Astrophysics I Unit 12 ReviewActive 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
• Includes quasars, blazars, Seyfert galaxies, and radio galaxies, each with distinct observational characteristics
• Accretion disks around SMBHs convert gravitational potential energy into thermal and non-thermal radiation
• AGN jets are collimated outflows of relativistic plasma extending up to megaparsec scales
  • Jet formation involves extraction of rotational energy from the SMBH or accretion disk
• AGN variability on timescales from minutes to years provides insights into the compact central engine
• AGN feedback can significantly impact the evolution of host galaxies and the intergalactic medium

Historical Background

• In the 1940s, Carl Seyfert discovered highly luminous nuclei in a small fraction of galaxies, later known as Seyfert galaxies
• Quasars, the most luminous AGN, were first identified as radio sources in the 1950s and optically in the 1960s
  • Initially thought to be stars due to their point-like appearance and high redshifts
• Advances in radio astronomy (Very Long Baseline Interferometry) revealed the presence of jets and compact central engines in some AGN
• X-ray observations in the 1970s established the presence of hot, accretion-powered emission in AGN
• Unified models developed in the 1990s aimed to explain the diverse appearance of AGN as a result of viewing angle effects
• Recent multi-wavelength surveys (Sloan Digital Sky Survey) have greatly expanded our understanding of AGN demographics and evolution

Types of Active Galactic Nuclei

• Seyfert galaxies are spiral galaxies with bright nuclei and strong emission lines
  • Seyfert 1 galaxies show broad and narrow emission lines, while Seyfert 2 galaxies show only narrow lines
• Quasars are the most luminous and distant AGN, outshining their host galaxies
  • Radio-loud quasars have powerful jets, while radio-quiet quasars lack strong jet emission
• Blazars are AGN with jets pointed close to our line of sight, exhibiting rapid variability and polarization
  • Include BL Lac objects (featureless spectra) and Flat-Spectrum Radio Quasars (strong emission lines)
• Radio galaxies are elliptical galaxies with extended radio lobes fed by jets
  • Fanaroff-Riley I (FR I) radio galaxies have lower luminosities and more diffuse lobes than FR II radio galaxies
• 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 the gravitational pull is so strong that nothing, not even light, can escape once it crosses the event horizon
• Characterized by three properties: mass, spin (angular momentum), and electric charge
• Stellar-mass black holes ($\sim3-100 M_{\odot}$) form from the collapse of massive stars
• Supermassive black holes (SMBHs; $10^6-10^{10} M_{\odot}$) reside at the centers of most galaxies and power AGN
  • SMBH formation mechanisms include gas accretion, mergers, and direct collapse of massive gas clouds
• Schwarzschild radius: $R_S = \frac{2GM}{c^2}$, where $M$ is the black hole mass, $G$ is the gravitational constant, and $c$ is the speed of light
• Innermost stable circular orbit (ISCO) depends on the black hole spin and ranges from $1-9 R_S$
• Black holes are described by the Kerr metric, which accounts for the effects of rotation on spacetime curvature

Observational Techniques

• Optical and UV spectroscopy reveal emission lines from ionized gas in the broad-line region (BLR) and narrow-line region (NLR)
  • Doppler broadening of BLR lines indicates high-velocity gas motion near the SMBH
• X-ray spectroscopy probes the hot inner accretion disk and corona
  • Fe Kα emission line at 6.4 keV is a key diagnostic of the accretion disk structure and SMBH spin
• Radio interferometry (VLBI) resolves jets and can image the event horizon of nearby SMBHs (Event Horizon Telescope)
• Polarimetry distinguishes between thermal (unpolarized) and non-thermal (polarized) emission components
• Reverberation mapping uses light travel time delays between continuum and emission line variations to map the BLR structure and estimate SMBH masses
• Gravitational microlensing can probe the inner accretion disk structure in distant quasars

AGN Structure and Components

• Supermassive black hole (SMBH) with a mass of $10^6-10^{10} M_{\odot}$ is the central engine powering the AGN
• Accretion disk extends from the ISCO to $\sim1000 R_S$ and emits thermal UV/optical radiation
  • Accretion disk can be geometrically thin and optically thick (standard disk) or thick and advection-dominated (ADAF)
• Hot corona above the accretion disk Compton scatters disk photons to X-ray energies
• Broad-line region (BLR) consists of high-velocity gas clouds at $\sim0.01-0.1$ pc from the SMBH
  • Photoionized by the accretion disk and corona, producing Doppler-broadened emission lines
• Dusty torus at $\sim1-10$ pc absorbs and reprocesses UV/optical radiation into infrared
  • Torus geometry (opening angle, clumpiness) affects the observed AGN type
• Narrow-line region (NLR) extends to $\sim100-1000$ pc and emits narrow forbidden lines from low-density, photoionized gas
• Relativistic jets emanate from the SMBH or inner accretion disk and can extend up to megaparsec scales
  • Jet composition can be leptonic (electrons/positrons) or hadronic (protons)

Accretion and Energy Production

• Accretion onto SMBHs is the primary energy source in AGN
• Gravitational potential energy of infalling matter is converted into kinetic energy and dissipated as heat through viscous processes
• Accretion rate $\dot{M}$ determines the accretion mode and radiative efficiency
  • Eddington accretion rate: $\dot{M}_{\rm Edd} = \frac{4\pi GM_{\rm BH}m_p}{\epsilon\sigma_T c}$, where $\epsilon$ is the radiative efficiency and $\sigma_T$ is the Thomson cross-section
• Standard thin disk (Shakura-Sunyaev disk) operates at high accretion rates ($\dot{M} \gtrsim 0.01 \dot{M}_{\rm Edd}$) and has a radiative efficiency of $\epsilon \approx 0.1$
• Advection-dominated accretion flow (ADAF) occurs at low accretion rates ($\dot{M} \lesssim 0.01 \dot{M}_{\rm Edd}$) and has a lower radiative efficiency
• Jet power can dominate over accretion disk luminosity in radio-loud AGN
  • Jet launching mechanisms include the Blandford-Znajek process (extraction of SMBH spin energy) and the Blandford-Payne process (magnetocentrifugal acceleration)
• AGN feedback can be radiative (quasar mode) or mechanical (radio mode) and affects the host galaxy and surrounding environment

AGN Evolution and Cosmological Implications

• AGN population evolves strongly with cosmic time, peaking at redshifts $z \approx 2-3$
  • Quasar density increases by a factor of $\sim100$ from $z = 0$ to $z = 2$
• AGN evolution is closely tied to the growth history of SMBHs and their host galaxies
  • SMBH mass correlates with host galaxy bulge properties (M-σ relation), suggesting co-evolution
• Hierarchical galaxy formation models predict that galaxy mergers trigger AGN activity and SMBH growth
• AGN feedback can quench star formation in the host galaxy and prevent overcooling in galaxy clusters
  • Radiative feedback can drive powerful outflows and expel gas from the galaxy
  • Mechanical feedback from jets can heat the intracluster medium and maintain the hot gas reservoir
• AGN can serve as cosmological probes, using their luminosity function and clustering properties to constrain cosmological parameters
• Reionization of the Universe at $z > 6$ may have been driven by early AGN, although their contribution relative to star-forming galaxies remains uncertain