roberts

Black Holes and Active Galaxies : 

Black Holes and Active Galaxies Doug Roberts, Ph. D. Adler Planetarium & Astronomy Museum Northwestern University

Basic concepts of gravity: 

Basic concepts of gravity Gravity is created by mass. Gravity is always attractive. Gravitational attraction is proportional to the sum of the masses of both objects. Gravitational attraction increases as two objects come closer to each other.

All due to Isaac Newton: 

All due to Isaac Newton In 1665, the plague had shut down Cambridge University where Newton had been working. He subsequently worked from home on circular motion and other ideas. When the university reopened two years later, Newton used Kepler’s laws and his own observations to derive the Universal Law of Gravitation.

Newton’s Universal Law of Gravitation: 

Newton’s Universal Law of Gravitation F: Gravitational attraction (Force) G: Gravitational constant = 6.67 × 10-11 m3kg-1s-2 M: Mass of central object m: Mass of smaller object r: Distance between the objects F =GMm/r 2

Dark stars (a.k.a black holes): 

Dark stars (a.k.a black holes) English geologist Rev John Michell realized that it would be theoretically possible for gravity to be so overwhelmingly strong that nothing – not even light could escape. In his 1783 paper to the Royal Society Michell wrote: If the semi-diameter of a sphere of the same density as the Sun in the proportion of five hundred to one, and by supposing light to be attracted by the same force in proportion to its [mass] with other bodies, all light emitted from such a body would be made to return towards it, by its own proper gravity.

Dark stars (a.k.a black holes): 

Dark stars (a.k.a black holes) At the time, the necessary conditions for “dark stars” (as Michell called them) seemed physically impossible. In 1796, the great French mathematician, Pierre Laplace proposed similar ideas to those of Michell in his famous paper ‘Exposition du Systeme du Monde’. In the early 1800’s experiments on optical interference led to the predominance of the wave theory of light and the end of the corpuscular theory. Since light waves were thought to be unaffected by gravitation, interest in the hypothetical “dark stars” ceased.

General relativity: 

General relativity 1915 Einstein published his General Theory of Relativity. The General Theory was a new theory of gravitation and one of its fundamental predictions was the effect of gravity on light.

General relativity: 

General relativity According to the theory, matter causes space-time to curve. The paths followed by light rays or matter is determined by the curvature of the space-time and allowed a modern scientific proof of Mitchell’s hypothesis.

General relativity: 

General relativity Soon after Einstein developed general relativity, Karl Schwarzschild discovered a mathematical solution to the equations of the theory that described such an object. It was only much later, with the work of such people as Oppenheimer, Volkoff, and Snyder in the 1930’s that people thought seriously about the possibility that such objects might actually exist in the universe.

General relativity: 

General relativity Einstein himself vigorously denied their reality, believing, as did most of his contemporaries, that black holes were a mere mathematical curiosity. He died in 1955, before the term “black hole” was coined or understood and observational evidence for black holes began to mount.

General relativity: 

General relativity Near a black hole, this distortion of space-time is extremely severe and causes black holes to have strange properties. In particular, a black hole has an event horizon, which is a spherical surface that marks the boundary of the black hole.

Event horizon: 

Event horizon You can think of the horizon as the place where the escape velocity equals the velocity of light. Outside of the horizon, the escape velocity is less than the speed of light. But if you find yourself inside the horizon, the escape velocity would be larger than the speed of light, thus there is no escape.

Black holes have no hair: 

Black holes have no hair Black holes, unlike most objects can only have three characteristics Mass Angular momentum or spin Electric charge

The event horizon and the Schwarzschild radius : 

The event horizon and the Schwarzschild radius For a nonrotating black hole, the horizon is located at the Schwarzschild radius (Rs) G: Gravitational constant = 6.67 × 10-11 m3kg-1s-2 M: Mass of the black hole c: Speed of light = 3 × 108 km s-1 Rs =2GM/c 2

The event horizon and the Schwarzschild radius : 

The event horizon and the Schwarzschild radius For a mass as small as a human being, the gravitational radius is of the order of 10-23 cm, much smaller than the nucleus of an atom. For a typical star such as the Sun, it is about 3 km (2 miles).

