Binary Black Hole Coalescence in Galaxy Mergers Steinn Sigurdsson Penn State 30 Oct ’02 CGWP Sources
Cosmological motivation • Hierarchical formation – Mergers Rates, mass ratios (cf Thorne & Braginsky ‘76) does each galaxy merge once, more or less? rate ~ 1 y − 1 if each galaxy merges once and a BH merger follows each galaxy merger. Does every galaxy have a black hole? MBH (Mgalaxy (z)) ? Do we observe BH, binary BH? Mass ratio, time since last merger, P(q=M2/M1) 30 Oct ’02 CGWP Sources
MBH whatn • Empirically: – MBH Mspheroid − 0.12% of mass – MBH 4 − for suitably defined velocity dispersion (G. et al ‘00, F&M et al ‘00, T. et al ‘00) – MBH “n” − Sersic index where I(R) R1/n (Graham etal ‘02) • No correlation with galaxy total mass. Disk mass decoupled. Projection of fundamental plane. Co − variant with true correlation? Contradictory? What is redshift evolution of correlation and underlying mechanism. 30 Oct ’02 CGWP Sources
Formation Hierarchy • Big galaxies formed from assembly of smaller units. Big galaxies are rare. • Smaller galaxies may have undergone 0 − 1 significant mergers (recently) cf Milky Way • Mergers correlate − if you merged you’re more likely to merge again. If you never merged you’re more likely to never merge in future. • Most likely merger has M2 ~ 0.1 M1 (cf Haenhelt et al, Madau et al ‘01 − ’02). • Mergers affect spin (ref ???) 30 Oct ’02 CGWP Sources
Source Analysis • Rates − do we see 0, 1 or many. LISA vs super − LISA/LISA − II/LISA − III • Waveforms, S/N − very high! • Extractable physics − BH properties and host properties. Counterparts??? • Fundamental physics − distinguish GR violations from classical perturbations mimicking signal • Complications − don’t mess up signal, add new signal in physics. Need to start thinking about 30 Oct ’02 CGWP Sources that.
What is the problem • BBR − “the last parsec” − – Dynamical ejection, loss − cone depletion – Loss − cone refilling? Gas? Other…? • Observational evidence for lack of binaries • Few “hung − up” close double nuclei − few % • Misaligned jets, core properties − cores due to binary BH interactions or initial conditions? (cf Quinlan ‘96, Q&H ‘97, M&M ‘01 − ’02, Hemsendorf et al ‘02, Merrit & Ekers ‘02) 30 Oct ’02 CGWP Sources
Lost Cones • Initial stage is dynamical friction hardening. Followed by 3 − body interaction. Large recoil of stars − depletion of stars on orbits that can interact. M ~ few M2 − ejection velocity ~ BH orbital velocity, carries off binding energy. Stars bound to either BH until late stages (cf Milky Way). • Is there eccentricity growth or not??? Aarseth ‘02 vs M&M ‘02 vs Hemsendorf et al ‘02 vs Quinlan ‘96 vs Ebizusaki et al • Kinematic signature. Radial orbits. Core depletion (observed? Gebhardt et al ‘02) 30 Oct ’02 CGWP Sources
Replenished cones • Diffusion replenishes loss − cone − slowly, relaxation times are long in most cases • At large radii, low J, pinhole regime, stars walk in. • Recycling: radial orbits return to core if bound to galaxy − any small non − monopole potential gradient deflects these orbits. • Black holes recoil − v ~ N* (m/M) vorb − small but finite. Move loss cone, keep going. − BH carries some stars with it, not clear this works well enough. 30 Oct ’02 CGWP Sources
…continued • Triaxiality − real galaxies are not smooth spheres or even axisymmetric. Post − merger remnants strongly non − axisymmetric. Large J on few dynamical time scales. • Triaxiality persists to small enough radii? Maybe − (cf Holley − Bockelmann et al ‘01, 02; Poons &Merritt ‘01). • Eccentricity growth: subtract J, not E. Sensitive to initial e? Sensitive to N! Kick in (1 − e2)7/2 to shrink time scale. Aarseth finds “final” e ~ 0.998 − very hard numerical problem. 30 Oct ’02 CGWP Sources
Interacting Stars − why is it hard • Dynamical friction vs individual super − elastic scatterings. Time scales are short, orbits extreme. • Star swallowing, tidal disruptions (during merger? − a problem or a solution?) • Global galaxy dynamics − need relation before and during merger, not current quiescent correlations. • Triple BH? If it hangs, and mergers correlate, then third BH will enter. SMBH ejections and naked BH in space? (cf Roos, Valtonen − Trentham?) 30 Oct ’02 CGWP Sources
SMBH as glorified planets • Gas dynamics − if SMBH carry disks, then there are BHi − diskj torques − Armitage & Natarajan ‘02 • Gas mass issues − need gas mass ~ M2 − is that plausible. Does a align with L before plunge? Well, to double AGN mass, need gas within r~ tS vS ~ 30 pc ?! Compare with hang − up radius Is gas stable − does it matter (cf Escara et al ‘02)? Partial disk + clumps + ambient gas Hard problem. 30 Oct ’02 CGWP Sources
A unified paradigm ; − ) • Mergers − BH hang − gas flows in from 100 − 1000 pc scales and accumulates. Triggers gas driven mergers when gas mass ~ BH mass. always have post − merger AGN phase with accretion leading to sub − doubling of mass, then BH merger, consistent with QSO lifetime estimates − solves lifetime conspiracy? • Implies substantial gas accretion just before merger, therefore a ~ 1? • Merger triggers short − lived outflows? GPS? 30 Oct ’02 CGWP Sources
The new spin • Spin signatures of current and past mergers − what was “a” just before merger − old gas accretion vs recent gas accretion vs previous merger − a1 not same as a2 − primary has different merger/accretion history! • Gas before during or after − late stages gas doesn’t affect BH but BH affect gas! Modulated accretion signal just before merger. Observable (retroactively?) • Test hierarchical models, “feeding the monster” theories, RQ vs RL QSOs, physics of spin • Physics of hardening − eccentricity growth, recoil, global galaxy properties 30 Oct ’02 CGWP Sources
What to look for • Potentially very high S/N, but low rate • E&M signatures − outflow, modulated accretion • Spin signatures − pre as well as post merger • Eccentricity − dynamical e final ever high enough? • Bound and unbound star perturbations − low amplitude, but high S/N. Can we see stars bound to individual BH during in − spiral. Merger rate during spiral − in enhanced enough? See in quasi − periodic phase rather than plunge? 30 Oct ’02 CGWP Sources
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