational potential energy is converted into kinetic energy and heat
• Accretion disks emit thermal radiation from infrared to X-rays
  • Temperature decreases with increasing distance from the SMBH
• Inverse Compton scattering in the hot corona produces X-rays
  • Low-energy photons from the disk gain energy by scattering off relativistic electrons
• Synchrotron radiation is produced by relativistic electrons spiraling in magnetic fields
  • Synchrotron emission is observed in radio jets and lobes
• Bremsstrahlung (free-free emission) occurs when free electrons are accelerated in the vicinity of ions
  • Bremsstrahlung contributes to the X-ray emission in AGN
• Photoionization of gas clouds by the central continuum source produces emission lines
  • Broad lines originate from the BLR, narrow lines from the NLR
• Relativistic beaming enhances the observed luminosity of jets pointed towards the observer
  • Doppler boosting and relativistic aberration are responsible for the beaming effect

Impacts on Host Galaxies

• AGN feedback can significantly influence the evolution of host galaxies
• Radiative feedback occurs when AGN radiation interacts with the surrounding gas
  • Radiation pressure can drive outflows and regulate star formation
• Mechanical feedback is driven by jets and winds from the AGN
  • Jets can heat and expel gas from the host galaxy, suppressing star formation
• Positive feedback can trigger star formation by compressing gas clouds
  • Jet-induced star formation has been observed in some radio galaxies
• AGN feedback is thought to play a crucial role in the co-evolution of SMBHs and their host galaxies
  • M-sigma relation suggests a tight correlation between SMBH mass and host galaxy properties
• AGN feedback may help explain the lack of extremely massive galaxies in the local Universe
  • Feedback can prevent gas from cooling and forming stars in massive galaxies

Current Research and Open Questions

• Understanding the role of AGN feedback in galaxy evolution
  • How does AGN feedback affect star formation and galaxy growth over cosmic time?
• Investigating the origin and growth of supermassive black holes
  • How did SMBHs form in the early Universe, and how do they grow over time?
• Studying the connection between AGN activity and galaxy mergers
  • What role do galaxy mergers play in triggering AGN activity?
• Probing the accretion process and accretion disk physics
  • How does the accretion flow behave close to the SMBH, and how does it affect the observed properties of AGN?
• Exploring the role of magnetic fields in jet formation and collimation
  • What is the mechanism responsible for launching and collimating relativistic jets?
• Searching for intermediate-mass black holes (IMBHs) and their relation to AGN
  • Do IMBHs exist, and how do they relate to the growth of SMBHs?
• Investigating the nature of changing-look AGN and tidal disruption events
  • What causes the rapid changes in AGN appearance, and how do tidal disruption events contribute to AGN variability?