17
$\begingroup$

A supermassive black hole was discovered recently that is 13 billion years old. This blackhole is 1.6 billion times the mass of the sun. How can this blackhole have formed so quickly after the big bang? Do theories predict supermassive black holes forming so quickly?

$\endgroup$
2
  • $\begingroup$ My guess is that a cluster of massive stars formed in a large (more than 100 million solar masses of material) nebula. One of them collapsed into a black hole, upwards of 1000-5000 solar masses. Then the other stars were sucked into the black hole or merged with the black hole. The black hole then drew in most of the remaining matter in the nebula. I don't know this for sure, just a wild guess. $\endgroup$ – fasterthanlight Jan 21 at 17:27
  • 1
    $\begingroup$ If a body forms a black hole (by implosion via supernova or something), it's not more likely to "suck" distant objects in as the gravitational attraction at a given distance from the center of the black hole wouldn't change. I'd go for the likelihood that things were just more crowded at the start before accretion thinned out the neighborhood. The furthest galaxies are also seen in the more distant past so they're accumulating stuff in the black hole fast enough to be quasars, less distant are Seyfert galaxies, and nearby galaxies are relatively quiescent. $\endgroup$ – Arthur Kalliokoski Jan 21 at 21:25
23
$\begingroup$

The answer to this is unknown at the present time.

The issue is that an accreting "seed black hole" can only accrete at a limited rate. The limitation is provided by radiation pressure from the material it is accreting. This provides negative feedback and defines a maximum accretion rate for spherical accretion known as the Eddington limit, which defines a maximum luminosity and hence a maximum accretion rate (since accretion provides the luminosity).

The net result is that the black hole can grow exponentially, with a growth timescale of about 30-40 million years. That is, it can roughly double in mass in that timescale.

Now the first, primordial stars may have been much more massive than big stars in the universe today, as a result of their pure hydrogen/helium composition. Even if they were able to form up to a few hundred solar mass black holes, it would still take almost a billion years to build a $10^9 M_\odot$ black hole, even if it accreted at the maximum Eddington rate for all of that time.

This is a bit of a stretch so alternative solutions are being sought. These include

  1. Maybe you can form even bigger black holes from early collapsing gas clouds. There are some ideas that if the primordial gas gets ionised by the very first massive stars then it doesn;t fragment into "stellar" objects. This can enable you to form very, very massive "stars" (sometimes called "quasi-stars"), which have black holes at their centres rather than nuclear-fusing cores. These might grow rapidly to be of order $10^5 M_{\odot}$ This can shave of many exponential growth timescales by starting off with a more massive seed.

  2. Maybe you can merge black holes. We now know from gravitational wave detections that mergers can proceed between black holes with tens of solar masses to form $\sim 100M_\odot$ black holes. Perhaps in the early universe there may have been clusters of high-mass black holes that managed to merge with each other on short timescales. These would then form a massive seed black hole that could grow relatively quickly.

  3. There are some ideas about how the Eddington limit can be circumvented and mass accretion can proceed at higher rates, perhaps by suppressing the radiative efficiency of the accreting black hole (e.g. Li et al. 2012).

$\endgroup$
8
  • $\begingroup$ Aren't black holes not subject to the Eddington limit, since an unlimited amount of energy can be lost down the hole without contributing to overall luminosity? $\endgroup$ – Vikki - formerly Sean Jan 22 at 1:33
  • $\begingroup$ @Sean cramming matter into the event horizon inevitably leads to it heating up and emitting radiation. Most of this radiation comes from well outside the event horizon. What you are suggesting is that the matter could be "advected" into the black hole without heating up. That isn't really possible. $\endgroup$ – ProfRob Jan 22 at 8:31
  • $\begingroup$ Why would the growth of the hypothetical black holes in the midst of gas clouds not be restricted by the same radiation pressure as that of "standard" black holes? Or do you mean that the black hole starts at 10^5 solar masses? $\endgroup$ – Peter - Reinstate Monica Jan 22 at 15:46
  • 1
    $\begingroup$ @Peter-ReinstateMonica The Eddington limit assumes that the accreting gas is not being "forced in" (other than by gravity). In a quasi-star the black hole sits in the middle of a massive "star", so my understanding is that the Eddington rate in this case is that for the TOTAL mass of the object, not just the black hole, so it can grow to $10^5M\odot$ quickly. $\endgroup$ – ProfRob Jan 22 at 16:50
  • 1
    $\begingroup$ @Peter-ReinstateMonica I'll come back with an edit later. I need to read a paper. I probably badly expressed it. The fact you have the weight of the envelope pressing the stuff inwards. I believe that term is neglected in the standard Eddington treatment. $\endgroup$ – ProfRob Jan 22 at 17:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.