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