Quasars, incredibly bright cosmic beacons, peaked in activity about 10 billion years ago. Their rise and fall mirror the evolution of galaxies, with intense activity during mergers and interactions. Today, most massive galaxies host quiet supermassive black holes.

The relationship between galaxies and their central black holes is a cosmic dance of growth and regulation. Black holes feed on galactic gas, triggering powerful outflows that shape star formation. Meanwhile, galactic activity fuels black hole growth, creating a feedback loop of cosmic proportions.

Quasar Activity and Galaxy Evolution

Timeline of quasar activity

• Quasar activity peaked ~10 billion years ago (redshift z ~ 2-3)
  • Corresponds to epoch of maximum star formation and galaxy growth (cosmic noon)
• Rapid decline in quasar activity after peak
  • Quasar population dropped by factor of ~100 from z ~ 2 to present day (z = 0)
• Quasars rare in early universe (z > 6, <1 billion years after Big Bang)
  • Early quasars extremely luminous, powered by massive black holes (>1 billion $M_{\odot}$)
• Quasars relatively uncommon in local universe (z < 0.5)
  • Most massive galaxies host quiescent supermassive black holes with little or no accretion activity (Milky Way, Andromeda)

Galaxies and black holes interaction

• Galaxies and central black holes co-evolve over cosmic time
• Black hole growth driven by accretion of gas and dust from host galaxy
  • Accretion most efficient during galaxy mergers and interactions, funneling gas to central regions
• Black hole accretion can trigger powerful outflows and jets (quasar feedback)
  • Quasar feedback heats and expels gas from host galaxy, regulating star formation and galaxy growth
  • Feedback responsible for observed correlation between black hole mass and galaxy bulge properties (M-sigma relation)
• Star formation and galaxy growth influence black hole growth
  • Stellar winds and supernovae provide steady supply of gas for black hole accretion
  • Rapid star formation in galaxy mergers triggers intense quasar activity

Theories of early black hole formation

• Seed black holes could form from collapse of first massive stars (Population III stars)
  • Population III stars very massive (>100 $M_{\odot}$) and short-lived
  • Upon death, these stars leave behind black holes with masses of ~10-100 $M_{\odot}$
• Direct collapse of primordial gas clouds could form more massive seed black holes
  • Requires specific conditions, such as suppression of molecular hydrogen cooling
  • Direct collapse creates seed black holes with masses of ~$10^{4}$-$10^{5}$ $M_{\odot}$
• Seed black holes must grow rapidly in early universe to explain existence of billion-$M_{\odot}$ quasars at z > 6
  • Growth occurs through combination of gas accretion and mergers with other black holes

Active quasars vs quiescent black holes

• Active quasars powered by rapid accretion onto supermassive black holes (active galactic nuclei)
  • Accretion rates in quasars close to Eddington limit, maximum rate at which radiation pressure balances gravitational attraction
  • High accretion rates lead to formation of luminous accretion disk and often powerful jets
• Quiescent black holes have low or negligible accretion rates
  • Most supermassive black holes in local universe are quiescent (Sagittarius A in Milky Way)
  • Low accretion rates result in little or no observable emission from black hole
• Difference in emission levels primarily due to availability of gas for accretion
  • Quasars reside in gas-rich galaxies, often undergoing mergers or interactions that funnel gas to central regions
  • Quiescent black holes found in gas-poor galaxies where most gas consumed by star formation or expelled by feedback processes
• Transition from active quasar to quiescent black hole a natural consequence of galaxy evolution
  • As galaxies consume or expel gas reserves, fuel for black hole accretion becomes increasingly scarce, leading to decline in quasar activity

Quasars as Cosmic Probes

• Quasars serve as bright beacons for studying the early universe and cosmic structure
  • The Lyman-alpha forest in quasar spectra traces the distribution of neutral hydrogen in the intergalactic medium
  • Quasars play a crucial role in cosmic reionization, ionizing the neutral gas in the early universe
• Large-scale surveys like the Sloan Digital Sky Survey have significantly increased our quasar catalog
• Gravitational lensing of distant quasars allows study of intervening mass distributions and the cosmic web structure