Formation of Supermassive Black Holes

Home , Quasar

by Stefan Taubenberger

1 Galaxies with BHs in their center

2 Centaurus A

3 Centaurus A

4 M87 / Virgo A

5 M87 / Virgo A

6 Index

1) Observational constraints Quasars found at z~6 Quasar number density σ MBH - relation Recent observation of Black Hole binaries Quiescent SMBHs

2) 3 basic scenarios of SMBH formation Collapse of a gas cloud or a supermassive star to a SMBH in the early universe Runaway growth by accretion Mergers of two Black Holes

3) One possible model of SMBH formation

Observation Scenarios possible model 7 Quasars at z~6 z = 5,8

Sloan Digital Sky Survey (SDSS) 2002: quasar at z = 5,8

Quasars Ù massive accreting BHs z = 4,75 First massive BHs must have existed 1 Gyr after the Big Bang

⇨ very rapid formation process required

Observation: z~6 quasars Scenarios possible model 8 Quasar number density

Quasar density peaks at z ~ 2 (~ 3 billion years after Big Bang) since then decrease in quasar density:

Seyfert galaxies (active galaxies, but less luminous than quasars) show different behaviour: maximum shifted to lower redshifts (z ~ 1) maximum is higher ⇨ Seyfert galaxies more common than quasars What is the physical difference? SMBH formation model should explain both phenomena

Observation: number dens. Scenarios possible model 9 Quasar number density

Quasar density peaks at z ~ 2 (~ 3 billion years after Big Bang) since then decrease in quasar density:

Observation: number dens. Scenarios possible model 10 σ MBH – relation

First idea: correlation between BH mass and bulge luminosity of the host galaxy

correlation not very tight

Gebhardt, Bender et al.

σ Observation: MBH – Scenarios possible model 11 σ MBH – relation

Much better: correlation betw. BH mass and stellar velocity dispersion σ in the galaxy

Ferrarese & Merrit:

Gebhardt, Bender et al.

σ Observation: MBH – Scenarios possible model 12 σ MBH – relation

Much better: correlation betw. BH mass and stellar velocity dispersion σ in the galaxy

Ferrarese & Merrit:

Note: observed stars are not in BH’s sphere of influence, i.e. the region where gravity is dominated by the BH

Gebhardt, Bender et al.

σ Observation: MBH – Scenarios possible model 13 Black Hole binaries

recent discovery of BH binary systems (Komossa et al.) located in merging galaxies

NGC 6240

Observation: binaries Scenarios possible model 14 Black Hole binaries Combination of Ground based HST & Chandra Optical Image Images

HST Chandra Image X-ray Image

Observation: binaries Scenarios possible model 15 Black Hole binaries

BH binary system Chandra X-ray Image

Observation: binaries Scenarios possible model 16 Quiescent Black Holes

not all SMBHs are luminous objects like quasars most of them live in their galaxy emitting only little radiation

⇨ it has taken a long time to see that each galaxy hosts a supermassive BH

formation model must explain why BHs don’t accrete larger amounts of matter all the time

most famous example of a non-accreting quiescent BH: Sgr A*, the center of our Milky Way

Galactic center

Observation: quiescent Scenarios possible model 17 Sgr A* (Galactic center)

Observation: quiescent Scenarios possible model 18 Scenario: supermassive star

Idea: SMBH forms by collapse of a supermassive star (SMS) with

Simulations show: star would not explode but collapse completely to a BH Problem: star formation simulations always predict masses ⇨ SMS should not form!

More realistic in the early universe (zero metallicity): formation of massive stars with (Bromm et al. 2002)

Observation Scenarios: supermassive star possible model 19 Scenario: supermassive star

Evolution of massive stars:

formation of stars with not observed nowadays

reason: collapsing gas clouds undergo fragmentation process (since cooling becomes more effective then)

massive star formation only possible in the early universe, when the metallicity is zero

H2 cooling is the only cooling mechanism available ⇨ impact on fragmentation ⇨ formation of massive stars

Observation Scenarios: supermassive star possible model 20 Scenario: supermassive star

short lifetime: ~

final evolution depends sensitively on the initial star mass

possible scenarios:

complete collapse to a massive BH with

complete explosion (metal release, no remnant)

Observation Scenarios: supermassive star possible model 21 Scenario: supermassive star

Heger,Abel et al. 2002

Observation Scenarios: supermassive star possible model 22 Scenario: supermassive star

Process does not explain the formation of BH with

Observation Scenarios: supermassive star possible model 23 Scenario: growth by accretion

Idea: BH grows by accretion of gas, dust & compact objects (stars)

σ σ 5 could possibly explain the MBH- relation (MBH ∝ ) Maximal accretion rate determined by Eddington limit (radiation pressure = gravitational attraction):

exponential growth:

works as long as there’s enough fuel to feed the BH

Observation Scenarios: accretion possible model 24 Accretion in an AGN

Observation Scenarios: accretion possible model 25 Scenario: growth by accretion

Problem: Most SMBHs almost don’t accrete at all (→ Sgr A*) Reason: Stars/particles have stable orbits with certain angular momentum matter density too low to get rid of angular momentum by friction good model: our solar system (negative feedback to perturbations)

What triggers accretion of a SMBH?

Observation Scenarios: accretion possible model 26 Scenario: BH mergers

Black Holes merge when galaxies collide !

