Quasi-stars and the Cosmic Evolution of Massive Black Holes

Transcription

1 Quasi-stars and the Cosmic Evolution of Massive Black Holes Marta Volonteri and Mitchell C. Begelman 2010 MNRAS 409:1022 David Riethmiller January 26, 2011

2 Outline Two different methods for MBH formation: Quasi-Stars vs. Pop III Progenitors Angular momentum is the primary factor in governing accretion efficiency in QSs JWST expected to detect several QSs per field, at z ~ MBH Mass function is double peaked; one peak exists for each formation mechanism

3 How to Form a Supermassive Black Hole SMBHs grew from remnants of early population of stars believed to have formed in pregalactic mini-haloes at z ~ 20. Precursors to SMBHs formed by direct collapse of large amounts of gas in much larger haloes at later times. Pop III Quasi-Star (QS)

4 Supermassive Stars Star at center of forming galaxy accumulates mass due to infall. Infall rates higher than 1M /yr: star never relaxes thermally during MS lifetime develops convective core core surrounded by convectively stable envelope

5 Quasi-Stars Only small fraction of mass in supermassive star has low enough angular momentum to collapse directly to BH. BH growth must be accompanied by outward angular momentum transport, which transfers energy into surrounding envelope, inflating it. Resulting object dubbed Quasi-Star.

6 Pop III Progenitors Population I: hot young metal-rich stars Population II: old red metal-poor stars Population III: early high-mass, zero-metal stars MBHs may also be remnants from Pop III stars, formed from pregalactic minihaloes at z 20 formed early, time to grow congregated and merged in cores of merging haloes models reproduce current population of SMBHs problems reproducing z 6 IMBHs too many remnants ejected from halo cores accretion rates depressed by shallow potential wells

7 BH Seed Growth Assumptions 1. MBHs in galaxies undergoing major mergers (mass ratio >1:10) accrete mass and become active. 2. Each MBH accretes mass M related to final velocity dispersion of merger. 3. Rate of mass accretion scales with Eddington rate for MBH. 4. When two galaxies hosting MBHs merge, the MBHs themselves merge within the merger time-scale of the host haloes.

8 Halo Assumptions 1. Inflow triggered by gas-rich major mergers. 2. Inflow may occur if: gas fraction of merger remnant above threshold angular momentum below threshold 3. Central mass accretion rate M = v 3 c / G 4. If inflow above 1M /yr, QS and seed MBH may form 5. QS formation is suppressed if MBH already present.

9 Angular Momentum Threshold For inflow, halo gas must be rotating slow enough that angular momentum does not impede gravitational collapse. λ thr = J he h 1/2 (GM h ) 5/2

10 Constraints on Angular Momentum Threshold: Lower Limit Number density constraint: Recent survey at z 6 of quasars powered by 1x10 9 M BHs gives number density of 1/Gpc. Goal: grow a BH that reaches this mass by z = 6.

11 Constraints on Angular Momentum Threshold: Upper Limit If accretion is very efficient, excess radiation may be effectively trapped in host halo. Solid: supermassive star contribution Dashed: QS remnant contribution Dotted: normal stellar contribution Estimate maximum permitted contribution of supermassive stars to reionization. Require 1 < n ion /n bar < 3.

12 Number Density of MBHs Thick curves: active MBHs only. Pop III remnant seeds are more often actively accreting - effect of imposing self-regulation of MBH mass (PopIII can t exceed M BH -σ relation; QS born above this relation). In the low-efficiency case (λ thr = 0.01) the number density of MBHs is dominated by the Pop III channel, if available; the opposite is true for the high-efficiency case.

13 JWST Sensitivity Model QS as blackbody with T=4000K, calculate counts in 2-10 µm band. JWST HUDF JWST should detect up to several QSs per field at z ~ 5-10.

14 λ thr = Pop III MBHs λ thr = Pop III MBHs Pop III MBHs Only Merloni & Heinz 2008 MBH Mass Function

15 Mass function is double-peaked. One peak for each of the BH formation mechanisms.

16 Conclusions Two different methods for MBH formation: Quasi-Stars vs. Pop III Progenitors Angular momentum is the primary factor in governing accretion efficiency in QSs JWST expected to detect several QSs per field, at z ~ MBH Mass function is double peaked; one peak exists for each formation mechanism

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