3.4 Quasars

Quasars, discovered in the 1950s, are incredibly luminous and distant objects powered by supermassive black holes. They exhibit high redshifts, compact appearance, broad emission lines, and brightness variability, making them unique cosmic entities.

Quasars play a crucial role in understanding the early universe and cosmic evolution. Their immense energy output, driven by efficient matter accretion, allows astronomers to probe the distant cosmos and study large-scale structures, providing valuable insights into the history and expansion of the universe.

Discovery of quasars

• Quasars, or quasi-stellar radio sources, were first discovered in the 1950s during radio surveys of the sky
• Initially identified as radio sources with no corresponding visible object, later found to have faint star-like optical counterparts
• In 1963, Maarten Schmidt measured the redshift of quasar 3C 273, revealing it to be at a vast distance and extremely luminous

Observational properties of quasars

High redshifts

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• Quasars exhibit high redshifts, indicating they are located at great distances from Earth
• Redshifts are caused by the expansion of the universe, with more distant objects showing higher redshifts
• Quasar redshifts range from z ~ 0.1 to z > 7, corresponding to distances of billions of light-years

Compact appearance

• Despite their high luminosity, quasars appear point-like or compact in optical images
• This compact appearance suggests that the immense energy output originates from a relatively small region
• The compact size is a key distinguishing feature of quasars compared to other luminous astronomical objects

Broad emission lines

• Quasar spectra show broad emission lines from highly ionized elements such as hydrogen, carbon, and magnesium
• These broad lines indicate gas moving at high velocities (thousands of km/s) in the vicinity of the quasar
• The width of the emission lines provides insights into the physical conditions and dynamics of the quasar's gas

Variability in brightness

• Quasars exhibit variability in their brightness on timescales ranging from days to years
• This variability suggests that the emitting region is compact, as larger regions would not be able to change in brightness so rapidly
• Studying quasar variability helps constrain the size and structure of the emission region

Quasar energy source

Accretion onto supermassive black holes

• The extraordinary energy output of quasars is attributed to accretion of matter onto supermassive black holes at their centers
• As matter spirals inward and falls onto the black hole, it forms an accretion disk where gravitational energy is converted to heat and radiation
• The accretion process can convert up to ~10% of the rest-mass energy of the infalling matter into radiation, making it highly efficient

Efficiency of energy production vs stars

• Accretion onto black holes is a far more efficient energy production mechanism compared to nuclear fusion in stars
• While stars typically convert ~0.7% of their mass into energy through fusion, accretion can convert ~10% of the accreted mass into energy
• This high efficiency explains how quasars can outshine entire galaxies while being much smaller in size

Quasar structure

Accretion disk

• The accretion disk is a flattened, rotating disk of gas and dust that surrounds the central supermassive black hole
• As matter in the disk spirals inward, it heats up due to friction and viscous dissipation, producing thermal radiation
• The accretion disk is the primary source of continuum emission in quasars, with temperatures ranging from ~10^5-10^6 K

Broad-line region

• The broad-line region (BLR) is a compact region of gas close to the accretion disk, moving at high velocities
• The high velocities of the BLR gas, typically thousands of km/s, are responsible for the broad emission lines seen in quasar spectra
• The BLR is thought to be photoionized by the intense radiation from the accretion disk, producing the observed emission lines

Narrow-line region

• The narrow-line region (NLR) is a more extended region of gas, located farther from the central black hole than the BLR
• The gas in the NLR moves at lower velocities (~hundreds of km/s), resulting in narrower emission lines compared to the BLR
• The NLR is also photoionized by the quasar's radiation, but the lower gas densities and velocities lead to different emission line properties

Jets and radio lobes

• Some quasars, particularly radio-loud ones, feature jets of plasma emanating from the central region at relativistic speeds
• These jets, believed to be powered by the spin of the central black hole and the accretion disk magnetic fields, can extend far beyond the host galaxy
• The jets can terminate in large, diffuse radio lobes, which are regions of radio-emitting plasma that can span hundreds of thousands of light-years

Types of quasars

Radio-loud vs radio-quiet

• Quasars can be classified as radio-loud or radio-quiet based on the strength of their radio emission relative to their optical emission
• Radio-loud quasars are characterized by strong radio emission and the presence of jets and radio lobes
• Radio-quiet quasars have much weaker radio emission and lack prominent jets or lobes, but still produce strong optical and X-ray emission

Optically violent variable quasars

• Optically violent variable (OVV) quasars are a subclass that exhibits rapid, large-amplitude variability in their optical brightness
• OVV quasars can change in brightness by more than a magnitude on timescales of days to weeks
• This extreme variability is thought to be related to relativistic beaming effects in the jet directed close to our line of sight

