Astrophysics from Space Lecture 6: Supermassive black holes

1. Astrophysics from Space Lecture 6: Supermassive black holes Prof. Dr. M. Baes (UGent) Prof. Dr. C. Waelkens (KUL) Academic year 2013-2014

2. Active Galactic Nuclei • More than 10% of the galaxies have abnormal nucleus • extremely bright • non-stellar spectrum (broad emission lines instead of absorption lines) • strong sources in radio,UV, X-ray, far-infrared… • strong variability on timescales of months or even days

3. Fornax A in optical radiation

4. Fornax A in radio continuum

5. The engine of AGNs Enormous luminosities: can be brighter than an entire galaxy Non-thermal spectrum: no stars • Engine must be compact • not resolved, not even with HST • variability on the time-scale of months/weeks/days Accretion of matter on a supermassive black hole is the only plausible explanation for the existence of AGN.

6. The AGN unified model

7. Quasars / QSOs • Extremely bright AGN that outshine their entire host galaxy • From the ground: look like stars • quasi-stellar radio sources (quasars) • quasi-stellar objects (QSOs) • HST (and ground-based AO observations): can resolve the host galaxies

8. The cosmic quasar density • Quasars are ideal cosmological probes • extremely bright • strong emission lines

9. The cosmic quasar density • Large surveys such as SDSS have been instrumental to determine the cosmic quasar density • SDSS DR7 quasar cataloguecontains 77429 QSOswith reliable redshifts. • Potential problems: • Malmquist bias • K-corrections • comovingdensities There were 100-1000 times more luminous quasars at z ≈ 2.5 than today…

10. Where have all the quasars gone ? • Two options: • The SMBHs have disappeared (Hawking radiation?) • Accretion has stopped (lack of fuel, conservation of angular momentum) Hawking radiation is extremely inefficient.The Local Universe must be full of sleeping supermassive black holes

11. Detecting sleeping SMBHs Sleeping SMBHs can only be detected by studying the dynamics of tracer populations. Sphere of influence: radius where the potential of the SMBH dominates the potential of the stars and gas. For a typical galaxy: rh = 10 pc At the distance of 15 Mpc: rh = 0.15 arcsec Resolution of HST is necessary to detect SMBHs in nearby galaxies

12. Stellar kinematics The shift and broadening of the stellar absorption lines reveal the kinematics of stars (Doppler effect). These kinematics can be modeled using the equations of stellar dynamics to determine the gravitational potential (and hence the mass distribution).

13. Stellar kinematics • Advantages • stars are always present • only gravity matters • Disadvantages • absorption features are weak • we have to make 3D models from 2D information • many unknowns: M/L,inclination, anisotropy • computation-intensive

14. Stellar kinematics: Cen A Stellar kinematics of Cen A can be reproduced best by a model with an SMBH with MBH ≈ 2 x 108Msun

15. Ionized gas kinematics • Ionized gas is often seen to reside in a Keplerian disc in the nucleus of nearby galaxies. • Advantages • can be studied by emission lines(easier than absorption lines) • modeling easier (disc) • Disadvantages • not for all galaxies • non-gravitational forces

16. Ionized gas kinematics : M84

17. Black hole demography Supermassive black holes have been detected in (nearly) all nearby galaxies. This can explain the scarcity of QSOs in the Local Universe. It implies that all galaxies must have gone through an AGN phase – also our own Milky Way. Relations between SMBHs and host galaxies imply that SMBHs play a key role in galaxy evolution.