Black hole classifications: 

Black hole classifications Black holes are theorized to come in three different sizes Small (“mini” or “primordial”) Medium (“stellar”) Large (“supermassive”).

Observations of black holes: 

Observations of black holes How can you check whether something is a black hole or not? The first thing you’d like to do is measure how much mass there is in that region. If you've found a large mass concentrated in a small volume, and if the mass is dark, then it's a good guess that there's a black hole there.

Observations of stellar black holes: 

Observations of stellar black holes One class of black-hole candidates are stellar-mass black holes, which are thought to form when a massive star ends its life in a supernova explosion.

Stellar black holes: 

Stellar black holes Stellar evolution: low mass stars end up as white dwarves Moderate mass stars end up as neutron stars and pulsars The highest mass stars become black holes

Observations of stellar black holes: 

Observations of stellar black holes Another possibility is that black holes might form as a merger of two neutron stars. Credit: Wai-Mo Suen, Malcolm Tobias, Mark Miller, et. al.

Observations of stellar black holes: 

Observations of stellar black holes Merger from loss of gravitational radiation Credit: J. Faber & F. Rasio, Northwestern U.

Observations of stellar black holes: 

Observations of stellar black holes If such a stellar black hole were to be off somewhere by itself, we wouldn't have much hope of finding it. However, many (probably most) stars come in binary systems – pairs of stars in orbit around each other.

Observations of stellar black holes: 

Observations of stellar black holes If one of the stars in such a binary system becomes a black hole, we might be able to detect it. In particular, in some binary systems containing a compact object such as a black hole, matter is sucked off of the other object and forms an “accretion disk” of stuff swirling into the black hole.

Observations of stellar black holes: 

Observations of stellar black holes The matter in the accretion disk gets very hot as it falls closer and closer to the black hole, and it emits large amounts of radiation. Accretion from companion onto compact object. Credit: Gamma-Ray Astronomy Program Working Group, NASA.

Observations of stellar black holes: 

Observations of stellar black holes Because of the intense heat created as the mass falls into the accretion disk, most of the radiation we observe is in the X-ray part of the spectrum. Many such “X-ray binary systems” are known, and some of them are thought to be likely black-hole candidates.

Observations of stellar black holes: 

Observations of stellar black holes In order to determine if an unseen compact object is a black hole, you need to do is to estimate its mass. By measuring how fast the visible companion orbits the center of mass of the system (together with a few other things), you can figure out the mass of the invisible companion. The technique is quite similar to the one for supermassive black holes in galactic centers: the faster the star is moving, the stronger the gravitational force required to keep it in place, and so the more massive the invisible companion.

Observations of stellar black holes: 

Observations of stellar black holes If the mass of the compact object is found to be very large very large, then there is no kind of object we know about that it could be other than a black hole. An ordinary star of that mass would be visible. A stellar remnant such as a neutron star would be unable to support itself against gravity, and would collapse to a black hole.

Observations of stellar black holes: 

Observations of stellar black holes The combination of such mass estimates and detailed studies of the radiation from the accretion disk can supply powerful circumstantial evidence that the object in question is indeed a black hole. Many of these “X-ray binary” systems are known, and in some cases the evidence in support of the black-hole hypothesis is quite strong.

Stellar black holes: Cygnus X-1: 

Stellar black holes: Cygnus X-1 Cygnus X-1 was the name given to a source of X-rays in the constellation Cygnus, discovered in 1962 with a primitive X-ray telescope flown on a rocket. By 1971, the location of the X-ray source in the sky had been measured more precisely, using rocket and satellite observations.

Stellar black holes: Cygnus X-1: 

Stellar black holes: Cygnus X-1 A faint star appears to be the companion to Cygnus X-1. Astronomers studying the light of this companion star have found two important facts: HDE 226868 is a blue supergiant star – a massive, normal star near the end of its life the star is orbiting another massive object in a 5.6-day orbit.

Stellar black holes: Cygnus X-1: 

Stellar black holes: Cygnus X-1 The explanation or “model” which best fits these facts is that the companion is a black hole of about 10 solar masses – the corpse of a massive star which was once the companion of the observed star.