Broad absorption line quasars

• Broad absorption line (BAL) quasars show broad absorption features in their spectra, in addition to the typical broad emission lines
• These absorption features are believed to originate from outflowing winds or gas clouds along our line of sight to the quasar
• BAL quasars provide valuable insights into the gas dynamics and composition of quasar outflows

Quasar evolution

Peak quasar activity in early universe

• Observations show that quasar activity peaked at redshifts around z ~ 2-3, corresponding to a cosmic age of ~2-3 billion years
• This peak in quasar activity suggests that supermassive black holes underwent rapid growth and accretion during this epoch
• The peak coincides with a period of intense star formation and galaxy assembly in the early universe

Decline in quasar population over time

• The number density of quasars has declined significantly since the peak at z ~ 2-3
• This decline is thought to be due to a combination of factors, including the depletion of gas available for accretion and feedback processes that suppress further accretion
• The decline in quasar activity is mirrored by a decline in the cosmic star formation rate over the same period

Connection to galaxy evolution

• The evolution of quasars is closely linked to the evolution of their host galaxies
• The peak in quasar activity coincides with a period of rapid galaxy growth and assembly through mergers and gas accretion
• Feedback from quasars, in the form of radiation and outflows, can have a significant impact on the gas content and star formation in their host galaxies

Role of quasars in cosmology

Probing the early universe

• Quasars are among the most luminous and distant objects known, making them valuable probes of the early universe
• By studying quasars at high redshifts, astronomers can investigate the properties of the universe when it was only a few billion years old
• Quasars provide insights into the formation and growth of the first supermassive black holes and the evolution of cosmic structure

Measuring the expansion rate

• Quasars can be used as standard candles to measure the expansion rate of the universe at different cosmic epochs
• By comparing the luminosity distances of quasars derived from their observed brightness to their redshifts, astronomers can constrain cosmological parameters
• Quasar measurements complement other methods, such as Type Ia supernovae and the cosmic microwave background, in determining the expansion history of the universe

Studying large-scale structure

• The spatial distribution of quasars traces the large-scale structure of the universe
• By mapping the positions of quasars on the sky and their redshifts, astronomers can study the clustering and evolution of matter on cosmic scales
• Quasar surveys, such as the Sloan Digital Sky Survey (SDSS), have provided detailed maps of the cosmic web and its evolution over billions of years

Quasars as probes of intervening matter

Absorption lines from intergalactic medium

• As light from quasars travels through the intergalactic medium (IGM), it can be absorbed by intervening gas clouds
• This absorption manifests as narrow absorption lines in quasar spectra, superimposed on the quasar's broad emission features
• By studying these absorption lines, astronomers can probe the distribution, composition, and evolution of gas in the IGM

Lyman-alpha forest

• The Lyman-alpha forest is a series of absorption lines in quasar spectra caused by neutral hydrogen in the IGM
• These absorption lines are produced when the quasar's light is redshifted into the Lyman-alpha transition wavelength of hydrogen at different distances along the line of sight
• The Lyman-alpha forest provides a detailed map of the distribution and density of neutral hydrogen in the universe and its evolution over cosmic time

Metal absorption lines

• In addition to hydrogen, quasar spectra can also show absorption lines from heavier elements or "metals" in the IGM
• These metal absorption lines are produced by gas clouds that have been enriched by stars and supernovae in galaxies
• By studying the strengths and ratios of different metal absorption lines, astronomers can trace the chemical evolution of the universe and the feedback processes that transport metals from galaxies to the IGM

Unified model of active galactic nuclei

Orientation effects

• The unified model of active galactic nuclei (AGN) proposes that the observed differences between various types of AGN, including quasars, are largely due to orientation effects
• According to this model, the appearance of an AGN depends on the viewing angle relative to the accretion disk and the presence of obscuring material
• For example, when viewed from certain angles, the broad-line region may be obscured, leading to the appearance of a narrow-line AGN or a Type 2 Seyfert galaxy

Relation to Seyfert galaxies and blazars

• Seyfert galaxies and blazars are two other classes of AGN that are related to quasars within the unified model framework
• Seyfert galaxies are typically less luminous than quasars and are often found in nearby galaxies, with Seyfert 1 galaxies showing broad emission lines and Seyfert 2 galaxies showing only narrow lines
• Blazars, which include BL Lac objects and flat-spectrum radio quasars (FSRQs), are characterized by rapid variability, high polarization, and strong gamma-ray emission, believed to be due to a relativistic jet pointed close to our line of sight