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Chapter 27

Hubble Ultra-Deep Field

  • Description: The Hubble Ultra-Deep Field provides the deepest image of the sky in visible light, showcasing numerous galaxies within a minuscule area, only 1/100th the area of the full Moon.
  • Contrast: The image taken in X-rays shows many point-like quasars, identified as supermassive black holes at the centers of galaxies.
  • Credits:
    • Left: modification of work by NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), Z. Levay (STScI)
    • Right: modification of work by ESO/Mario Nonino, Piero Rosati, ESO GOODS Team

Chapter Outline

  • 27.1 Quasars
  • 27.2 Supermassive Black Holes: What Quasars Really Are
  • 27.3 Quasars as Probes of Evolution in the Universe

Introduction to Quasars

  • Historical Context: In the first half of the 20th century, astronomers saw the universe as peaceful, believing galaxies evolved slowly over billions of years.
  • Changing Perception: This view changed radically towards the end of the 20th century, revealing the universe's violent nature with supernovae explosions, galaxy collisions, and energy outputs around massive black holes.
  • Key Discovery: Quasars were identified as pivotal in shifting our understanding of the universe.

27.1 Quasars

Learning Objectives

  • Describe how quasars were discovered.
  • Explain how redshifts determined the distances of quasars.
  • Justify the claim about the energy production of quasars in a small spatial volume.

Definition and Origin

  • Quasar Meaning: The term “quasar” is short for “quasi-stellar radio sources,” where quasi-stellar means “sort of like stars.”
  • Initial Discovery: Originating in the 1950s from surplus World War II radar equipment leading to the discovery of point-like radio sources resembling stars. No known stars emitted such strong radio radiation.

Redshift and Spectral Analysis

  • Visible-Light Spectra: Astronomers obtained spectra for two faint blue “stars” with strong radio waves. Their emission lines proved unidentified with known substances.
  • Historical Context: By the 1960s, spectra analysis had been advanced by a century of research. Mysterious lines indicated a new category of objects: quasars.
  • Identifying Quasars: In 1963, Maarten Schmidt analyzed the spectrum of the radio star 3C 273:
    • Emission lines matched the Balmer lines of hydrogen but were significantly redshifted, indicating a recession speed of 45,000 km/s (15% the speed of light).

Characteristics of Quasars

  • Quasars were found to emit large redshifts and bright emissions across infrared and X-ray spectra, classifying them as quasi-stellar objects (QSOs).
  • Over a million quasars detected, with their spectra showing purely redshifts, indicative of their distances and high energies.
  • Example: Quasar 3C 273 with a redshift indicating its light took about 2.5 billion years to reach Earth.

Doppler Shift Formula for Quasars

  • Doppler Shift Definition: Used to calculate recession speeds, where z = rac{ iny{ ext{ extlambda}}o - iny{ ext{ extlambda}}e}{ iny{ ext{ extlambda}}_e}; with ext{ extlambda} being the wavelength.
  • Example Calculation:
    • Wavelength of a galaxy line at 393 nm becomes redshifted by 7.86 nm observed at 400 nm.
    • Redshift shows this galaxy is receding at approximately 2% of the speed of light.

Hubble Law and Quasars

  • Quasars demonstrate compliance with Hubble's law, indicating they are part of galaxies and confirming their distances.
  • Observational evidence shows quasars exist in the centers of both spiral and elliptical galaxies, exhibiting redshifts co-existing with their host galaxies.

27.2 Supermassive Black Holes: What Quasars Really Are

Learning Objectives

  • Describe characteristics of quasars.
  • Justify supermassive black holes as energy sources for quasars.
  • Explain the energy production process in quasars.

Characteristics of Quasars

  • Power and Size: Quasars emit immense power, greatly exceeding the total luminosity of stars within our galaxy and are roughly the size of our solar system.
  • Jets: Some quasars emit jets of particles at close to light-speed in tight beams, contributing to radio and gamma-ray emissions.
  • Energy Mechanism: Must derive energy through processes more efficient than nuclear fusion.

Black Holes in Active Galaxies

  • Relevance: Supermassive black holes are essential for explaining quasar properties and geophysical interactions in galaxies.
    • Estimated that most galaxies, especially elliptical ones and those with nuclear bulges, harbor central black holes.

Energy Production from Accretion

  • Accretion Disk Dynamics: When material falls onto a black hole, forming an accretion disk, energy is released as radiation proportional to the black hole's mass and accretion rate.
    • Material decreasing at about 10 solar masses per year can yield energy outputs comparable to multiple galaxies.

Observational Evidence for Black Holes

  • Methodologies include measuring radial velocities of gas surrounding a suspected black hole, (v \ ext{ with } \ ext{R} \ ext{ and Kepler's Third Law} ).
  • Example: The extraordinary rotation speed of gas around M87’s black hole indicates a mass of at least 3.5 billion solar masses in a confined area.

27.3 Quasars as Probes of Evolution in the Universe

Learning Objectives

  • Trace the evolution of quasars.
  • Describe interactions between galaxies and black holes.
  • Explain the formation of early black holes.
  • Justify quiescent states of black holes.

Evolutionary Significance of Quasars

  • Quasars serve as indicators for the universe’s evolution, illustrating a vibrant state in which they were most active, particularly when the universe was 20% of its current age.
  • With significant quasar counts, early instrumentation has uncovered formative structures before the galaxies settled into larger formations.

Galaxy-Black Hole Interdependency

  • Black holes deplete stellar materials through gravitational pulls, impacting star formation rates. Simultaneously, quasars can either support star formation through energy input or suppress growth via radiative interactions.

Quasar Activity Over Cosmic Time

  • Interactions: Quasars predominantly provided insights into early galactic environments, where formations and collisions of smaller galaxies fed into larger ones, supplying the necessary fuel for black holes, and re-triggering quasar states.
  • Mortality of quasars and their eventual quiescent phases due to their energy depletions is observable over time scales leading up to current observations.

Black Hole Formation Challenges

  • The origins of supermassive black holes remain questionable, with hypotheses on how those with masses in the billions formed soon after the universe began.
  • Theories suggest rapid assembly through stellar remnants or collapsing gas clouds captivating enough material before conditions disallowed further accrual.

Key Terms

  • Active Galactic Nuclei (AGN): Galaxies almost as luminous as quasars, exhibiting similar properties.
  • Quasar: A highly luminous object encompassing extragalactic stars and defined by a significant redshift, known also as quasi-stellar object (QSO).

Summary Highlights

  • Quasars, first discovered as star-resembling radio sources, present significant redshifts and lie at the centers of galaxies, supporting the Hubble Law.
  • The energy emitted from quasars originates from in-falling materials around supermassive black holes, explaining their compact sizes and immense luminosities.
  • Quasars act as pivotal markers for cosmic evolution, their alteration traces guiding future explorations into the ongoing interactions between black holes and surrounding galaxies.