1. Cosmology and extragalactic astronomy Mat Page Mullard Space Science Lab, UCL 7. Quasars

2. This lecture: Quasars, Seyferts and radio galaxies: –emission line spectra –radio emission –similarities and differences –What are they? Slide 2

3. First signs of quasars Very early radio astronomy, (1940s) –strong radio source in Cygnus 1951: odd looking, dim galaxy at position of Cygnus A (Palomar Observatory) –strong emission lines Slide 3

4. Emission line spectra Active galaxy Normal galaxy Slide 4

5. Quasars are seen at high redshift In 1963 3C273 identified as a 13th magnitude ‘star’ through lunar occultation by Cyril Hazard Maarten Schmidt from Caltech took a spectrum of 3C273 and recognised Balmer series –but redshifted to z=0.158 –Doppler v=44000 km/s –too fast to be in Milky Way - redshift must be cosmological Slide 5

6. Seyfert galaxies Carl Seyfert in 1943 noticed that some galaxies have unusually bright nuclei and emission line spectra. This type of galaxy is called a Seyfert galaxy Seyfert’s original sample included NGC4151, NGC1068, NGC4051. Seyfert galaxies are like quasars but not as luminous Slide 6

7. 3C273 spectrum Slide 7

8. 3C273 was the most distant object ever seen, hundreds of Mpc away –and it is 13th magnitude! –Must be a very luminous source of both radio and visible light. Cygnus A at 230 Mpc, radio luminosity ~ 10 7 times the Milky Way. Objects like 3C273 which look like stars, but emit in the radio were termed “quasi- stellar radio sources” –later shortened to quasars Quasars are powerful! Slide 8

9. Since quasars are seen to vary on a timescale of days, the radiation must be coming from a region no larger than a few light days. Only thing that can make hundreds of galaxies worth of emission from a region a bit larger than the solar system is a massive accreting black hole –(famous paper by Cambridge astronomer Donald Lynden Bell in 1963) What is the power source? Slide 9

10. 90% are radio quiet, but can be identified by their strong emission lines. Most quasars are seen at large redshifts (z>0.3) Current record is z=7.1 Characteristics of quasars Slide 10

11. What can we tell about quasars? Emission lines - excited hydrogen Lots of UV radiation - material of 100,000 K –(c.f. stars 2,500-50,000K) Bright X-ray sources Broad Balmer lines - 10,000 km/s or more 1980s: “Fuzz” around nearby quasars has absorption lines - quasars are at the centres of galaxies. Quasars live mostly in massive elliptical galaxies. Slide 11

12. Radio galaxies Almost always elliptical galaxies. Not necessarily any emission lines Jets emit polarised, non- thermal radiation - synchrotron radiation. Slide 12

13. …Radio galaxies Usually have two radio lobes, one on each side of the elliptical galaxy Some radio galaxies have a quasar visible in the middle. Slide 13

14. Blazars Prototype is BL Lacertae –highly variable star classified in 1929 –factor 15 variability in a few months Bright core shows no emission or absorption lines –synchrotron radiation High-res images from the Very Large Array show diffuse radio emission around a bright core We are looking down the jet of a radio galaxy Slide 14

15. How could jets be made? Slide 15

16. AGN classification Very luminous (M B <-23) - Quasar Less luminous - Seyfert If it has radio jets - radio loud If not - radio quiet If it has broad lines - Seyfert 1 If it has only narrow lines - Seyfert 2 Slide 16

17. Unified model of active galaxies (this one radio loud) Slide 17 Warning: this is a rather fanciful picture!

18. AGN structure Narrow lines Broad lines Slide 18

19. Some key points about quasars: Very luminous objects. Powered by massive black holes. Can be seen to very large distances. Categorised according to –luminosity –radio properties –emission line properties (orientation) Slide 19