Glossary

are incredibly bright cosmic beacons powered by . Discovered in the 1960s, these distant objects shine with the intensity of entire galaxies, despite being no larger than our solar system.

help astronomers peer into the early universe, revealing clues about galaxy evolution and cosmic structure. Their extreme energy output and compact size make them fascinating subjects for studying the most powerful phenomena in the cosmos.

Quasars

Discovery and significance of quasars

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  • Quasars first discovered in 1960s during radio astronomy surveys
    • Appeared as point-like sources of radio emission (, )
    • Did not correspond to any known celestial objects at the time
  • Optical counterparts to radio sources identified
    • Showed unusual emission lines in their spectra (, )
    • Lines highly redshifted, indicating great distances (billions of light-years)
  • Significance in astronomy
    • Quasars are the most luminous and energetic objects in the universe (up to 104710^{47} erg/s)
    • Can be used as probes to study the early universe and its evolution (back to 1 billion years after Big Bang)
    • Provide insights into the growth and evolution of supermassive black holes (SMBHs)
    • Help in understanding the large-scale structure of the universe (, galaxy clusters)

Quasar redshifts and cosmic distances

  • Quasar spectra exhibit high redshifts
    • is the shift of spectral lines towards longer wavelengths (red end of spectrum)
    • Caused by the expansion of the universe (cosmological redshift)
  • relates redshift to distance
    • v=H0×dv = H_0 \times d, where vv is recession velocity, H0H_0 is , and dd is distance
    • Higher redshifts indicate greater distances (millions to billions of light-years)
  • Quasars have redshifts ranging from 0.056 to 7.642
    • Corresponding to distances of millions to billions of light-years (most distant known quasar: at z=7.642)
  • High redshifts imply that quasars are some of the most distant objects observed
    • Provide a glimpse into the early universe (, )
    • Help study the evolution of galaxies and the universe over cosmic time (star formation history, chemical enrichment)
    • Allow for the study of and their properties at different cosmic epochs

Energy output vs size of quasars

  • Quasars have extremely high luminosities
    • Can outshine entire galaxies by a factor of 100 or more (Milky Way luminosity: ~104410^{44} erg/s)
    • Typical luminosities range from 104010^{40} to 104710^{47} erg/s
  • Compact size of quasars
    • Inferred from rapid variability in brightness (hours to days)
    • Variability on timescales of days to weeks suggests small size (light-travel time argument)
    • Size comparable to the solar system, despite high luminosity (< 1 light-year)
  • Energy output mechanism
    • Accretion of matter onto supermassive black holes (SMBHs)
    • Gravitational energy of infalling matter converted to radiation ()
    • Accretion disks around black holes are highly efficient energy sources (up to 40% of rest-mass energy)
    • Maximum luminosity for a given mass before radiation pressure overcomes gravity (LEdd=3.2×104(M/M⊙)L⊙L_{Edd} = 3.2 \times 10^4 (M/M_{\odot}) L_{\odot})
    • Quasars often emit close to or at the Eddington limit (feedback, outflows)
    • Implies the presence of supermassive black holes with masses of 10610^6 to 101010^{10} solar masses
  • Quasars are a subset of
    • AGN are powered by accretion onto supermassive black holes at the centers of galaxies
    • Highly collimated outflows of plasma ejected from the vicinity of the black hole
    • Can extend for thousands of light-years and emit synchrotron radiation
    • AGN with jets oriented close to our line of sight
    • Exhibit rapid variability and high polarization due to relativistic beaming effects
  • of AGN
    • Proposes that different types of AGN are intrinsically similar but appear different due to viewing angle and obscuration
    • Explains the diverse observational properties of quasars, galaxies, and radio galaxies

Key Terms to Review (25)

