BLACK HOLES IN GALAXIES

Bryan Nozaleda

Aside from stars and the interstellar medium, what else do galaxies contain?

• Galaxies often have large magnetic fields that run through and around their disks or bulges. Although at any particular location, the fields may be weak, the overall effect of those fields can be tremendous, affecting the motion of charged particles and interstellar medium throughout galaxies. Galaxies can also contain black holes.

Does the Milky Way contain a supermassive black hole?

• It most certainly does. The center of the Milky Way is in the direction of the constellation Sagittarius; right at the center, there is an object called Sag A* (pronounced “Sagittarius A-star”) that emits much more X-rays and radio waves than expected for a star-sized body.

Does every galaxy contain a black hole?

• There are two general categories of black holes that have been observed: stellar black holes and supermassive black holes. Every galaxy that has contained very hot, luminous stars—stars 20 times or more the mass of the Sun—almost certainly contains stellar black holes.

Does every galaxy contain a supermassive black hole?

•No, but based on current observations, the majority of galaxies do contain one. Among nearby galaxies, more than 90 percent of all galaxies that have been measured so far appear to contain a supermassive black hole.

ACTIVE GALAXIES

Bryan Nozaleda

What is an “active galaxy,” or an “active galactic nucleus” (AGN)?• If a supermassive black hole exists at the nucleus of a galaxy, it

may accumulate matter from the stars and gas that surround it. If this matter is accumulated rapid-ly—at a rate of a few Earth-masses per second or greater—tremendous amounts of energy can be generated as the matter falls toward the black hole. The energy that is released in this way can be much greater than that of the nuclear fusion of a star. In fact, such a supermassive black hole system can radiate more energy in a few seconds than our Sun can produce in thousands or even millions of years. These systems are called active galactic nuclei, or AGNs.

Who first discovered and studied active galaxies?

• The American astronomer Carl Seyfert (1911–1960) is credited with the discovery of active galaxies. Seyfert’s general area of astronomical expertise was determining the spectroscopic properties, colors, and luminosities of stars and galaxies.

How many different kinds of active galaxies are there?

•AGNs can occur in any type or shape of galaxy—spiral, elliptical, or irregular. Depending on exactly how the energy radiates from the AGN, they can have very different appearances.

What determines the luminosity of an AGN?

•The single most important determinant of an AGN’s luminosity is the rate at which matter falls toward its central supermassive black hole.

What are radio galaxies?

•Radio galaxies are simply galaxies—usually very ordinary-looking elliptical galaxies when viewed by visible light—that radiate an unusually large amount of radio waves.

What is the unified model of active galactic nuclei?

• All AGNs have the same basic structure: they hava a QSO sitting in the middle of a galaxy.

• The QSO will have a different spectroscopic signature from our point of view. Furthermore,

• The QSO host galaxy could be spiral, elliptical, or peculiar, and we could be seeing the QSO through a screen of interstellar dust, or lots of gas, or a lot of stars of differing colors and luminosities.

What determines the luminosity of an AGN?

•The single most important determinant of an AGN’s luminosity is the rate at which matter falls toward its central supermassive black hole.