Binary Black Hole Coalescence in Galaxy Mergers Steinn Sigurdsson - - PowerPoint PPT Presentation

30 Oct ’02 CGWP Sources

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.

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 ???)

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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 that.

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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)

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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.

• 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)

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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.

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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?)

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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

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.

• Implies substantial gas accretion just before merger, therefore a ~ 1?
• Merger triggers short−lived outflows? GPS?

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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

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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?