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

Active Galactic Nuclei More than 10% of the galaxies have an 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

Fornax A in optical radiation

Fornax A in radio continuum

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

The AGN unified model

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

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

The cosmic quasar density Large surveys such as SDSS have been instrumental to determine the cosmic quasar density SDSS DR7 quasar catalogue contains QSOs with reliable redshifts. Potential problems: Malmquist bias K-corrections comoving densities There were times more luminous quasars at z ≈ 2.5 than today…

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

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: r h = 10 pc At the distance of 15 Mpc: r h = 0.15 arcsec Resolution of HST is necessary to detect SMBHs in nearby galaxies

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).

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

Stellar kinematics: Cen A Stellar kinematics of Cen A can be reproduced best by a model with an SMBH with M BH ≈ 2 x 10 8 M sun

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

Ionized gas kinematics : M84

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.