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Formation of Supermassive Black Holes

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Formation of Supermassive Black Holes Formation of Supermassive Black Holes 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: 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 Hasinger et al. 2003 should explain both phenomena 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
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