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

Quasars 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

• Quasars first discovered in 1960s during radio astronomy surveys
  • Appeared as point-like sources of radio emission (3C 273, 3C 48)
  • Did not correspond to any known celestial objects at the time
• Optical counterparts to radio sources identified
  • Showed unusual emission lines in their spectra (Lyman-alpha, CIV)
  • 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 $10^{47}$ 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 (cosmic web, galaxy clusters)

Quasar redshifts and cosmic distances

• Quasar spectra exhibit high redshifts
  • Redshift is the shift of spectral lines towards longer wavelengths (red end of spectrum)
  • Caused by the expansion of the universe (cosmological redshift)
• Hubble's law relates redshift to distance
  • $v = H_0 \times d$, where $v$ is recession velocity, $H_0$ is Hubble constant, and $d$ 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: J0313-1806 at z=7.642)
• High redshifts imply that quasars are some of the most distant objects observed
  • Provide a glimpse into the early universe (cosmic dawn, reionization)
  • Help study the evolution of galaxies and the universe over cosmic time (star formation history, chemical enrichment)
  • Allow for the study of host galaxies 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: ~$10^{44}$ erg/s)
  • Typical luminosities range from $10^{40}$ to $10^{47}$ 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 disk)
  • Accretion disks around black holes are highly efficient energy sources (up to 40% of rest-mass energy)
• Eddington limit
  • Maximum luminosity for a given mass before radiation pressure overcomes gravity ($L_{Edd} = 3.2 \times 10^4 (M/M_{\odot}) L_{\odot}$)
  • Quasars often emit close to or at the Eddington limit (feedback, outflows)
  • Implies the presence of supermassive black holes with masses of $10^6$ to $10^{10}$ solar masses

Active Galactic Nuclei (AGN) and Related Phenomena

• Quasars are a subset of active galactic nuclei (AGN)
  • AGN are powered by accretion onto supermassive black holes at the centers of galaxies
• Relativistic jets
  • Highly collimated outflows of plasma ejected from the vicinity of the black hole
  • Can extend for thousands of light-years and emit synchrotron radiation
• Blazars
  • AGN with jets oriented close to our line of sight
  • Exhibit rapid variability and high polarization due to relativistic beaming effects
• Unified model 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, Seyfert galaxies, and radio galaxies