Quasars with a “Kick”

1. Quasars with a “Kick” Greg Shields1, Erin Bonning2, Sarah Salviander1 1U. Texas 2Obs. Meudon Image: NASA / CXC / A. Hobart

2. Abstract Recent simulations of merging black holes with spin give recoil velocities from gravitational radiation up to several thousand km/s. A recoiling supermassive black hole can retain the inner part of its accretion disk, providing fuel for a continuing QSO phase lasting millions of years as the hole moves away from the galactic nucleus. Observational manifestations include QSOs displaced from the galactic nucleus, an x-ray flare from the reforming disk, and QSO emission lines shifted in velocity from the host galaxy. We discuss candidate QSOs from the Sloan Digital Sky Survey (SDSS) that have broad emission lines shifted by more than 1000 km/s relative to the narrow lines.

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 or 2500 km/s or more due to anisotropic emission of gravitational waves.1 • 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.2

4. 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 is sufficient to fuel prolonged QSO activity: Mb = (108.0 M) -1-4/5M83/2 (dM/dt)07/10v1000-5/2, tQSO = (108.0 yr) -1-4/5M83/2 (dM/dt)0-3/10v1000-5/2 where (dM/dt)0 is the mass accretion rate (M yr-1).

5. Indications of Kicks • Thermal x-ray flare from reforming disk. • Wandering QSO phase could last comparable time to pre-kick phase giving: • QSO displaced many kpc from galactic nucleus. • Broad emission-line lines shifted in velocity relative to host galaxy and narrow emission lines. Orphan quasar Hosted quasar

6. X-ray Flares • Marginally bound material will fall back into the disk with velocity ~ vrecoil. • This will produce a brief but powerful thermal x-ray flare with luminosity Lxf = (1047.1 erg s-1) -1-4/5M81/2 (dM/dt)07/10v10005/2, with a temperature kT = (0.7 keV) v10002 and duration txf = (300 yr) M81/2 v1000-3. • This flare may exceed the x-ray luminosity of the AGN itself. However, its short duration is such that only ~100 flares at redshift < 2 may be in play at one time, where fxf is the fraction of mergers of black holes of ~108 solar masses that occur during an active QSO phase. The flare may be heavily absorbed by the infalling material.

7. 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.3 • Objects with shifts > 1000 km/s between H and [O III] peak velocities were inspected for good quality spectra and symmetrical broad lines.

8. 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.

9. 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.

10. 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 expectation3: 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

11. 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).

12. Conclusions • Schnittman, J. & Buonanno, A. 2007, astro-ph/0702641, and ref. therein • Loeb, A. 2007, astro-ph/0703722; Hoffman & Loeb 2006, ApJ, 638, L75 • Salviander, S. et al. 2006, ApJ, in press • Bogdanovic, T. 2007, astro-ph/0303054 • 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 x-ray flare. • 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 gas4 during the orbital decay. References