Celestial Coordinates All extended sources

1. Galactic Coordinates Point source catalog 2MASS Data, near-IR Celestial Coordinates All extended sources

4. Image of Galactic Center region = 7.7 arcsec

5. Radio image, central 3 ly • Center is the red ellipse at the center • Called Sgr A*

6. X-ray image, central 3 ly Sgr A* is the bright object in the center of the image.

7. CXOGC J174539.7-290020 The Central Parsec 0.25 pc Sgr A* CXOGC J174539.4-290031 (AFNW) IRS13

9. The unseen object at the center of our galaxy is probably a black hole with mass of about 3 million solar masses

10. What are black holes? • Objects so massive & dense that their gravity basically cuts them off from the universe as a whole • Very small -- • Escape velocity exceeds the speed of light -- so no radiation or information can get out. • Seen in both stellar mass variety as well as supermassive variety. • Often the prime mover for various high-energy, active phenomena in the regions surrounding the BH.

11. Short-period flares in galactic center -- mass accreting onto the black hole • Flares seen in IR, X-ray typically ~once per day • In 2003, IR flare lasting 16 minutes at position of Sgr A* -- the central black hole. • Short period requires Doppler boosting of hot gas in last stable orbit

12. X-ray flaring activity – timescales as short as ~30 minutes!

14. Can we be confident this is a black hole? • Mass density is so high no other model works!

15. What lurks at the center of other galaxies? • Are the centers of most galaxies just stars and gas, or is there more? • The main way we can look at this is by taking a long-slit spectrum of the central regions, as pictured at right. • In a galaxy (or bulge) with velocity dispersion , the black hole dominates the motions inside a radius rBH, given by: • Look for clear divergence inside rBH

16. Most or all bright galaxies have black holes

17. AGN: What are they? • Basic Picture of Active Galactic Nuclei: • Some of the black holes that lie at the centers of bright galaxies are actively accreting material from surroundings • Gas accreting onto the black hole will release a large amount of gravitational PE: • Efficiency Accretion rate • Both the efficiency and accretion rate vary by orders of magnitude across different object types. • Accretion onto a black hole can produce a host of observational phenomena, e.g.: • Very high luminosity from a small region • Small physical size allows rapid variability • Broad Spectral lines due to Doppler shift of gas orbiting the black hole • X-ray emission from high-temperature plasma cluse to the black hole • Mechanical power in the form of outflows and jets from the central regions

18. The discovery of the AGN phenomenon NGC 5548 Optical Spectrum UV Spectrum • 1907: E. A. Fath obtains a spectrum of NGC 1068 at Lick Observatory. He notes the presence of strong emission lines in its spectrum (later confirmed by Slipher at Lowell Observatory).

19. The discovery of the AGN phenomenon 1918: Curtis Heber noticed a ‘curious straight ray’ emanating from the nucleus of the galaxy M87. By the 1950s, it was identified with one of the brightest radio sources in the entire sky, called Virgo A.

20. AGN: Zooming In (Artists Impression) SuperMassive Black Hole (106 - 1010 M) Accretion of Matter PE (+Mag Energy) KE (thermal)  Luminosity  Outflow KE (+Angular Momentm) Relativistic Jets UV, X-& -rays + launch of outflow Stars, gas & dust (& starbursts)

21. AGN: The big picture The Unified Model • Supermassive black hole (106-1010M) M= 108MRG~ 3x1013 cm. • Accretion disk; thermal UV/X & lines from highly ionized atoms (3-100 RG). • High velocity (>103 km/s) broad-line clouds (R~103-4RG). • Dusty torus, which orbits in/near plane of accretion disk (R~104-5 RG). • Lower velocity (few hundred km/s) narrow-line clouds (R~105-7 RG). • Relativistic jet ( ~ 5-30), which may be collimated on ~50 RG scales. Observed properties vary with viewing angle (Urry & Padovani 1995)

22. Reverberation mapping • Use simultaneous monitoring of AGN in X-rays, UV, optical continuum and various UV/optical emission lines to measure structure • The idea: UV/X-ray emission from the accretion disk is what drives variability in the broad-line emission • Use delays between UV/X continuum response & BL continuum response to • infer BLR radial structure • Can also use it to obtain structure of accretion disk further out (from optical continuum delays)

24. Another way that’s specific to AGN • Reverberation mapping • Taking advantage of the dynamical link between accretion disk and line variability to measure black hole mass

25. Radio-Loud AGN & Relativistic Jets • Radio-loud AGN are characterized by relativistic jets, which: • Emerge relativistically from center of galaxy (bulk ~5-30) • Transport matter, energy well beyond bounds of host galaxy (to lobes) • Emit synchrotron & inverse-Compton radiation. • Are natural particle accelerators: e--have  up to 106-7. Owen et al. 1999 B

26. The M87 Jet (Marshall et al. 2002)

28. Madrid 2009

29. Velocity Structure of HST-1 • Four moving components, motions tracked for over three years • One knot – C – split in 2005, coinciding with main flare. Also location of flux peak Birth of a new component in jet during flare.

30. Superluminal motion in quasar jets: an optical illusion Positions of knot when two pictures were taken, one year apart. Speed of knot (close to the speed of light) Light paths: B Small angle: the knot’s motion is mostly along the line of sight. A Not drawn to scale! 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 for the knot to appear to move that fast.)

31. Polarization & Spectral Behavior • Strong correlation of polarization with flux magnetic field involved in particle accel • Complicated relationship between flux and spectral index Epochs 4-9 – “hard lagging” Epochs 13-17 – “soft lagging” • The latter is more common; implies shorter acceleration timescales than cooling timescales. • The latter normally requires the opposite relationship … but X-ray is also synchrotron • Possibility: most energy losses are actually in inverse-Comptonizing external photons near Klein-Nishina limit. Perlman et al. 2011

32. Shocks Compression ratio k

33. Masers between 0.16-0.3 pc from BH

34. Conclusions • The center of our galaxy is a dense, exciting environment • Dense stellar field • Different in each band, many different features • At its heart - a super-massive black hole, mass ~ 4 million suns. • The black hole can’t be seen - but the results of accretion onto it can be seen in the form of infrared and X-ray variations. • Observations of other nearby galaxies reveal that central super-massive black holes are common in bright galaxies. • Most are not actively accreting (like our own) … just taking in whatever matter is there and so have a low luminosity • But when a collision between two galaxies occurs, material is forced into the center … active accretion and an AGN! • Many associated phenomena, including jets, accretion disk, line emission regions, dust regions.