February 29, 2024

Lukmaan IAS

A Blog for IAS Examination




THE CONTEXT: While acknowledging that the formation of new stars in galaxies can also produce radio waves, the research team focused on galaxies with minimal or no star formation. Of the 587 nearby galaxies scrutinized, all 40 of the largest galaxies examined were found to emit radio waves.


  • Radio galaxies are a distinct class of galaxies characterized by their substantial emissions of radio waves that extend far beyond their visible structures.
  • These phenomena are rooted in active galactic nuclei (AGNs) housing supermassive black holes and generate colossal lobes of gas emitting radio waves.

Characteristics of Radio Galaxies

  • Radio galaxies emit intense radio waves originating from expansive lobes of gas, extending millions of light-years beyond the visible galaxy structure.
  • These lobes, typically occurring in pairs, are a consequence of AGNs—regions of extraordinary brightness where supermassive black holes actively consume surrounding matter, emitting glowing radiation.
  • Around 15% to 20% of galaxies hosting AGNs exhibit “radio loud” characteristics, as distinguished by their prominence in radio emissions.
  • The differentiation between “radio loud” and “radio quiet” AGNs remains an ongoing area of investigation due to similarities in emissions across various wavelengths.

Factors Influencing Radio Loudness

  • Research hints at potential connections between radio loudness and the type of host galaxy.
  • Notably, radio galaxies predominantly manifest in the form of massive elliptical galaxies, possibly formed through mergers of smaller galaxies.
  • Additionally, the rotational dynamics of central black holes may contribute to the formation of powerful jets, influencing radio emissions.

Examples of Radio Galaxies

  • Prominent instances of radio galaxies include:
    • Cygnus A, exhibiting bright lobes of gas surrounding a galactic nucleus;
    • Messier 87, characterized by twin jets emanating from a luminous core; and
    • Centaurus A, an elliptical galaxy intersected by a dust lane.

Types of Radio Galaxies

  • Distinctive optical emissions define two types of radio galaxies:
    • broad-line radio galaxies
    • narrow-line radio galaxies
  • Broad-line radio galaxies display broad-line emissions from ionized oxygen, hydrogen, and silicon in their optical spectra.
  • Whereas narrow-line radio galaxies lack such emissions but showcase narrow emission lines from hydrogen and oxygen.

Differences from Normal Galaxies

  • Unlike typical galaxies, radio galaxies possess AGNs fueling the emission of intense radio waves.
  • The presence of supermassive black holes driving these AGNs distinguishes them.
  • These AGNs can produce enormous dual lobes extending thousands of light-years from the galactic center, sometimes exceeding the width of the Milky Way.

Formation and Structure of Radio Galaxies

  • The prevalent model suggests that massive dual radio lobes derive energy from supermassive black hole jets.
  • As these jets feed into the lobes, pressure builds within them, causing expansion.
  • These lobes, primarily symmetrical elliptical structures aligned with the galaxy’s center, comprise immense clouds of plasma, constituting some of the most substantial structures observed in astronomy.

Radiation and Energy Emissions

  • Radio lobes emit synchrotron radiation generated by accelerated electrons in powerful magnetic fields.
  • These emissions span a broad spectrum, ranging from radio waves to infrared, optical, ultraviolet, and even X-ray radiation.
  • The energy released by these lobes can be tremendously high, exceeding the total energy emitted by the Milky Way galaxy.


  • The study of radio galaxies unveils the dynamic interplay between supermassive black holes, AGNs, and the colossal emissions of radio waves.
  • Understanding their formation, structure, and energy emissions remains an intriguing area of astronomical research, offering insights into the most extreme phenomena within galaxies and the universe at large.

SOURCE: https://www.space.com/what-are-radio-galaxies

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