Sub-Parsec Black Hole Binaries

http://www.noao.edu/outreach/press/pr09/pr0901.html

Advanced Seminar July 7th 2009

Lydia Moser

An Overview

• Introduction/Motivation• Theory behind Black Hole

Binaries:–Structure formation in the

Universe–Evolution of Black Hole Binaries

(BHB)• Observation of BHB candidates• Summary & Outlook

Introduction

• Observed growth of galaxies suggests:– Black holes (BH) at galactic centers find each

other and merge– Prediction of theory of structure formation in

the Universe• Galaxies grow by mergers• Gravitation and dynamical friction drag BHs to

bottom of potential well → coalescence?• dynamics at binary separations smaller than one

pc not fully understood → research• Binaries as test for general relativity (GR)

Hierarchical structure formation

• Short after the Big Bang:→ Universe inhomogeneous due to quantum

fluctuations → density fluctuation on larger scales as the Universe

inflates• Density fluctuation → self gravitation: Cold Dark

Matter (CMD) collapses and clumps to filamental structures and leaves voids

• Baryons follow these potential wells after decoupling from photons

• Galaxy formation in or in vicinity of potential wells• Small structures form first and merge to larger

ones (bottom-up model)

dark matter structures

http://www.guzzlingcakes.com/images/dark_matter.jpg

Conclusions for galaxy formation

• galaxies surrounded by spherical dark matter halos

• “cosmic overdensities”: supermassive black holes (SBH) likely to reside in centers of galaxy clusters & galactic nuclei (supported by simulations)

• SBH mass correlates with host galaxy´s bulge mass → coevolution of SBH and galaxy

• structures and evolution in the Universe (distribution of galaxies, mass distribution in galaxies) given by the structure of CDM

• galaxies grow by agglomeration of smaller oneshttp://www.youtube.com/wa http://www.youtube.com/watch?v=lZEoa5hoSZM&hl=de tch?v=lGwEQEnkbJc&hl=de

http://www.youtube.com/watch?v=lGwEQEnkbJc&hl=de

Evolution of Black Hole Binaries

http://www.youtube.com/watch?v=lGwEQEnkbJc&hl=de

Mergers & Black Hole Binaries

• Two galaxies capture each other by their gravitation and interact

• Violent relaxation: gas and stars lose orbits and are mixed up due to gravitational torques and dynamical friction → energy distributed randomly, chaotic orbits → new galaxy is elliptical

• SBHs sink towards center of the new galaxy via dynamical friction and gravitational torques

• capture each other by their potential wells

• Binary hardens, i.e. its separation decreases due to loss of orbital energy an angular momentum

• Mechanisms: gravitational slingshot, gas dynamical friction and torques

• At certain separation agw: emission of gravity waves (GW)

→ coalescence

Gravitational slingshot (gas-poor galaxies)

• SBH capture stars passing their immediate vicinity

→ eject them at much higher velocities at cost of their binding energy

• Finite supply of slingshot stars? Stalling decay?

→ once ejected stars may return to central region

→ new stars diffuse into interaction volume

http://cr4.globalspec.com/blogentry/5867/Relativistic-Slingshot-Gravity-Assist

Circum-binary disk model (gas-rich galaxies)

• circum-binary disk of gas forms (interstelIar medium (ISM) & tidally disrupted accretion disk gas)

• Inflow from circum-binary disk onto accretion disks (especially secondary for SBH with different masses)

• Gravitational and viscous torques – loss of angular momentum & sustained inflow

http://www.sochias.cl/reunion/jorgecuadra.pdf

Gravitational Wave Emission

Fundamental prediction of GR:

• At agw emission of GW efficient enough to take away last orbital energy and angular momentum

• Massive objects induce space-time curvatures

• two orbiting massive objects induce ripples in space-time

• GW propagate at speed of light

http://hera.ph1.uni-koeln.de/~heintzma/NS1/gr-strahlung.htm

Final parsec problem• final parsec problem:

– Decrease of supply of star or gas– decay mechanisms become

inefficient– effect of GW emission to weak – decay in a time shorter than the

age of the Universe?→ decay stalls

→ exploration of sub-pc binaries

• To coalesce or not to coalesce

Coalescence and Recoil• Emission of gravitational waves at

coalescence not symmetric in general→ remnant can receive significant recoil

• Spins parallel or anti-parallel to orbital angular momentum or low spin → in-plane kick less than 500 km/s

• Spins oppositely directed in the orbital plane → out of plane kicks up to 4000km/s (for equal mass SBH, max. spin)

http://astro1.phys.uniroma1.it/ericeschool/lectures/campanelli2.pdf

Observations of Binaries

Why are binaries important to investigate?