Stellar black holes: Cygnus X-1: 

Stellar black holes: Cygnus X-1 The X-rays are produced as gas from the atmosphere of the blue supergiant star falls into the collapsed object and is heated. The collapsed object cannot be a white dwarf or neutron star, because these objects can’t have masses greater than 1.4 and 3 solar masses, respectively.

Similar stories for other X-ray binary systems: 

Similar stories for other X-ray binary systems LMC X-3 Nova Muscae 1991 V616 Mon (A0620-00)

Supermassive black holes: 

Supermassive black holes Some point like sources of radio emission are not related to any optical star or galaxy. In 1963, the location of the radio source (called 3C273) was identified and coincided with a very distant star-like object in the visual wavelengths.

Supermassive black holes: 

Supermassive black holes CHANDRA X-RAY (Credit: NASA/HST, Jodrell Bank Observatory, & NASA/CXC/SAO/H. Marshall et al.)

Supermassive black holes: 

Supermassive black holes They called these objects quasi-stellar radio sources – quasars, for short – because they looked like stars, and produced large amounts of radio waves as well as light.

Supermassive black holes: 

Supermassive black holes If you had radio eyes you would see these quasars and the centers of active galaxies all over the sky in stead of stars. Credit: image courtesy of NRAO/AUI

Supermassive black holes: 

Supermassive black holes Astronomers also realized that, although quasars were rare, there were many other objects – apparently galaxies of stars – which showed less extreme versions of the same phenomenon: very large power from a very small volume.

Supermassive black holes: 

Supermassive black holes These objects shared another remarkable property: jets of high-energy particles emitted from their cores. Credit: NASA.

Supermassive black holes: 

Supermassive black holes Credit: C.M. Urry & P. Padovani

Supermassive black holes: 

Supermassive black holes More recent studies have confirmed that QSO’s lie at the hearts of galaxies which are themselves too dim to be visible. QSO’s are thought to be of the order of size of our solar system, but radiate more than 1000 times as much energy as our entire galaxy. The current explanation is that they result from a supermassive black hole which is consuming matter from its surrounding galaxy.

Supermassive black holes: 

Supermassive black holes The observed power output could be explained if material the mass of our Sun were to fall into the black hole each year. This amount of material which could easily come from the orbiting gas and winds from massive stars near the core of the galaxy.

Supermassive black holes: 

Supermassive black holes The jets of particles in active galactic nuclei are produced by material spiraling into a disk around the black hole; jets are emitted from the top and bottom of the disk. Cygnus A radio galaxy taken at the Very Large Array. Credit: Image courtesy of NRAO/AUI

Supermassive black holes: 

Supermassive black holes This explanation for the “central engine” in an active galactic nucleus has been strongly supported by images obtained by the Hubble Space Telescope, Chandra X-ray observatory and radio telescopes such as the Very Large Array.

Supermassive black holes: 

Supermassive black holes 2D MHD Simulation of Jet (Mach number=10, Jet density=0.01 of external medium), Top: Gas Density; Bottom: Magnetic Pressure. Credit: I. L. Tregillis, T. W. Jones & Dongsu Ryu.

Supermassive black holes: 

Supermassive black holes 3D MHD Simulation of Jet (Mach number=6), Synchrotron (radio) surface brightness. Credit: I. L. Tregillis, T. W. Jones & Dongsu Ryu.

How do supermassive black holes form?: 

How do supermassive black holes form? Some theories hold that the first generation of stars included a large proportion of very massive stars, all of which formed black holes which somehow merged. Other theories hold that a single “seed” black hole accreted stars and gas, growing more and more massive with time.

Observations of supermassive black holes: 

Observations of supermassive black holes Many galaxies have been observed to contain such massive dark objects in their centers. The masses of the cores of these galaxies range from one million to several billion times the mass of the Sun.

Observations of supermassive black holes: 

Observations of supermassive black holes The mass is measured by observing the speed which stars and gas orbit around the center of the galaxy: the faster the orbits, the stronger the gravitational force required to hold them in their orbits. This is the most common way to measure masses in astronomy. For example, we measure the mass of the Sun by observing how fast the planets orbit it, and we measure the amount of dark matter in galaxies by measuring how fast things orbit at the edge of the galaxy.

