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24 May 2011

The First Massive Black Hole Seeds and Their Hosts

Link to Paper
Authors: Jillian Bellovary, Marta Volonteri, Fabio Governato, Sijing Shen, Thomas Quinn, James Wadsley

The paper attempts to investigate the haloes in which massive seed black holes (MBH) can form by looking at local properties of the halo. The MBH are put in by using an efficiency factor on the same prescription that is used to make stars. In the study, MBH refers to 10^4 SM black holes which can be considered as direct collapse remnants or Pop III remnants that grew over time to reach this mass via mergers or accretion. Accretion onto black holes that are populated in this fashion is not accounted for. The aim is to track the formation histories of MBH seeds in haloes and make predictions for future surveys on the occupation fraction of MBH in various type of galaxies.


  1. 1 - The results presented in Fig.1 seem to indicate a bimodal formation of seed BHs, with a main pic of formation before z=10 and more quiescent and extended formation until z=5. In the latter case, I'm wondering if this seeds come from mini-haloes accreted at latter time or if some part of the main halo (still composed of pristine materials) are only collapsing latter?
    A related question I had was about resolution issues: from an higher resolution run, one can expect to resolve better the collapse of the first mini-haloes, and hence form the first generation of stars (individual ones?) even earlier... In this scenario, the late formation of massive seeds could either be anticipated or even removed? I guess this would strongly depend on the efficiency of the chemical enrichment...

  2. 2 - I know this work use a phenomenological argument to assume the a massive BH is able to form either via accretion and merging of PopIII remanent BHs or direct collapse BHs, but does the authors verified that in the former scenario, 100 Msun seeds have enough time to accrete and form a MBH consistent with their resolution (~10^4 Msun)?

  3. 3 - What about the BH conversion probability? Is there any indications towards a recommended value to be used?

  4. Ans 2-
    from what i know, a single or a few 100 solar mass blackhole(s) growing to 10^4 SM by redshift 12 via Eddington accretion is not entirely unrealistic. However, the timescales are not discussed in the paper explicitly but a reference to Li et al. 2007 is mentioned.

  5. Ans 1-
    Part (i): They argue that the bimodal nature (which is observed in the most massive case 'hz1' in their work) might be due to the fact that early on, the MBHs first form in the central region (peak at low halo mass) and then also, later in the outskirts of the halo (second peak at the large halo mass).

    Part (ii) : They also agree that this might be a resolution effect and they plan to study this in more detail in the future. They also mention the possibility of higher density threshold for the formation of MBHs, which could also cause them (MBHs) to disappear in the simulations.

  6. 1: The bimodality in MBH formation is seen in all three simulations in the paper, which is interesting. On average, the first peak represents MBH formation in the first collapsing halos, while the second peak is due to later-collapsing halos and MBHs forming in halo outskirts. Determining whether this bimodality is a real feature will have to wait until we finish simulating a uniform volume, which will give us more robust statistics. Regarding resolution, we do see that halos collapse earlier (and MBHs form sooner) at higher resolution, but the effect is not that large.

    2: Since we can't yet do cosmological simulations with particles of mass 100 Msun, we haven't verified whether a 100 Msun seed can quickly grow to 10^4. As was mentioned above, it's not unreasonable to believe this happens, though it's not obvious it does either. I think many of us are eager for the answer to this question.

    3: Determining an optimal value for the seed formation efficiency requires simulations to z=0 including accretion and feedback, which we are working on now. Based on the local M-sigma relation and halo occupation fraction, our preferred value is likely somewhere in the middle of the range explored in this paper. Stay tuned for z=0 results.