Today in Astronomy 142: supermassive black holes in active-galaxy nuclei

Transcription

1 Today in Astronomy 142: supermassive black holes in active-galaxy nuclei Active-galaxy nuclei (AGNs) Relativistic and superluminal motion in quasar jets Radio galaxies, quasars and blazars: the same objects seen in different orientations Seyferts: active spiral galaxies AGN accretion disks Masses of AGN black holes Hubble-ACS image of quasar MC , showing that it lies at the center of an elliptical galaxy with peculiar shell-like collections of bright stars (Gabriela Canalizo, UCR). 9 April 2013 Astronomy 142, Spring

2 Active galaxies and active galactic nuclei (AGNs) These kinds of galaxies have active nuclei: Quasars Radio galaxies Both discovered originally by radio astronomers. Thousands of each are now known. Seyfert galaxies Blazars (a.k.a. BL Lacertae objects) Both discovered originally by visible-light astronomers. Hundreds of each also now known. We know lots of them, but active galaxies are quite rare, in the sense that they are vastly outnumbered by normal galaxies. 9 April 2013 Astronomy 142, Spring

3 Active galaxies and active galactic nuclei (AGNs) (continued) Different classes of active galaxies have a lot in common, despite their different appearances. The two most obvious common features: All have some sort of star-like object at their very centers, that dominates the galaxies luminosities. They are all much more luminous ( ) than normal galaxies of the same Hubble type, and are therefore all thought to involve accreting, supermassive black holes. We discussed quasars briefly last class. The distinguishing characteristics of a quasar: Starlike galaxy nucleus with extremely large luminosity. One-sided jet. 9 April 2013 Astronomy 142, Spring

4 The archetypal quasar, 3C 273 In X rays, by CXO. In visible light, by HST. In radio, by MERLIN. In each case, the quasar (upper left) is starlike (despite the spreading glare in the visible and X-ray images), and much brighter than anything else in the image. No counterjet is seen on the other side of the quasar from the jet in this image. 9 April 2013 Astronomy 142, Spring

5 3C 273 (continued) Hubble-ACS photo-negative image of 3C 273. Beyond the glare of the quasar one sees the starlight from the elliptical galaxy that plays host to the quasar. (John Bahcall, Princeton U.) 9 April 2013 Astronomy 142, Spring

6 Superluminal (apparently faster-thanlight) motion in quasar jets The innermost parts of the radio jet in 3C 273 consists mainly of small knots with separation that changes measurably with time, as shown in these radio images taken over the course of three years (Pearson et al. 1981). The brightest (leftmost) one corresponds to the object at the center of the quasar. One tick mark on the map border corresponds to 20.2 light years at the distance of 3C 273. Thus the rightmost knot looks to have moved about 21 light years in only three years. Movie It moves at seven times the speed of light? 9 April 2013 Astronomy 142, Spring

7 Superluminal motion in quasar jets: an optical illusion Positions of knot when two pictures were taken, one year apart. θ v Speed of knot (close to the speed of light) Light paths: Small angle: the A knot s motion is mostly along the Not drawn to scale! line of sight. Light path B is shorter than path A. If the knot s speed is close to the speed of light, B is almost a light-year shorter than A. This head start makes the light arrive sooner than expected, giving the appearance that the knot is moving faster than light. (Nothing actually needs to move that fast.) 9 April 2013 Astronomy 142, Spring B

8 Superluminal motion (continued) As you ll show in this week s Workshop: vsinθ vsinθ v,apparent = = v 1 cosθ 1 β cosθ c 1 v = v = γ v max 2 1 β ( ),apparent Thus apparent speeds in excess of the speed of light can be obtained. However: the apparent speeds only turn out to be much in excess of the speed of light if the actual speed of the radio-emitting knots is pretty close to the speed of light. 9 April 2013 Astronomy 142, Spring

9 Radio galaxies Discovered in the 1950s at about the same time as quasars. Quite distinct from quasars, which in the 50s were seen to be point-like and associated only with pointlike optical objects: radio galaxies consisted of a pair of extended radio lobes on either side of a visible, elliptical galaxy. As radio interferometric techniques improved, radio galaxies were shown also to possess compact, pointlike central objects coincident with the galaxy nuclei, connected to the lobes by narrow, usually straight jets. The lobes themselves have fine, filamentary structure; many have hot spots at the ends of the jets. 9 April 2013 Astronomy 142, Spring