Observations of supermassive black holes: 

Observations of supermassive black holes These massive dark objects in galactic centers are thought to be black holes for at least two reasons. First, it is hard to think of anything else they could be: they are too dense and dark to be stars or clusters of stars.

Observations of supermassive black holes: 

Observations of supermassive black holes Secondly, the only promising theory to explain the enigmatic objects known as quasars and active galaxies suggests that such galaxies have supermassive black holes at their cores. If this theory is correct, then a large fraction of galaxies – all the ones that are now or used to be active galaxies – must have supermassive black holes at the center.

Exotic technique: gravitational radiation: 

Exotic technique: gravitational radiation The existence of curved spacetime opens up the possibility that ripples or waves can exist in the spacetime continuum. These ripples are called gravitational waves. Gravity waves could be detected from colliding black holes, supernova explosions and the black hole at the core of our Galaxy.

Supermassive black holes: M87: 

Supermassive black holes: M87 Hubble measurements show the disk at the center of M87 is rotating very rapidly. Scientists believe it contains a massive black hole at its hub. Though the black hole weigh as much as 3 billion of our Suns, it is concentrated into a space no larger than our solar system. A brilliant jet of high-speed electrons that emits from the nucleus is believed to be produced by the black hole’s “engine.”

Supermassive black holes: M87: 

Supermassive black holes: M87 Credit: Image courtesy of NRAO/AUI.

Supermassive black holes: M87: 

Supermassive black holes: M87 Hubble Space Telescope image of a spiral-shaped disk of hot gas in the core of active galaxy M87. Credit: STScI WFPC2.

Supermassive black holes: NGC 4261: 

Supermassive black holes: NGC 4261 A composite image of the active galaxy NGC 4261, showing jets of radio-emitting particles spurting from the core of the galaxy. A false-color image (right) from the Hubble Space Telescope, shows a dark, doughnut-shaped structure surrounding a possible supermassive black hole. Credit: Walter Jaffe, Leiden Observatory; Holland Ford, STScI, NASA

Supermassive black holes: NGC 4258: 

Supermassive black holes: NGC 4258 NGC 4258 was found to have a system of “water masers” near its nucleus. Using the technique of very-long-baseline [radio frequency] interferometry, researchers were able to determine the motion of the gas very accurately. From this they can conclude that the massive object at the center of this galaxy is less than half a light-year in radius. It is hard to imagine anything other than a black hole that could have so much mass concentrated in such a small volume. Credit: Harvard Smithsonian CfA, NRAO/AUI.

Supermassive black holes: The Milky Way: 

Supermassive black holes: The Milky Way There has been growing evidence that our own Galaxy harbors a black hole in its center. Credit: Image courtesy of NRAO/AUI

Supermassive black holes: The Milky Way: 

Supermassive black holes: The Milky Way Credit: Image courtesy of NRAO/AUI

Supermassive black holes: The Milky Way: 

Supermassive black holes: The Milky Way Credit: Image courtesy of NRAO/AUI

Supermassive black holes: The Milky Way: 

Supermassive black holes: The Milky Way Credit: Image courtesy of NRAO/AUI

Proper motion of hot gas in the Galactic Center: 

Proper motion of hot gas in the Galactic Center Credit: D. Roberts, F. Yusef-Zadeh & W.M Goss

Proper motion of stars in the Galactic Center: 

Proper motion of stars in the Galactic Center Credit: R. Genzel, A. Eckart, T. Ott, MPE.

Proper motion of stars in the Galactic Center: 

Proper motion of stars in the Galactic Center Credit: UCLA Galactic Center Group.

Supermassive black holes: The Milky Way: 

Supermassive black holes: The Milky Way Research that investigated the motion of gas and stars around the center has shown that the enclosed mass is constant to within a few times the size of our solar system.

Supermassive black holes: The Milky Way: 

Supermassive black holes: The Milky Way The only known object that could produce this effect is a supermassive black hole (although this is the smallest supermassive one known) that is about 2 million times more massive than the sun. The big mystery in our Galaxy is why; with such a large black hole in the center don’t we see our core as an active galactic nucleus.