🪐Intro to Astronomy Unit 27 – Active Galaxies and Supermassive Black Holes

What's the Big Deal?

• Active galaxies emit enormous amounts of energy across the electromagnetic spectrum, far exceeding the output of normal galaxies
• Supermassive black holes, with masses millions to billions of times that of the Sun, power the extreme luminosity of active galaxies
• Studying active galaxies and supermassive black holes provides insights into the formation and evolution of galaxies throughout cosmic history
• Accretion of matter onto supermassive black holes releases gravitational potential energy, fueling the prodigious energy output of active galaxies
• Jets and outflows from active galactic nuclei can extend hundreds of thousands of light-years into intergalactic space, impacting the surrounding environment
• Feedback from active galactic nuclei can regulate star formation and the growth of the host galaxy (self-regulation)
• Supermassive black holes are thought to reside at the centers of most, if not all, massive galaxies, making them a fundamental component of galaxy evolution

Galaxy Basics

• Galaxies are vast collections of stars, gas, dust, and dark matter held together by gravity
• The Milky Way, our home galaxy, is a spiral galaxy containing hundreds of billions of stars
• Galaxies come in various morphologies, including spiral galaxies (Milky Way, Andromeda), elliptical galaxies (M87), and irregular galaxies (Large Magellanic Cloud)
• The size of galaxies spans a wide range, from dwarf galaxies with a few billion stars to giant elliptical galaxies with trillions of stars
• The interstellar medium (ISM) within galaxies consists of gas and dust, providing the raw material for star formation
• Galaxies often interact and merge with one another, leading to the formation of larger structures and influencing their evolution
• Dark matter, an invisible form of matter that interacts gravitationally, dominates the mass of galaxies and plays a crucial role in their formation and structure

Active Galaxies: The Showstoppers

• Active galaxies are characterized by extremely luminous nuclei that outshine the combined light of all the stars in the host galaxy
• The extraordinary energy output of active galaxies spans the electromagnetic spectrum, from radio waves to gamma rays
• Quasars, the most luminous class of active galaxies, can be seen from the farthest reaches of the observable universe
• Seyfert galaxies, another type of active galaxy, exhibit bright nuclei and strong emission lines in their spectra
• Blazars, a subset of active galaxies, display rapid variability and polarized emission due to relativistic jets pointed towards Earth
• The unified model of active galactic nuclei suggests that the observed differences among various types of active galaxies can be explained by viewing angle and the presence of obscuring material (torus)
• Active galaxies provide a unique laboratory for studying extreme physics, including accretion processes, relativistic jets, and the effects of strong gravity

Supermassive Black Holes: The Main Event

• Supermassive black holes, with masses ranging from millions to billions of solar masses, reside at the centers of most massive galaxies
• The Milky Way hosts a supermassive black hole named Sagittarius A* (Sgr A*), with a mass of about 4 million solar masses
• Supermassive black holes form through the accretion of matter and mergers with other black holes over cosmic time
• The sphere of influence of a supermassive black hole extends to the region where its gravitational potential dominates over the collective potential of the surrounding stars
• The event horizon of a black hole is the boundary beyond which nothing, not even light, can escape its gravitational pull
• The accretion disk surrounding a supermassive black hole consists of hot, ionized gas spiraling inwards, releasing enormous amounts of energy
• Supermassive black holes play a crucial role in the formation and evolution of their host galaxies through feedback processes

Feeding the Beast: How Black Holes Grow

• Supermassive black holes grow primarily through the accretion of matter from their surroundings
• As matter falls towards the black hole, it forms an accretion disk, where gravitational potential energy is converted into heat and radiation
• The accretion process is highly efficient, with up to ~40% of the accreted mass being converted into energy (compared to ~0.7% for nuclear fusion in stars)
• Eddington limit: the maximum luminosity an accreting black hole can achieve when the outward radiation pressure balances the inward gravitational force
• Black hole mergers, resulting from galaxy collisions, can lead to the formation of even more massive black holes
• Tidal disruption events occur when a star ventures too close to a supermassive black hole and is torn apart by tidal forces, providing a temporary boost in accretion
• The growth of supermassive black holes is regulated by feedback processes, such as jets and outflows, which can heat and expel surrounding gas

Cosmic Fireworks: Jets and Outflows

• Relativistic jets are highly collimated streams of plasma expelled from the vicinity of supermassive black holes at near the speed of light
• Jets are powered by the extraction of rotational energy from the spinning black hole (Blandford-Znajek mechanism) or the accretion disk (Blandford-Payne mechanism)
• Synchrotron radiation, produced by electrons spiraling around magnetic field lines, is responsible for the radio emission observed in jets
• Inverse Compton scattering of low-energy photons by relativistic electrons in the jet can produce high-energy X-rays and gamma rays
• Lobes and hotspots form at the ends of jets, where they interact with the surrounding intergalactic medium, creating spectacular radio structures (radio galaxies)
• Outflows, such as winds and bubbles, can also be driven by the intense radiation and magnetic fields near the black hole
• Feedback from jets and outflows can have a profound impact on the host galaxy, regulating star formation and the availability of cold gas for future accretion

Observing the Invisible

• Supermassive black holes are studied through their interaction with surrounding matter, as they themselves emit no light
• Accretion disks around supermassive black holes emit radiation across the electromagnetic spectrum, from radio to X-rays
• X-ray observations (Chandra, XMM-Newton) probe the innermost regions of the accretion disk, where matter is heated to extreme temperatures
• Radio telescopes (VLA, ALMA) map out the structure and kinematics of jets and lobes, revealing the impact of the black hole on its surroundings
• Infrared observations (Spitzer, JWST) can penetrate the obscuring dust and gas, allowing the study of hidden active galactic nuclei
• Gravitational wave detectors (LIGO, Virgo) have opened a new window to study the mergers of supermassive black holes
• Event Horizon Telescope, a global network of radio telescopes, has captured the first image of a black hole's event horizon (M87*)

Impact on Galactic Evolution

• Supermassive black holes and their host galaxies co-evolve over cosmic time, influencing each other's growth and properties
• The mass of the central black hole correlates with the properties of the host galaxy's bulge (M-sigma relation), suggesting a fundamental link between them
• Feedback from active galactic nuclei can quench star formation in the host galaxy by heating and expelling gas (negative feedback)
• AGN feedback can also trigger star formation by compressing gas clouds (positive feedback), leading to the formation of new stars
• Mergers of galaxies and their central black holes can disrupt the host galaxy's structure and lead to the formation of elliptical galaxies
• The energy output of active galactic nuclei can ionize the surrounding intergalactic medium, contributing to the reionization of the universe
• Studying the evolution of supermassive black holes and their host galaxies provides crucial insights into the formation and growth of cosmic structures