• Solution of final pc-problem (sub-pc domain)

• Test and deeper understanding of large-scale structure-formation in the Universe (if theory right, BH mergers should be common)

• Test of general relativity:– Dynamics in extreme gravitational fields– Gravitational wave emission

How to find BHB?• Problem: sub-pc black hole binaries difficult

to find:– Limitation of today´s observatories

→ higher resolution needed– Information from spectra only

→ indirect → difficult to interpret

• Quasars best for finding BHB– Supermassive black hole actively

accreting gas– Galaxy merger remnant

→ second SBH possible & large gas supply → Gas accretion might play keyrole for last stages of orbital decay (final pc)

Quasars• Most luminous version of class active

galactic nuclei (AGN), up to 100 times brighter than Milky Way– accretion onto SBH

• AGN spectra: broad and narrow emission lines – originate from distinct regions of

orbiting gas

• Broad line region (BLR): ~ 0.01 pc from BH– Line widths: Keplerian orbital speeds

• Narrow line region (NLR): ~ 10 pc from BH

Sub-pc BHB Candidates

• spectra of 17500 quasars from Sloan Digital Sky Survey: – only two show signs of a BHB

• further quasar: periodic flux variation

→ binary orbital motion

Quasar 1/3: J0927Bogdanovic et al. 2009

• Model corresponds to the circum-binary disk model

• accreting secondary SBH main source of ionizing radiation

• primary much fainter or quiescent

• Velocity shift represents projected orbital velocity of 2nd SBH relative to center of mass

• Broad emission lines: BLR of 2nd SBH

• Narrow emission lines: NLR in circum-binary disk

• Less broad lines/broader narrow lines: inflow streams from NLR

Counter-proposal: recoiling merger remnant (Komossa et al. 2008) moving away at velocity of shift allmost all bulges contain SBH

SDSS J092712.65+294344.0: Recoiling black hole or a sub-parsec binary candidate?, Tamara Bogdanovic et al., 2009

Quasar 2/3: J1536

Boronson & Lauer 2009

• model: two BLR around separated, gravitationally bound SBH in single host galaxy

• NLR associated with host galaxy

• Mass estimate (highly uncertain): Mr =108.9Msun, Mb =107.3Msun

• T<500yr, Rorbit ≈RBLR<<RNLR

• Counter-proposal: superposition of two quasars (possilility: 0,3%)

Quasar 3/3: OJ287Valtonen et al. 2008

• Quasi-periodic pattern: Outbursts in pairs separated by 1-2yrs, occur 12yrs apart

• Model: SBHB – secondary pierces primary´s accretion disk → two impact-flashes per period

• Model fitted to data of former outburst sequences → prediction for next outburst turned out to be true

• Test of general relativity:– Gravitational wave emission

(most probable for orbital decay)

– Without: deviations of weeks

http://www.astro.utu.fi/news/img/oj287.jpg

Conclusion From Observation

• Last two quasars definitely in sub-pc domain

• Circum-binary disk-model favorized• Model seem quite robust but have to

be confirmed against counter-proposals and improved

• OJ287 supports theory of structure formation best (inevitable coalescence)

• More observation necessary

Summary

• structure formation: Galaxies grow by mergers, their central SBHs coalesce

• Different mechanisms for orbital decay of BHB:– interaction with stars, gas dynamical

processes, gravitational wave emission• Final pc-problem

• Observation of sub-pc BHB candidates: – Circum-binary disk likely at final pc scale– OJ287 most convincing evidence– counter-proposals must be checked and

open questions and curiosities analyzed– Support theory of structure formation & GR

Outlook

• BHB difficult to identify: uncertainties in structure and properties of nuclear region, signatures circum-binary accretion inflow not clear or not understood

• Further theoretical & observational work needed

• Spectroscopic monitoring to study orbital motion (change in line shifts and widths) – direct test of binary hypothesis

• Higher spectral resolution• Spectroscopic and imaging observations at

high angular resolution (morphology)• Future GW observatories (LISA & LIGO)

Thank you for your attention!

Questions?

Bibliography• Einführung in die Extragalaktische Astronomie und Kosmologie, Peter

Schneider

• THE DEMOGRAPHY OF MASSIVE DARK OBJECTS IN GALAXY CENTERS, JOHN MAGORRIAN et al., 1997

• LONG-TERM EVOLUTION OF MASSIVE BLACK HOLE BINARIES, Milos Milosavljevic et al., 2003

• Capturing black-hole pairs, Jon M. Miller, NATURE | Vol 458|5 March 2009

• SDSS J092712.65+294344.0: RECOILING BLACK HOLE OR A SUB-PARSEC BINARY CANDIDATE?, TAMARA BOGDANOVIC et al., 2009

• A candidate sub-parsec supermassive binary black hole system, Todd A. Boroson & Tod R. Lauer, Vol 458|5 March 2009| doi:10.1038/nature07779

• A massive binary black-hole system in OJ 287 and a test of general relativity, M. J. Valtonen et al., Vol 452 | 17 April 2008 | doi:10.1038/nature06896