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--- courses:ast100:2.2 [2026/02/24 00:14] – [2.2. Types of galaxies] asad
+++ courses:ast100:2.2 [2026/02/24 01:18] (current) – [Active galaxies] asad
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 ===== Active galaxies =====
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+Active galaxies are highly luminous systems whose extraordinary energy output—which cannot be explained by the combined light of stars—originates from a compact central region known as an active galactic nucleus (AGN). According to the accepted standard model of AGN (pictured above), the central engine powering all AGNs is a supermassive black hole that is actively consuming material from a rapidly rotating, superheated accretion disk. Gas orbiting very close to the black hole's intense gravitational field moves at incredibly high speeds, generating "broad emission lines" in the object's spectrum, while gas located further out moves more slowly and produces "narrow emission lines". This inner accretion disk and broad-line region are typically enveloped by a thicker, donut-shaped torus of dust and gas, and the black hole may also eject high-speed jets of particles into space.
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+Because of this geometry, AGNs are classified under different names depending on their intrinsic brightness and our specific viewing orientation relative to the torus and jets. If viewed from a more face-on angle where the central accretion disk is unobscured, the object prominently displays broad emission lines; depending on its luminosity, this is classified as a quasar or Type I Seyfert galaxy. Conversely, if viewed edge-on, the dusty torus blocks our direct view of the brilliant core and broad-line region, meaning we primarily observe narrow lines (though broad lines are sometimes faintly visible via reflection)—a variation the sources detail as Type II Seyfert galaxy. Finally, if the AGN is oriented such that its high-speed jet points directly at Earth, the object's intense emission outshines its spectral lines and it is classified as a "blazar".
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+A real picture of an active galaxy is shown above. Superimposing observations from three telescopes at three different wavelength ranges provides a comprehensive view of Centaurus A, revealing the powerful lobes and high-energy jets emanating from its central supermassive black hole. Visible light data captured by the Wide Field Imager at the La Silla Observatory shows the galaxy's stars and its characteristic dark dust lane in near "true color". Layered over this, X-ray data from the Chandra X-ray Observatory is shown in blue, capturing the high-energy jets blasting outward. Finally, radio-microwave data from the APEX telescope is superimposed in orange to map the extended lobes. Together, these three distinct wavelengths beautifully expose the complete, violent anatomy of this active galaxy.