Black Holes and Dark Matter in the Cosmic Dark Ages

The Growth of the FirstBlack Hole SeedsFabio Pacucci

In collaboration with: Andrea Ferrara (SNS), Marta Volonteri (IAP)EWASS 2015 Tenerife22/06/2015

Good afternoon everybody, I am Fabio Pacucci, a PhD student at the Scuola Normale Superiore in Pisa and today I have the pleasure to present my recent work about the Growth of the First Black Hole Seeds.1Key Question

Credit: ESO/UKIDSS/SDSSULAS J1120+0641z = 7.085Observations of SMBHs less than 800 Myr after the Big Bang:How is it possible?

One possible solution: Direct Collapse Black HolesFormation possible only between . Requires:Atomic cooling ( ) halosMetal free gas (Z=0)Strong Lyman-Werner (11.2 13.6 eV) flux

This work develops from a key question, which is still unanswered. Recent observations have detected the presence of quasars at redshift higher than 6, like this very famous object at z=7. These observations suggest the presence of black holes with mass 10^9 \Msun or even 10^10 \Msun less than 800 Myr after the Big Bang. For the standard theory of black hole growth, it is very challenging to build up such massive objects from a low-mass black hole seed. Instead, black hole seeds could have been very massive, 10^4-5\Msun: one possibility to build up such massive objects already at z \sim 10-15 is through the Direct Collapse Black Hole scenario. The collapse of a primordial atomic-cooling halo may lead, with the presence of a strong Lyman-Werner flux to destroy the hydrogen molecules, to the formation of DCBHs with a birth mass function peaked at 10^5 \Msun.2Questions we aim at answeringHow do the first black hole seeds shine? Physical Framework:Numerical Framework:

Radiation-hydrodynamic code:

Physically intuitiveSpherical symmetryIntermediate spatial scalesSolves Eulers equationsFrequency-integrated RTCooling: BS and atomicOpacity: Thomson and B-FHow do the first black hole seeds grow? The main objective of our work is to understand how the first black hole seeds grow and shine, in order to test the DCBH hypothesis. This is the general physical framework of our work. A high-redshift (z=10) black hole seed with a given initial mass is located at the center of a dark matter halo with primordial composition and total mass \sim 10^8 \Msun. We analyze the accretion process on scales comparable with the Bondi radius through a radiation/hydrodynamic code that we developed, whose main features are reported in this list. I can provide more information about the code to the interested people.3How do the First Black Hole Seeds Grow?Black hole massAccretion modelMain growth parameters:Duty cycleAccretion rate depend on

StandardSlim-diskMass accreted:

Time scale:

Mass accreted:

Time scale:

Let's move to our first question. The main parameters which control the black hole growth are the duty cycle, i.e. the fraction of time spent accreting, and the accretion rate, i.e. the amount of mass accreted per unit time. In our analytical model, they depend on the black hole mass and on the accretion model, i.e. the relation between the accretion rate and the emitted luminosity. In the standard model this relation is linear. The duty-cycle, shown as red points on the right axis as a function of the mass of the black hole seed, shows a mass-dependence, while the accretion rates, shown as green points on the left axis, are well approximated by the Eddington rate at large masses. The seed is able to accrete \sim 10% of the gas mass of the host halo in a time scale 100 Myr, the remaining gas being expelled with outflows. The alternative scenario assumes that the relation between accretion rate and emitted luminosity is logarithmic, to model environments with strong radiation trapping. The seed accretes almost the entire mass of the halo on a time scale 10 Myr. In this case the accretion is continuous independently on the mass of the seed and the rates largely exceed the Eddington value.4The Spatial Structure of the Accretion Flow

Transition Radius:

