Thursday, October 14, 2021

Learning galaxies

Interaction between galaxies

 Stars within a galaxy do not collide because the distance between them is 100,000 to millions of times their size.  However, distances between galaxies can be on the order of tens of times larger than the size of galaxies, not considering satellite galaxies.  The Milky Way, within a radius of twice its size, has several small galaxies orbiting around it;  two of them are the Small and Large Magellanic Clouds, satellite galaxies that are interacting with the Galaxy.  The closest galaxy of about the size of ours is the Andromeda Galaxy, which is 30 diameters away from the Milky Way.  Galaxies in rich clusters are even closer together.

 As such, collisions between galaxies are common.  But the time scales are too long to compare with our lifetime - tens of millions of years.

 Active Core Galaxies

 We define active core galaxies as those that have a very luminous core, far above the normal luminosity of galaxies, and that emit light in all or almost the entire spectrum.

 quasars

 They are objects that in 1960, when they were first observed, were believed to be blue stars, due to their optical appearance.  However, strangely they had a high radio emission, which does not occur in stars.  In optical, these objects had emission lines, without absorption lines characteristic of this type of star.  quasar or a star?

 Schmidt (1963), comparing the 3C 273 radio source with the hydrogen spectrum, noted that although very red-shifted, the object's spectrum reproduced the regularity of the lines in the hydrogen spectrum.  He was shocked when calculating the object's recession speed: 47 400 km/s (almost 16% of the speed of light!).  Hubble's law said that this object did not belong to the Galaxy.  These quasi-stellar objects were called quasars. Another difference: the continuum of the spectrum did not obey the Planck blackbody distribution.

 Quasars are superluminous active galaxies located at great distances.

 active galaxies

 Galaxies whose luminosity is between the luminosities of typical galaxies and powerful quasars, and which possess

 nuclear activity.

 Seyferts galaxies

 They were discovered by Carl Seyfert, who first noticed the peculiarities of the spectra of this type of galaxy.  They are spiral galaxies with a compact and very bright core, which produce a non-thermal continuum spectrum with emission lines.

 Some Seyferts have narrow emission lines, rather than broad lines, and are bright in the infrared.  These Seyferts are called Seyfert 2 and the classic ones with extended lines Seyfert 1.

 Radio Galaxies

 They are galaxies whose nucleus emits strongly in radio;  they also have radio lobes that extend as shown below.  Most radio galaxies are elliptical.

 A third type of active galaxy, called BL Lacertae, is probably radio galaxies with the jet along our line of sight.

 Where does the luminosity of active galaxies and quasars come from?

 One possible mechanism to explain the high nuclear luminosity of these galaxies is that this radiation intensity would be produced by a hot gas in an accretion disk around a black hole.  The hot gas spirals around the black hole and is dredged up, releasing energy.

 The black hole must be supermassive, with a mass on the order of 100 million solar masses.  The accretion disk is on the order of light months.

 Unified Model

 Around the core of radio galaxy NGC 4261 is a ring of gas and dust about 400 light-years across and a jet perpendicular to the plane of the dust and gas disk.  The black hole is too small to be resolved at a distance from this galaxy.

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