10 The archetypal radio galaxy, Cygnus A Not to be confused with Cygnus X-1. Top: X-ray image, by the CXO (Wilson et al. 2000). Middle: visible-light image, from the HST-WFPC2 archives. Bottom: radio image, by Rick Perley et al. (1984), with the VLA. 9 April 2013 Astronomy 142, Spring

11 3C353 Swain & Bridle, with the VLA 9 April 2013 Astronomy 142, Spring

12 Radio galaxy 3C 175 VLA image by Alan Bridle (NRAO), April 2013 Astronomy 142, Spring

13 Double-nucleus radio galaxy 3C 75 = NGC 1128 Composite radio (red, green) and X- ray (blue) image at right; visible-light image below. (Dan Hudson et al. 2006). 9 April 2013 Astronomy 142, Spring

14 Blazars (BL Lacertae objects) Bright and starlike. Only recently has very faint luminosity been detected around them to indicate that they are the nuclei of galaxies. Smooth spectrum: hard to measure Doppler shift. Thus it was not realized at first that these objects were far enough away to be galaxy nuclei. Most are strong point-like radio sources. (Stars aren t; this was the first real indication that blazars are distant galaxies.) Violently variable brightness: large luminosity produced in a very small volume. (Sounds like a quasar so far.) No long jets seen. 9 April 2013 Astronomy 142, Spring

15 Seyfert galaxies Discovered in the 1940s by Carl Seyfert: spiral galaxies with starlike nuclei, often brighter than the rest of the galaxy, with ionized gas associated with these centers. Some found with very broad (thousands of km/s wide) recombination lines associated with the nuclei: these are called Type 1 Seyferts. NGC 4151 is one of these. Others have only narrow-line spectra (< a few hundred km/s at the nucleus): type 2 Seyferts. NGC 1068 is the paradigm of this class. There are intermediate types as well. The rotational and random speeds inferred from spectral lines near the centers indicate supermassive black holes for these galaxies, too, as we ll see in a minute. 9 April 2013 Astronomy 142, Spring

16 Seyfert galaxies (continued) Since spirals have a lot more interstellar gas and dust than the elliptical RGs and quasars, the jets don t make it out of the galaxy. Example: M106 (NGC 4258). Very short jet, oriented roughly along the galaxy s axis, seen in nucleus. Rest of jet apparently entrained in disk of galaxy. M106 in X rays from jet-driven shocks (blue) and visible light from normal galactic processes (red). From Y. Yang (UMd/CXO/NASA). 9 April 2013 Astronomy 142, Spring

17 Quasars, radio galaxies and blazars are the same thing, seen from different angles. Relativistic, accelerating electric charges beam the light they emit in the direction they re going. You ll learn why, in PHY 218. Thus the approaching jet should be much brighter than the receding one (counterjet). Under simple assumptions for steady jets one gets, in a difficult calculation, f f jet counterjet (e.g. Blandford & Königl 1979). Quasar jets are one-sided: radio galaxy jets viewed closer to head-on? Blazars: radio galaxies viewed precisely head on? 9 April 2013 Astronomy 142, Spring β cosθ 1 if θ 0, β 1. 1 β cosθ

18 Quasars, radio galaxies and blazars are the same thing, seen from different angles (continued) Galaxy Relativistic Jets An observer whose line of sight makes a small angle with the jet would see the object as a quasar. (For an extremely small angle, it appears as a blazar.) An observer whose line of sight is closer to perpendicular to the jet would see the object as a radio galaxy. 9 April 2013 Astronomy 142, Spring

19 Quasars, radio galaxies and blazars are the same thing, seen from different angles (continued) It is possible to predict from these suggestions what the relative numbers of quasars and radio galaxies should be. Suppose jets within 45 o of the line of sight appear as quasars (i.e. have one jet), and those outside as radio galaxies (two). For sake of argument: no jet = fjet f counterjet > 100. Then our chances of seeing a given AGN as a quasar are the same as our chances of being in the cones within 45 o of the jets. This, in turn, is the fraction of the AGN s sky s solid angle that the cones occupy. Similarly for radio galaxies and the solid angle outside the cones. 9 April 2013 Astronomy 142, Spring

20 Quasars, radio galaxies and blazars are the same thing, seen from different angles (continued) 45 o RG QSO QSO θ Line of sight Solid angle outside the cones: 2π ( ) 3 π /4 Ω RG = dω= dφ dθsinθ 0 π /4 3 π /4 π /4 = 2π cosθ = 2π 2 Solid angle of the cones: Jet axis ( ) Ω = 4π Ω = 2π 2 2 QSO RG 9 April 2013 Astronomy 142, Spring