Physically, the rapidity of the black hole growth is determined by the effectiveness with which the radiative feedback is able to disturb the gas inflow. This figure shows on the horizontal axis our spatial profile, from the inner to the outer boundary, and on the vertical axis the velocity. Each line correspond to a velocity profile at a different time (separated by 20 000 yr). Our analytical model predicts the existence of a transition radius which indicates the spatial scale above which the radiative feedback is highly effective in disturbing the gas inflow. The transition radius, depending on the mass of the black hole, increases with time. In the region close to the accretion boundary the inflow is very smooth, while the inflow starts to be disturbed, with frequent velocity inversions, in the region around the transition radius. In the outflow region the radiative feedback is dominant.5A Bimodal Evolution of the Black Hole Seeds

determinesContinuous growthIntermittent growthThe extension of the transition radius depends on the mass determines the rapidity of the black hole growth. We predicted the possibility of a bimodal evolution in mass of the black hole seeds. This Figure shows, in blue, an initial mass function for black hole seeds at z=10 and, in green, its cosmological evolution up to z=7, with the rules set by our model. The lower-mass seeds would go through a slow growth, with recurring episodes of strong outflows, accreting only a few percent of the gas reservoir. Higher-mass seeds would go through a very rapid growth, with outflows playing a negligible role, reaching the SMBH stage early in time. To conclude the first part of my talk, a question: are these objects at the high-mass end of the IMF the seeds which gave birth to the extremely massive objects we have observed at high redshifts?6How do the First Black Hole Seeds Shine?

Now we move to our second question. We employed our hydrodynamic simulations and post-processed them with a spectral synthesis code, in order to compute the time-evolution of the spectrum emerging from the host halo, shown in this video. The two horizontal axes show the observers frame (at redshift 9) and the rest frame, while the vertical axis shows the flux density. On the right you can see the time in Myrs and the column density of the host halo.Most of the energy emerges in observed X-ray band and infrared band. Photons with frequency shortwards than the Ly_alpha line are absorbed and reprocessed in the infrared. For a Compton-Thick gas, the continuum normalization in the infrared and in the X-rays increase with the bolometric luminosity of the source. When the gas becomes Compton-thin, the X-rays continuum rapidly increases, while the infrared one starts to decrease, because a smaller number of photons is reprocessed at lower energies. As can be seen from the flux thresholds shown, we predict that the JWST will be able to observe almost the entire accretion process, while ATHENA and CDF-S will only detect it around the peak luminosity.7Upper Limit for the High-z Black Hole Mass Density

AGN candidate at z = 6.26Three AGN-candidates observed by the CDF-S at z>6 (Giallongo et al. 2015)Our upper limits (Pacucci et al. 2015):StandardSlim-diskWe employed our prediction for the CDF-S observability to provide an upper limit for the high-redshift black hole mass density.The CDF-S has actually observed 3 AGN-candidate objects at redshift higher than 6. Coupling this observation with our results, we have been able to compute upper limits on the high-redshift black hole mass density of 2.5 x 10^{2} \Msun/ Mpc^3 assuming standard Eddington-limited accretion, and of 7.6 x 10^{3} \Msun/ Mpc^3 if instead accretion occurs predominantly in the slim disk, highly obscured mode.8Is CR7 the First Observed DCBH ?

Hypothesis: CR7 is a DCBH(Pallottini et al. 2015)

Main features of CR7 (Sobral et al. 2015):z=6.6No metal linesStrong Lya and He II linesStrong Lyman-Werner flux(Elvis et al. 2009)Another very recent application deals with the observation of CR7, a very bright Ly_\alpha emitter at z=6.6. The spectroscopic follow-up by Sobral shows that this object has no signature of metal lines, while the Lya and HeII emissions are very strong. In addition, it is irradiated by a Lyman Werner flux well in excess of the required threshold for DCBH formation. With our simulations, we showed that CR7 could be powered by accretion on a DCBH of initial mass 10^5 \Msun. The predicted Ly_\alpha and He II line luminosities perfectly match CR7 observations during 14% of the system lifetime. In addition, the predicted X-ray luminosity of CR7 is below the current upper limit (10^{44} erg/ s) set by Elvis et al. in 2009.9Take-Away PointsSlim-disk allows super-Eddington, continuous accretion of most (~ 100%) of halo mass on timescales of order 10 Myr.The black hole growth may be bimodal, diversifying between low-mass ( ) and high-mass ( ) black holes.Upper limit on the z ~ 10 black hole mass density: ---The z=6.6 Lya emitter CR7 may be compatible with being the first Direct Collapse Black Hole ever detected.

Finally, in this slide I leave the most important take-away points of my talk and I am available for questions. Thank you for you attention.10

Time-evolving spectrum emerging from the host halo computed from: How do the First Black Hole Seeds Shine?

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