3C 273: 3C 273 is the name of the first quasar ever discovered. It is an extremely luminous and distant active galactic nucleus, located approximately 2.4 billion light-years from Earth. 3C 273 is considered a prototypical quasar and has been extensively studied to understand the nature and properties of these enigmatic celestial objects.
3C 48: 3C 48 is a quasar, a highly luminous active galactic nucleus powered by a supermassive black hole at the center of a distant galaxy. It is one of the first quasars ever discovered and has been extensively studied to understand the nature of these enigmatic celestial objects.
Accretion Disk: An accretion disk is a rotating disk of dense, accreting material surrounding a central object, such as a star, black hole, or neutron star. It is formed by the gravitational attraction and conservation of angular momentum of material falling towards the central object.
Active Galactic Nuclei (AGN): Active Galactic Nuclei (AGN) refer to the extremely luminous and energetic centers of some galaxies, powered by supermassive black holes that are actively accreting matter. AGN are key to understanding the evolution of galaxies and the universe as a whole.
Blazars: Blazars are a type of active galactic nucleus (AGN) that emit powerful jets of high-energy particles and radiation, often in the direction of Earth. They are among the most luminous and energetic objects in the universe, powered by supermassive black holes at the centers of distant galaxies.
CIV: CIV refers to the ionization state of carbon, specifically the quadruply ionized carbon atom (C^4+). This highly ionized state of carbon is an important diagnostic feature in the study of quasars, as it provides insights into the extreme physical conditions present in these active galactic nuclei.
Cosmic Dawn: Cosmic Dawn refers to the earliest stages of the universe, a period when the first stars and galaxies began to form after the cosmic dark ages. It marks the transition from a universe filled with neutral hydrogen to one with the first luminous structures that eventually led to the diverse cosmic structures we observe today.
Cosmic Web: The cosmic web is a large-scale structure of the universe, composed of galaxies, galaxy clusters, and filaments of matter that are separated by vast empty spaces called voids. It is a complex network that describes the distribution and organization of matter on the largest scales in the universe.
Curtis: Curtis refers to Heber Doust Curtis, an American astronomer who played a significant role in the early 20th-century debate about the nature of spiral nebulae. Curtis advocated for the idea that these nebulae were actually separate galaxies beyond the Milky Way, which was a foundational concept for understanding quasars and active galaxies.
Eddington Limit: The Eddington limit, also known as the Eddington luminosity, is the maximum luminosity that a star can reach before the outward radiation pressure overcomes the star's gravitational attraction, leading to the ejection of the star's outer layers. This concept is crucial in understanding the properties and evolution of various astronomical objects, particularly quasars and active galactic nuclei.
Host Galaxies: Host galaxies are the galaxies that contain and provide the environment for other astronomical objects, such as quasars. They serve as the backdrop and provide the necessary conditions for these active galactic nuclei to exist and be observed.
Hubble constant: The Hubble constant is the rate of expansion of the universe, measured in kilometers per second per megaparsec (km/s/Mpc). It provides a relationship between the distance of galaxies and their recessional velocity due to cosmic expansion.
Hubble Constant: The Hubble constant is a fundamental parameter in cosmology that describes the rate of expansion of the universe. It represents the relationship between the distance to a galaxy and its recessional velocity, providing a measure of the expansion rate of the observable universe.
Hubble law: Hubble's Law describes the observation that galaxies are receding from us at velocities proportional to their distance. This relationship is a cornerstone in understanding the expanding universe and supports the Big Bang theory.
Hubble's Law: Hubble's Law is a fundamental principle in cosmology that describes the relationship between the distance of a galaxy from the Milky Way and its recessional velocity. It states that the farther a galaxy is from our own, the faster it is moving away from us, indicating an expanding universe.
J0313-1806: J0313-1806 is a quasar, which is an extremely luminous active galactic nucleus powered by a supermassive black hole at the center of a distant galaxy. It is one of the most distant known quasars, with a redshift indicating it existed when the universe was only about 800 million years old.
Lyman-alpha: Lyman-alpha is the spectral line emitted when an electron in a hydrogen atom transitions from the second energy level (n=2) to the first energy level (n=1). This transition produces ultraviolet radiation with a wavelength of 121.6 nanometers, which is a key feature in the study of both the formation of spectral lines and the properties of quasars.
Quasars: Quasars are extremely luminous active galactic nuclei powered by supermassive black holes at their centers. They emit massive amounts of energy, often outshining entire galaxies.
Quasars: Quasars are extremely luminous, compact objects at the centers of some distant galaxies. They are powered by supermassive black holes that are actively accreting matter, releasing enormous amounts of energy across the electromagnetic spectrum. Quasars are important for understanding the large-scale structure of the universe, the formation of spectral lines, the Doppler effect, evidence for black holes, observations of distant galaxies, and the composition of the universe.
Redshift: Redshift is the phenomenon where the wavelength of light emitted from a distant object is shifted towards longer, or redder, wavelengths compared to the original wavelength. This shift in the observed wavelength is caused by the relative motion between the object and the observer, as well as the expansion of the universe.
Reionization: Reionization is a critical phase in the early history of the universe when the neutral hydrogen that permeated the cosmos after the Big Bang was re-ionized, transforming the universe from a neutral, opaque state to a transparent, ionized state. This process had far-reaching implications for the formation and evolution of the first stars, galaxies, and large-scale structures in the universe.
Relativistic Jets: Relativistic jets are narrow, high-speed streams of plasma that are ejected from the vicinity of supermassive black holes at the centers of active galaxies, such as quasars and active galactic nuclei. These jets travel at a significant fraction of the speed of light, exhibiting relativistic effects.
Seyfert: Seyfert galaxies are a class of active galaxies with extremely bright nuclei, believed to be powered by supermassive black holes. They exhibit strong emission lines in their spectra, indicating high-energy processes at their cores.
Supermassive Black Holes: Supermassive black holes are incredibly dense, massive celestial objects found at the center of most, if not all, galaxies. These black holes have a gravitational pull so strong that nothing, not even light, can escape their event horizon, making them invisible to direct observation.
Unified Model: The unified model is a comprehensive theoretical framework that seeks to unify and explain the diverse phenomena observed in active galactic nuclei (AGN) and quasars. It proposes a common underlying structure and mechanism to account for the wide range of properties and behaviors exhibited by these celestial objects.
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