1. Do black holes really exist? Dr Marek Kukula, Royal Observatory Greenwich

2. Now have strong evidence for two classes of black hole Stellar-mass black holes: few times the mass of the sun. Found throughout our own Galaxy. Supermassive black holes: up to 10 billion times the mass of the sun. Found only in the centres of large galaxies.

3. What is a black hole? A region of space with such intense gravity that not even light can escape. First suggested in the 18 th Century by Laplace. Idea confirmed by Einstein’s General Theory of Relativity.

4. Escape velocity If enough mass is concentrated into a small enough volume its gravity will be so strong that even light will not be able to escape. Strength of gravity depends on: Mass of object Distance from centre of mass  A Black Hole

5. Event horizon Background light distorted by intense gravitational field close to the black hole. Event horizon: escape velocity = speed of light  Nothing can escape the gravitational pull inside this radius. Singularity: all matter inside the event horizon is crushed to a point of ZERO SIZE and INFINITE DENSITY.

6. How might black holes form? Where should we look for them?  very large mass  very small volume To be sure that we’ve found a black hole astronomers need to demonstrate the object has: Physicists and mathematicians might also like to see evidence for:  an event horizon  a singularity ()

7. Stellar mass black holes Nuclear reactions in the stellar core support a star against the inward force of its own gravity. When the star’s nuclear fuel runs out it should begin to collapse… Is this a way to form a black hole?

8. Death of a star like the sun When the the Sun’s helium fuel is exhausted it will have no further source of energy The outer layers of the star are gently expelled into space, forming a glowing “planetary nebula” The hot, dense stellar core is left behind to cool slowly over billions of years – a White Dwarf star Everything depends on the mass of the star…

9. White Dwarf star The mass of the sun in a volume the size of a planet. Composed of “degenerate matter”. … but it’s not a black hole

10. Planetary nebulae

11. Stars more massive than the Sun end their lives in Supernova explosions:

12. Much of the star’s mass is lost in the explosion A dense, compact core is left behind.

13. If the remaining stellar core has a mass less than 3 times the mass of the sun it will form a Neutron Star:

14. Neutron star: a ball of subatomic particles supported by nuclear forces Mass: 1.4  3 times the Sun Radius: 10 km Density: Ben Nevis per teaspoonful!

15. Do neutron stars really exist? Radio signals from the centre of supernova remnants: Lovell radio telescope, Jodrell Bank “pulsars”

16. The discovery of pulsars Jocelyn Bell-Burnell & Anthony Hewish 1967 Such rapid radio pulsations could only come from a very small, dense object with an intense magnetic field.  Exactly the properties expected for a rapidly spinning neutron star. But this still isn’t a black hole!

17. For really massive stars (> 10 solar masses) the remaining stellar core will have a mass more than 3 times that of the sun. even neutrons cannot support this amount of mass. The core is crushed down to a point of INFINITE DENSITY with a gravitational field so intense that even light cannot escape… A Black Hole

18. How can we detect them? Can’t see the black hole directly But can try to observe the effects of its gravity on its surroundings…

19. Binary star systems Many stars occur in binary pairs, orbiting each other. If one of the stars goes supernova, the collapsed core of the star will remain in orbit around its companion.

20. X-ray Binary Systems The collapsed stellar core is too small to be directly detected but we can infer its presence from its effect on the visible companion star. Gas is stripped from the companion star and heated as it spirals in towards the neutron star or black hole. This gas emits huge amounts of X-rays.

21. Anatomy of an X-ray binary system Gravity of compact object pulls matter off companion star Accretion disc: shines in X-rays Jets of material ejected at high speed, giving off radio waves

22. Measuring mass in X-ray Binaries Binary orbit around common centre of mass causes a wobble in the position of the visible star: If the mass of the compact companion is greater than 3 times the mass of the sun it CANNOT be a neutron star. The object must be a black hole. Speed of wobble gives mass of invisible compact companion.

23. Cygnus-X1: the best candidate for a stellar-mass black hole From the ‘wobble’ of the visible star we can weigh the mass of the companion to be ~10 solar masses. Astronomers are 95% certain that Cyg-X1 is a black hole. X-ray source associated with a binary star. 1 billion times more luminous in X-rays than the Sun.

24. 8 such black hole candidates are now known, with masses estimated at >3 solar masses The case for stellar-mass black holes looks good

25. The evidence for stellar mass black holes Intense X-ray emission from gas falling onto an extremely compact object (< 3km across) Wobble of companion star indicates a mass of over 3 times the mass of the Sun Physics suggests such an object can only be a black hole

26. Supermassive Black Holes

27. 1963: radio source 3C273 associated with a blue star-like object. Implied distance is 2 billion light years.  Optical luminosity 250 times brighter than the milky way. Quasi-stellar radio sources (Quasars) Many similar objects soon discovered, all with highly unusual properties. 3C273

28. Imaging quasars with Hubble  Quasars lie at the centres of distant galaxies

29. Quasar properties Luminous at all wavelengths Jets  compact, stable energy source Rapid variability  object is small

30. Powering quasars Extremely luminous Extremely small Only plausible energy source is an accretion disc around a black hole with millions of times the mass of the Sun.

31. The black hole’s accretion disc is only the size of the solar system, yet it emits more light than the 100 billion stars in the Milky Way.

32. X-rays from iron atoms Gas moving with velocities up to 100,000 km/s - exactly the speed we’d expect at the Event Horizon Gas moving with velocities up to 100,000 km/s - exactly the speed we’d expect at the Event Horizon Broad “emission tail”  evidence for gravitational redshift predicted by General Relativity close to a BH Broad “emission tail”  evidence for gravitational redshift predicted by General Relativity close to a BH High temperatures cause iron atoms to give off X-rays High temperatures cause iron atoms to give off X-rays High speeds close to the black hole change the frequency of these X-rays  “Doppler Shift” High speeds close to the black hole change the frequency of these X-rays  “Doppler Shift” X-Ray frequency 

33. More evidence from the Hubble Space Telescope Hubble finds signs of dormant black holes in most large galaxies, not just quasars Stellar velocities: very massive, very compact object in galaxy centre.

36. Is there a Supermassive Black Hole in the Milky Way? Radio image of the Galactic Centre Sag A*

37. Infrared images Reveal the central star cluster:

38. The La Silla Observatory Chile

39. The SHARP-1 Camera (Speckle-Interferometry) Special technique counteracts atmospheric blurring to give accurate positions for the stars in the Galactic centre.

40. High resolution infrared imaging of the galactic centre of the galactic centre 1994 19972000  can track the motions of individual stars

41. Stellar motions in the Galactic centre  mass of central object = 3 million suns

42. Chandra launch, July 23 1999 Measure X-ray emission from the Galactic centre

43. Our black hole takes a snack Before: After:

44. What does the black hole look like?

45. The Evidence for Supermassive Black Holes  Energy source for quasars  Quasar variability  Stability of radio jets  X-rays from iron atoms at the Event Horizon  Motion of gas in nearby galaxies  Stellar motions in centre of Milky Way  only plausible explanation is a black hole

46. So do black holes really exist? Extremely compact stellar-mass objects in X-ray binary systems Extremely massive compact objects in the centres of most galaxies We have found: Their properties are exactly what we’d expect if they are powered by black holes (BUT we still haven’t seen a black hole directly!)

47. Answer: yes (probably)

48. The End