class14

Class 14: Discovery of Quasars

ASTR350: Black Holes (Spring 2026)

Professor: Richard Mushotzky

Recap of Previous Classes

Part I: Physics of Black Holes

• Topics Covered:
    - Physics from Newton to Einstein
    - Special and General Relativity
    - Schwarzschild and Kerr Black Holes

Part II: Stellar Mass Black Holes

• Topics Covered:
    - Black holes as endpoints of massive stellar evolution
    - X-ray binary systems and accretion disks
    - Gamma-ray bursts as sources of stellar mass black holes

Summary of Last Lecture

Gamma-Ray Bursts

• Origin and Characteristics:
    - Discovered accidentally in 1967 through a US program aimed at detecting nuclear tests in space; public details released in 1973.
    - Characteristics:
      - Fairly common (about one per day across the sky).
      - Isotropic distribution on the sky.
      - Divided into two categories: short and long forms.
    - Counterparts in other wavelengths indicate cosmological origins with high redshifts.
    - Highly beamed emissions; observable when aligned with jets possessing Lorentz factors of about 100.
    - Long gamma-ray bursts arise from specific core-collapse supernovae; short bursts correspond to neutron star mergers.
    - Not all stellar mass black holes are produced through gamma-ray bursts.

Today's Class: Part III - Supermassive Black Holes

• Formed through different processes than stellar mass black holes.
• Powered by accretion; can be extraordinarily luminous (up to $10^{14}$ times the luminosity of the Sun).
• Today's focus:
    - Discovery of quasars.
    - Characterization and arguments supporting the existence of supermassive black holes.

Radio Sources

Definition

• Celestial objects that emit radio waves; must be luminous in radio bands for detection beyond the Milky Way galaxy.

Discovery Highlights

• Karl Jansky discovered radio waves from outside the Solar System, published in 1933; initially overlooked.
• Grote Reber followed up, spending approximately 15 years categorizing radio sources emitting signals at radio band wavelengths (~100s of MHz).

Evolution of Radio Astronomy

Technology Development

• Rapid advancement in the field using WWII radar technology.
• Challenges:
    - Positions of radio sources were imprecisely located (best at 20 square arc minutes).
    - Dim optical counterparts were difficult to identify.
• Initial surveys identified ~500 radio sources by the late 1950s.
• Precision improved with the release of the 3C catalog in 1959.

Quasi-Stellar Objects (QSOs)

Historical Context

• In 1959, radio astronomers from Cambridge published their 3rd Cambridge Catalogue listing about 470 radio sources.
• Catalog leveraged a radio interferometer of four linked telescopes across 1.5 km, operating at 1.89 m wavelength (159 MHz).
• Resolution limits influenced by diffraction, yielding an angular resolution of approximately 0.09 degrees (5.4 arc minutes).

Significance

• Discovery of radio sources marked a pivotal event in astronomy, expanding the techniques used beyond optical methods.
• Led to new forms of observational astronomy including X-ray, gamma-ray, and infrared astronomy.
• Resulted in the need for innovative technologies and understanding of physical processes for radio emissions.

Initial Identifications

• Early radio galaxies identified included M87 and Centaurus A in 1947.
• By 1963, approximately 70 extragalactic identifications were made, with 6 tied to supernova remnants, such as 3C58 (Crab Nebula).

Identifying Sources

Methodology

• Accurate positional measurements were conducted from New Zealand and Australia, leveraging lunar occultations to fine-tune celestial coordinates.
• Aerial observations during source rises and sets helped plot their celestial coordinates.

Modern Observations

Sky Imaging

• Advances allowed for maps of the sky where each dot represents a radio source.
• Noted the inconspicuous nature of optical counterparts typically observable with optical telescopes.

Discovery of Quasars

First Identified Quasar: 3C273

• 3C273 was discovered using lunar occultation techniques by Cyril Hazard.
• The process determined the position of the moon's edge, improving localization accuracy to 3 arc seconds.
• Occultations of 3C273 are infrequent, occurring roughly once every twenty years, with the latest recorded on February 12, 2020.

Quasar Characteristics

• Initially termed as a Quasi-Stellar Radio Source, 3C273 was found to have an optical spectrum with unique characteristics, involving redshift that implied high speeds.
• Observed redshift denoted a velocity of approximately 50,000 km/s (~1/6 the speed of light).

Hubble's Law

Overview

• Hubble's law connects the recession velocity $V_{shift}$ of galaxies to their distance $D$, represented by the equation:
  $V_{shift} = H_0 D$

Calculation for 3C273

• Modern Hubble constant value: $H_0 = 68 ext{ km/s/Mpc}$.
• For a redshift of 50,000 km/s:
    - Distance $D = 735 ext{ Mpc} (2.2 imes 10^{22} ext{ km})$.
• Implies formidable luminosity for 3C273, exceeding $10^{39} ext{ W}$, approximately 100 times that of our galaxy.

Spectral Lines and Black Hole Masses

Findings from 3C273's Spectrum

• Strong spectral lines originating from hydrogen show evidence of extreme redshifts.
• The broad lines indicate high velocities with a width of approximately $V_{width} ext{~ } 5000 ext{ km/s}$.
    - This is useful for estimating black hole mass, using the relationship:
  $V_{width} ext{~ } 200 \frac{M_7}{r}^{1/2} ext{ km s}^{-1}$,
    where $M_7$ is the mass in units of $10^7$ solar masses and $r$ is the radius in parsecs.

Variability of Quasars

Observational Insights

• Detected variability in intensity over time scales as short as weeks, indicating a limited size of the emitting region.
• Further observations show variability on days, stressing the need for efficient and compact energy processes.

Nature and Power of Quasars

Remarkable Characteristics

• Observations highlighted odd nature of brightness in distant quasars.
• Quasars such as 3C273 and 3C48 have visual magnitudes of 16 and 13, respectively, despite distances of about four billion light-years.
• Potentially signify novel physical processes or challenge our understanding of astrophysical phenomena.

Accreting Black Holes

Discussion

• Donald Lynden-Bell's 1969 proposition: supermassive black holes in galactic cores could drive the immense energy of quasars.
    - Estimated energy from radio outbursts reaches $10^{61} ext{ erg}$, with implications on mass and efficiency.
• Efficiency of accretion based on hole spin:
    - Non-spinning black holes can achieve efficiencies of up to 6%.
    - Rapidly spinning black holes could achieve up to 43%.

Conclusion and Future Directions

Future Questions

• What mechanisms underlie quasar power generation?
• How does mass transition into energy?
• Exploration of galaxy influences by active galactic nuclei (AGN) and the connection to general relativity predictions.