astrophysics from space lecture 6: supermassive black holes

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Astrophysics from Space Lecture 6: Supermassive black holes. Prof. Dr. M. Baes (UGent) Prof. Dr. C. Waelkens (KUL) Academic year 2013-2014. Active Galactic Nuclei. More than 10% of the galaxies have abnormal nucleus extremely bright - PowerPoint PPT Presentation


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Astrophysics from SpaceLecture 6: Supermassive black holes

Prof. Dr. M. Baes (UGent)Prof. Dr. C. Waelkens (KUL)

Academic year 2015-2016

Active Galactic NucleiMore than 10% of the galaxies have an abnormal nucleusextremely brightnon-stellar spectrum (broad emission lines instead of absorption lines)strong sources in radio, UV, X-ray, far-infraredstrong variability on timescales of months or even days

Fornax A in optical radiation2

Fornax A in radio continuum3Enormous luminosities: can be brighter than an entire galaxyNon-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 AGNsEngine must be compactnot resolved, not even with HSTvariability on the time-scale of months/weeks/days

The AGN unified modelQuasars / QSOsExtremely bright AGN that outshine their entire host galaxy

From the ground: look like starsquasi-stellar radio sources (quasars)quasi-stellar objects (QSOs)

HST (and ground-based AO observations): can resolve the host galaxies

The cosmic quasar densityQuasars are ideal cosmological probesextremely brightstrong emission lines

The cosmic quasar densityLarge surveys such as SDSS have been instrumental to determine the cosmic quasar density

SDSS DR7 quasar cataloguecontains 77429 QSOswith reliable redshifts.

Potential problems:Malmquist biasK-correctionscomoving 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 holesDetecting sleeping SMBHsSleeping 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 galaxiesStellar kinematicsThe 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 kinematicsAdvantages stars are always presentonly gravity matters

Disadvantagesabsorption features are weakwe have to make 3D models from 2D informationmany unknowns: M/L, inclination, anisotropycomputation-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 MsunIonized gas kinematicsIonized gas is often seen to reside in a Keplerian disc in the nucleus of nearby galaxies.

Advantagescan be studied by emission lines (easier than absorption lines)modelling easier (disc)

Disadvantagesnot for all galaxiesnon-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 wavesThe 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

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