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1446 Introductory Astronomy II. Chapter 17 Active Galaxies and Quasars R. S. Rubins Fall 2011. 1. Normal and Active Galaxies 1.
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1446 Introductory Astronomy II Chapter 17 Active Galaxies and Quasars R. S. Rubins Fall 2011 1
Normal and Active Galaxies 1 • Of an estimated 50 billion galaxies in the observable universe, most lie over a billion (109) ly away, which is too far for their Hubble classifications to be determined reliably. • However, by studying the radiation they emit, they may be classified as normalor active. • Active galaxies are a small minority of the observed galaxies. • A few are found closer than a hundred million (108) ly, but they become increasingly more common with increasing distance. • The more numerous normal galaxies emit thermal radiation, with intensity maxima occurring in the visible. • Since visible radiation emitted by normal galaxies is essentially that of its component stars, it is known as stellar radiation. 2
Active Galaxies 1 • Active galaxiesinclude Seyfert and radio galaxies • Active galaxies emitnonstellar(or nonthermal) radiation, with maximum emission occurring at IR or radio wavelengths. • Active galaxies have nuclei, which have luminosities more than 10,000 times that of the nucleus of the Milky Way, and more than ten times that of the entire galaxy. • When viewed at visible wavelengths, the nearer active galaxies appear similar to normal galaxies in both shape and visible luminosity, but often exhibit high-speed jets of particles. 3
Intensity vs. Wavelength Curves Peak at radio or IR wavelenghths Peak in the visible 4
Active Galaxies 2 • Active galaxies have nuclei that contain powerful energy sourcesor engines, which have been identifiedas supermassive black holesfor the following reasons: • i. broad emission lines resulting from a Doppler effect caused by objects spiraling rapidly about an extremely massive central object. • ii. an energy output which may fluctuate appreciably over months, indicating that energy source may be considerably smaller than a light year. Remember that an object cannot flicker in less time than it takes light to travel across its radius. 5
Flicker Time for a Pulsar The width of the signal gives the flicker time. 7
Flicker Time and Object Size An instantaneous change in brightness of an emitting object star appears to last for the time it took for the radiation to cross the visible part of the object. For the sun, this would be 2 seconds, indicating that the radius of the Sun cannot be more than 2 light seconds.. 8
Seyfert Galaxy NGC 1566 Seyfert galaxies resemble spiral galaxies in visible light, with their spiral arms emitting about the same amount of light. However, the nucleus may have the brilliance of 100 billion (1011) Suns at IR wavelengths. 9
Radio Galaxies • Radio galaxies resemble elliptical galaxies in visible light, although their EM emissions peak at radio wavelengths. • Lobe-radio galaxies (or double radio sources) have radio emissions from two huge regions of gas – the radio lobes – often formed at over a hundred thousand ly on either side of the nucleus. • The lobes result from the slowing down and spreading out of two oppositely directed narrow jets of charged particles, which originate in the compact nucleus. • If the galaxy is moving through the intergalactic medium with sufficient speed, then a head-tail radio galaxy, with trailing lobes, may be observed. • Core-halo galaxies appear to be lobe-radio galaxies observed along the line between their lobes. 11
Cygnus A: a Radio Lobe Galaxy • The radio lobes extend about 160,000 ly on either side of the of this galaxy. 12
Quasars 1 • First discovered in 1960, quasars(quasi-stellar radio sources) were defined to be extremely strong radio emitters with a star-like appearance. • More than 10,000 quasars are now known, with look-back times of just under a billion to over 10 billionyears. • About 90% are weak radio emitters, and so are better described by the term QSOs (quasi-stellar objects). • Quasars are possibly the most luminous continuously radiating objects in the universe, with 100-1000 times the luminosity of the entire Milky Way Galaxy. • The most luminous quasars are estimated to devour about a thousand solar masses per year, which may create huge cavities around them, thus limiting their lifetimes to possibly tens of millions of years. 15
Quasars 2 • The rapid luminosity changes observed from some quasars indicates that their energy emitting regions may be as little as several light-hours across; i.e. not much larger than the solar system. • The combination of huge energy emissions and a very small volume indicates that a quasar is a supermassive black hole. • A typical quasar spectrum shows highly red-shiftedemission lines superimposed on a nonstellar background radiation. • The extremely large redshifts indicate that quasars are among the most distant objects in the universe. • The “fuzz” around some quasar images show absorption spectra, indicating that quasar lies within a galaxy. 16
Quasar 3C 48 18
The Most Distant Object • In 2009, astronomers using NASA’s Swift γ-ray Telescope observed a 10 second gamma-ray burst from an object that exploded at a distance from us of about 13.1 billion ly from us, making it the most distant object yet observed. • This event, which occurred about 600 million ly after the Big Bang, is thought to have been associated with the death of a massive star, the center of which collapsed into a black hole. • Measurements of the afterglow, which lasted for about 10 days, made at Hawaii’s Gemini North Telescope, were hindered by cloud cover.
Galaxy Evolution through Mergers When spiral galaxies of comparable size merge, an eliptical galaxy may result. When a spiral galaxy absorbs a smaller galaxy, a larger spiral may result.
