Astrophysics from Space
Lecture 6: Supermassive black holes
Prof. Dr. M. Baes (UGent)
Prof. Dr. C. Waelkens (KUL)
Academic year 2015-2016
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 cataloguecontains 77429 QSOswith reliable redshifts.
Potential problems:• Malmquist bias• K-corrections• comoving densities
There were 100-1000 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: rh = 10 pc At the distance of 15 Mpc: rh = 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 MBH ≈ 2 x 108 Msun
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)
• modelling 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.
SMBH mergers and gravitational waves
The merging of SMBHs are among the most (gravitationally) violent events in the Universe.
Result: gravitational waves. Finally detected in Sep 2015, discovery announced on 11 Feb 2016
http://www.phdcomics.com/comics.php?f=1853
