Active Galactic Nuclei
Quasar
Most luminous objects in universe — galaxies fueled by supermassive black holes feeding voraciously
A quasar (quasi-stellar object) is the brightest type of active galactic nucleus (AGN) — galaxy with a supermassive BH actively accreting material. The accretion disk + jets emit luminosities up to 10¹⁴ L_sun (10,000× the Milky Way). At cosmological distances, quasars appear as bright "stellar" sources. Discovered 1963 (Maarten Schmidt: 3C 273 redshift). Most quasars at high redshift (z > 1; early universe). Modern view: SMBHs at galactic centers undergoing rapid growth phase. Cosmologically important — track SMBH evolution and galaxy formation.
- Discovery1963 (Maarten Schmidt, 3C 273 redshift)
- Peak luminosity~10⁴⁶-10⁴⁷ erg/s (~10⁹-10¹² L_sun)
- Most distantz = 7.6 (570 Myr after BB)
- SMBH mass10⁶-10¹⁰ M_sun
- Accretion rate~Eddington (~M_sun/yr)
- Lifetime~10⁷-10⁸ years (active phase)
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Why quasars matter
- SMBH growth. Active feeding phase of galaxies.
- Galaxy evolution. AGN feedback regulates star formation.
- Cosmology. Probe distant universe; high z.
- Accretion physics. Extreme conditions on display.
- Cosmological distances. Test universe expansion.
- Spectroscopy targets. Probe intervening matter.
- Lyman alpha forest. Quasar light absorbed by intervening clouds.
Common misconceptions
- Quasars are stars. Galactic nuclei (active SMBHs).
- Quasars are nearby. Most are very distant (early universe).
- All AGN are quasars. Quasar is brightest subset.
- Quasars last forever. Brief active phase (~10⁸ yr).
- Galaxies need to be massive for quasars. SMBH mass varies with galaxy mass.
- Quasars are dangerous. Far away — poses no threat.
Frequently asked questions
How was the first quasar discovered?
3C 273 — radio-bright source in catalog. Maarten Schmidt (1963) measured optical spectrum: emission lines at unfamiliar wavelengths. Realized: spectrum was redshifted. Implied huge distance (~620 Mpc). For object that bright at that distance, intrinsic luminosity must be enormous — ~10¹² L_sun. New phenomenon discovered.
What powers quasars?
Supermassive black hole + accretion. Material falls into BH; gravitational PE → heat → radiation. Efficiency: ~10% mass-energy conversion (better than fusion). Accretion disk reaches ~10⁵-10⁶ K — emits across spectrum. Jets (when present) carry away ~10% energy as relativistic outflow.
Why are most quasars far away?
Active phase is brief in galaxy lifetime (~10⁷-10⁸ yr). Most galaxies underwent quasar phases at z > 1 (early universe; ~10 Gyr ago). Today, most galaxies have quiet SMBHs (like Milky Way's Sgr A*). Quasar fraction declines with cosmic time. Light from past quasars reaches us today.
What's the Eddington luminosity?
Maximum luminosity at which radiation pressure equals gravity for accreting object. L_Edd = 4πGMm_p c/σ_T ≈ 1.3×10⁴⁶ (M/10⁹ M_sun) erg/s. Beyond this, radiation pressure pushes matter away. Most quasars accrete near Eddington — physical limit explains uniform brightness.
How do quasars vary?
Different types based on jet/disk orientation. Type 1 (face-on disk): broad emission lines, blue continuum. Type 2 (edge-on; disk obscured): narrower lines, redder. Radio-loud: jets prominent. Radio-quiet: most. Blazars: jets pointed at us — extremely bright variable. Same physics, different geometry.
How do quasars affect host galaxies?
Major influence on galaxy evolution. Radiation/jets heat ISM — slows star formation ("AGN feedback"). M-sigma relation: BH mass correlates with bulge stellar velocity dispersion. Quasar phase contributes to galaxy "quenching" — transition from blue (star-forming) to red (passive). Co-evolution of SMBH and galaxy.
What's the future of quasars?
Most galaxies will not have quasar phases again; SMBHs grow more slowly. New quasars rare in modern universe. Major discovery via JWST (2022+): quasars from very early universe (z = 7-10), ~few hundred Myr after BB. Surprising — implies SMBHs can grow fast. New questions for cosmology.