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Quasars with a “Kick”

Quasars with a “Kick”. Greg Shields 1 , Erin Bonning 1,2,3 , Sarah Salviander 1 1 U. Texas 2 Obs. Meudon 3 Yale Univ. Image: NASA / CXC / A. Hobart. Abstract. -- Gravitational radiation recoil => kicks up to four thousand km/s. -- A recoiling hole can retain accretion disk, remain AGN.

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Quasars with a “Kick”

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  1. Quasars with a “Kick” Greg Shields1, Erin Bonning1,2,3, Sarah Salviander1 1U. Texas 2Obs. Meudon 3Yale Univ. Image: NASA / CXC / A. Hobart

  2. Abstract -- Gravitational radiation recoil => kicks up to four thousand km/s. -- A recoiling hole can retain accretion disk, remain AGN. -- Reforming disk may give powerful flare in X-rays, optical-UV, and IR emission -- Wandering QSO will show emission lines shifted in velocity from the host galaxy. Limits derived from SDSS are below predictions.

  3. Introduction • Galaxy mergers lead to binary supermassive black holes (BH). • Binary may decay quickly in presence of gas. • Recent advances in BH merger simulations show vrecoil up to 4000 km/s due to anisotropic emission of gravitational waves. • Recoil could displace the BH from the galactic nucleus or eject it from the galaxy. • Possible observational manifestations include galactic nuclei with no BH, wandering BH, and wandering AGN.

  4. Motivation Image credit: HENZE/NASA

  5. How Many Big Kicks? • Final recoil comes from anisotropic emission of gravitational radiation • For random spin alignments, a*=0.9, mass ratio > 1:10 • 2% of kicks > 1000 km/s • 10% of kicks > 500 km/s. (Schnittman & Buonnano 1997) • Maximum kick of 4000 km/s happens for maximal spins anti-aligned and equal mass holes • Kick magnitude `small’ by velocities at last orbit, but very significant astrophysically. Movie by Campanelli, Lousto, Zlochower, RIT

  6. Image: Tim Jones/McDonald Observatory PIO

  7. Kicked-Out Quasars For BH merger in active QSO, accretion disk remains bound to recoiling BH inside radius where orbital velocity equals recoil velocity: Rb = (1018.1) cm M8v1000-2. Retained disk mass may be sufficient (beware self-gravity) to fuel prolonged QSO activity: Mb = (108.0 M) -1-4/5M83/2 (dM/dt)07/10v1000-5/2 where (dM/dt)0 is the mass accretion rate (M yr-1).

  8. Recoil Flares • Marginally bound material will rejoin the moving disk with impact velocity ~ vkick. • The resulting shocks produce a brief but powerful thermal x-ray flare with luminosity Lf = (1045.7 erg s-1) -1-4/5M81/2 (dM/dt)07/10v10005/2, with a temperature kT = (0.7 keV) v10002 and duration tf = (103.9 yr) M81/2 v1000-3. The high luminosity results from the large mass and short timescale, even though the binding energy per unit mass is only 10-5 c. Surrounding disk material will largely degrade X-rays to optical-UV emission lines and further to infrared continuum. • Roughly 100 flares at redshift < 3 may be in play at one time above vkick = 500 km/s. The flare may be heavily absorbed by the infalling material.

  9. Case 2: Moderate kick, disk partially bound

  10. Recoil flares: rate of occurrence • Rate of occurrence depends on rate of mergers, probability of a merger taking place in an active galaxy, probability of a kick of a given velocity, and flare duration (depends on kick velocity and BH mass). • Two estimates of rate: • Use computed merger treesto get merger rate up to z=3. • Expect ~ 15 events for vkick > 1000 km/s, ~500 events for vkick > 500 km/s for BH mass 107.5 • Get merger rate from QSO luminosity function assuming Eddington luminosity, 1 event/BH, Salpeter lifetime. • Expect ~ 3 events for vkick > 1000 km/s, ~100 events for vkick > 500 km/s • Major uncertainties: fraction of gas-rich mergers, fraction of mergers contemporaneous with QSO activity, fraction of flares unobscured. • Better understanding of spectral signature should lead to detection in surveys.

  11. Get Your Kicks from SDSS • Broad emission-line region (BLR) corresponds to bound disk • Broad lines will be shifted in velocity relative to host galaxy • Narrow emission-line region (NLR) will not follow BH; narrow lines will reflect velocity of host galaxy. • We examined 3000 QSOs from SDSS Data Release 5 (DR5) in the redshift range 0.1 < z < 0.81 with measurable H and [O III] and with successful fits by our automated program. • Objects with shifts > 1000 km/s between H and [O III] peak velocities were inspected for good quality spectra and symmetrical broad lines.

  12. Mg II Hb [O III] z = 0.4650 SDSS J091833+315621: H – [O III] ~ 2700 km/s. Green and black lines represent data before and after Fe II template subtraction; red line shows fit to data.

  13. Recoil Candidates from SDSS • Candidates ruled out for presence of • Large asymmetries in H • Strong Fe II emission • Our sample: 70 candidates out of 3000 • Displacements > 3 from the mean • 2.4% compared to 0.27% expected from Gaussian z = 0.268 SDSS J123215+132032: an asymmetrical H.

  14. Hb – [ O III] <shift> = 99 km/s FWHM = 503 km/s # of objects Hb – Mg II <shift> = 294 km/s Velocity Shift (km/s) Distribution of Shifts • Theoretical expectation: probability of line-of-sight velocity greater than v • P (v > 500) = 0.023 • P (v > 1000) = 0.008 • Our sample: fraction with velocity greater than v: • f500 = 0.05 • f1000 = 0.0058

  15. Results Incidence of kicks greater than line-of-sight velocity: 0.2% > 800 km/s (Mg II)* 0.04% > 2500 km/s (Hbeta) *Implies fraction < 1.2% for intrinsic kick > 2000 km/s (Based on SDSS QSO properties, assumed QSO lifetime, and slowing model. (Slowing of hole in galactic potential serious for v < 1500 km/s.)

  16. Blocked Kicks Reasons to doubt kick candidates: • Greater number of red than blue “kicks” • Greater shifts for wider and more asymmetrical lines • Mg II velocity often differs from H. • NLR spectral properties • Normal line intensities (surprising if QSO not in nucleus) • [Ne V] broader than [O III], but centered on [O III] velocity (suggests inner NLR feels BH gravity, so BH is in center of galaxy).

  17. Conclusions • Recoiling black holes could retain a massive accretion disk. • The disk could fuel a lasting QSO phase while the BH wanders far from the galactic nucleus. • The violent infall of material into the recoiling disk produces a brief but powerful thermal flare whose appearance depends on reprocessing. • Shifted broad lines in QSOs may reveal recoiling QSOs, but most shifts result from conditions in the BLR. The true incidence of kicks over 1000 km/s is, at most, several times less than theoretically expected for rapidly spinning holes with random orientations and similar masses. This could result from a dearth of mergers during an active QSO phase or alignment of BH spins by nuclear gas during the orbital decay.

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