21 Quasars, radio galaxies and blazars are the same thing, seen from different angles (continued) 45 o RG QSO QSO Jet axis θ Line of sight Thus: N( RG) ΩRG 2π 2 = = N( QSO) Ω 2π 2 2 Observations of QSOs and RGs in the 3CRR catalogue for z = give (Barthel 1989). QSO 1 = = N ( RG) N ( QSO ) = 2.44 ( ) 9 April 2013 Astronomy 142, Spring

22 AGN accretion disks A disk-shaped collection of matter surrounding the black hole in an AGN arises rather naturally, just as it does in galactic black holes and young stellar objects. Stars in a galaxy perpetually collide elastically, exchanging energy, momentum and angular momentum. Two stars, originally in similar orbits and undergoing such a collision, will usually find themselves pushed to different orbits, one going to a smaller-circumference orbit, and one going to a larger orbit. Thus some stars and gas clouds are pushed to the very center of the galaxy after a number of these encounters. There they are tidally sheared in the strong gravity near the black hole. 9 April 2013 Astronomy 142, Spring

23 Black hole AGN accretion disks (continued) Star View from high above, along orbit s axis. Quinn & Sussman April 2013 Astronomy 142, Spring

24 AGN accretion disks (continued) Eventually the tidally-disrupted material from many stellar encounters settles down into a flattened disk. Collisions among particles in the disk cause material to lose its spin and become accreted by the black hole. Perspective view Black hole horizon Accretion disk Rotation of disk Can contain M and extend 10 2 s of ly from the black hole. 9 April 2013 Astronomy 142, Spring

25 Operation of AGN accretion disks Recall that for non-spinning black holes, orbits with circumference less than 3C S are unstable, and no orbits exist with circumference less than 1.5C S. Within this volume the disk structure breaks down and material tends to stream in toward the horizon. A large amount of power, mostly in the form of X rays and γ rays, is emitted by the infalling material. Pressure exerted by this light slows down the rate at which accretion takes place. Much of this high-energy light is absorbed by the disk, which heats up and re-radiates the energy as longer wavelength light. Heated disk = compact central object seen in radio images of radio galaxies and quasars. 9 April 2013 Astronomy 142, Spring

26 Operation of AGN accretion disks (continued) Some of the particles absorbing the highest-energy light are accelerated to speeds approaching that of light. If their velocity takes them into the disk, they just collide with disk material and lose their energy to heat. If their velocity takes them perpendicular to the disk, they may escape (Blandford & Rees 1975). High-speed particles escaping perpendicular to the disk = jets seen in radio and visible images of radio galaxies and quasars. Accelerated by gravity and by radiation pressure and maybe even by magnetic effects the escaping material would be expected to be relativistic, just as observed in jets. 9 April 2013 Astronomy 142, Spring

27 Structure of an AGN accretion disk Not drawn to scale! Jet Innermost stable orbit Ingoing: matter, being accreted Outgoing: X and γ rays, heating disk and accelerating jets Horizon Accretion disk (crosssection view) 9 April 2013 Astronomy 142, Spring

28 Accretion disk in NGC ,000 light years 400 light years Left: radio (red) and visible (white) view with ground-based telescopes. Right: visible light image by the Hubble Space Telescope (NASA/STScI). 9 April 2013 Astronomy 142, Spring

29 Accretion disk in M106 In the center of M106, Miyoshi et al. (1995) detected a molecular disk about 1 ly in diameter, rotating at 1000 km/sec near the outer edge. Observation of the distribution and motion of water-vapor masers in this disk allowed the geometrical determination of its distance (e.g. Greene et al. 2010), discussed last lecture. 9 April 2013 Astronomy 142, Spring

30 Measurement of masses of black holes in galactic nuclei With high enough angular and spectral resolution, one can derive the masses of central black holes. When, on scales much smaller than the scale length of the central stellar cluster, random radial velocities v r peak very sharply, there is probably a point mass present, presumably a BH. Its mass 2 may be obtained basically with M = 6v r r G (see, e.g., Gültekin et al. 2009). a Keplerian rotation curve v 1 r is observed, a central point mass may again be inferred, and its mass 2 may be obtained basically with M= vrg (see, e.g., Greene et al. 2010). 9 April 2013 Astronomy 142, Spring

31 Measurement of masses of black holes in galactic nuclei (continued) Gültekin et al total, not just BH region Note that we didn t say that this could only be done for active galaxies! Many of the galaxies on this plot of supermassive central black holes are not active. An AGN requires a supermassive black hole and an ample supply of mass to be accreted. Not all SBHs have it, at the moment. 9 April 2013 Astronomy 142, Spring