Observation Scenarios: BH merger possible model 27 Scenario: BH mergers

“Major merger”: colliding galaxies have about the same size “Minor merger”: one of the colliding galaxies is much larger First take a look at major mergers: “Antenna galaxies”

Observation Scenarios: BH merger possible model 28 Scenario: BH mergers

Observation Scenarios: BH merger possible model 29 Merging of Antenna galaxies

2 spiral galaxies …which will go on collide growing by mergers

Antenna galaxies …forming an as we observe elliptical galaxy… them today The spiral structure is lost completely…

Observation Scenarios: BH merger possible model 30 Galaxy merger

Observation Scenarios: BH merger possible model 31 Galaxy merger

Observation Scenarios: BH merger possible model 32 Galaxy merger

Observation Scenarios: BH merger possible model 33 Scenario: BH mergers

BHs fall quickly towards the center of the new mass distribution (driven by gravity)

mechanism: energy loss through dynamical friction

high cross-section for BHs: each BH has a surrounding “swarm” of gravitationally bound stars ⇨ effective interaction with interstellar gas

When the black holes are close enough (≪ 1 ly): rapid coalescence through emission of gravitational waves (→ LISA)

Spurzem, Deiters 2003

Observation Scenarios: BH merger possible model 34 Scenario: BH mergers

Now consider minor mergers:

large galaxy only slightly disturbed

spiral galaxies could turn into bar galaxies

less spectacular event than major merger

Problem in both cases: sheer addition of the BH masses due to coalescence is much to inefficient to explain the formation of SMBHs alone !

Observation Scenarios: BH merger possible model 35 One possible model

Situation comparable to a puzzle:

we have the single pieces (collapse, merger, accretion)

now we must put them together in the right way

But nobody knows what the result will look like (there are as many ideas as physicists working on this topic)

What we look at here is just one possible idea!

Observation Scenarios possible model 36 One possible model

First step: collapse of a very massive star

formation of massive stars ( ) in the early universe

after 105 years a 100 solar mass star collapses completely…

…forming a seed BH of about

Observation Scenarios possible model 37 One possible model

Second step: further growth by accretion triggered by galaxy mergers

merger tree:

major mergers dominant in the early universe since the young galaxies have all got similar masses

Observation Scenarios possible model 38 One possible model

consequence of a major merger: seed BHs cross large amounts of interstellar gas…

…and accrete at the Eddington limit for several e-folding times (→ observed as quasar)

Observation Scenarios possible model 39 One possible model

Archibald et al. 2002

Mass Flux into Center

Eddington Limit

take for instance

One major merger is sufficient to form a SMBH !!!

Observation Scenarios possible model 40 One possible model

redshift dependence of major mergers: first increase in number density as galaxies just begin to evolve and to merge after some time the number density should decrease because

∝ (1 + z)5-6 space expands (n(z) ∝ (1 + z)6) galaxies become more and more unequal in their size

simulations yield a maximum at z ~ 2-3 Ù corresponds to the maximum in quasar activity!

Observation Scenarios possible model 41 One possible model

minor mergers: less numerous in the early universe become more important at later stages when galaxy masses differ more strongly peak at lower redshift Seyfert galaxies are probably correlated to minor mergers maximum at lower redshift than quasars less violent accretion ⇨ lower luminosity

Hasinger et al. 2003

Observation Scenarios possible model 42 One possible model

Third step: quiescent stage Between 2 galaxy mergers the central BH is quiescent and accretes only a minimal amount of matter (as discussed in the accretion scenario)

THE END

Observation Scenarios possible model 43 References

E.Archibald et al.: “Coupled spheroid and black-hole formation, and the multifrequency detectability of active galactic nuclei and submillimetre sources”, astro-ph 0108122 v2 (2002)

V.Bromm, A.Loeb: “Formation of the first Supermassive Black Holes”, astro-ph 0212400 v1 (2002)

L.Ferrarese, D.Merritt: “Supermassive Black Holes”, astro-ph 0206222 (2002)

L.Ferrarese, D.Merritt: “The M • -σ Relation for Supermassive Black Holes”, ApJ, 547, 140-145 (2001)

K.Gebhardt, R.Bender et al.: “A Relationship between Nuclear Black Hole Mass and Galaxy Velocity Dispersion”, ApJ, 539, L13-L16 (2000)

G.Hasinger et al.: “Formation and Evolution of Supermassive Black Holes in Galactic Centers: Observational Constraints”, in “The Emergence of Cosmic Structure: Thirteenth Astrophysics Conference ”, ed. by S.Holt and C.Reynolds

A.Heger, S.Woosley, I.Baraffe, T.Abel: “Evolution and Explosion of Very Massive Primordial Stars” in proc. MPA/ESO/MPE/USM Joint Astronomy Conference “Lighthouses of the Universe”, ed. by M.Gilfanov, R.Sunyaev, E.Churazov (Springer 2002)

M.J.Rees: “Formation and Growth of Supermassive Black Holes” in proc. MPA/ESO/MPE/USM Joint Astronomy Conference “Lighthouses of the Universe”, ed. by M.Gilfanov, R.Sunyaev, E.Churazov (Springer 2002)

R.Spurzem, S.Deiters: “Tanz der Schwarzen Löcher”, Sterne und Weltraum 2/2003

M.Volonteri, F.Haardt, P.Madau: “The Assembly and Merging History of Supermassive Black Holes in Hierarchical Models of Galaxy Formation”, ApJ, 582, 559-573 (2003)

MPG Presse Information PRI SP 15 / 2002 (118): “Schwarze Löcher im Doppelpack”, Team: S.Komossa, G.Hasinger et al.