An intermediate-mass black hole candidate in the Milky Way | Hacker News Reader

An intermediate-mass black hole candidate in the Milky Way

nature.com

103 pointsJerry220 comments

This could be a really interesting discovery if it holds up. Astronomers have long known the existence of stellar mass black holes and supermassive black holes. But intermediate mass black holes (between, ~10^3 -- 10^5 solar masses) have been much more controversial. Now there are probably a handful of solid detections of IMBHs, but they are all in globular clusters. This is the first IMBH found in the Galaxy itself.

This is important because the formation mechanism of both IMBHs and SMBHs are unknown. There are two main competing models for SMBH formation: direct collapse and accretion. In the case of direct collapse you have a big cloud of gas that just collapses into a SMBH as the galaxy is forming. In the case of accretion, the SMBH starts off as a collection of stellar mass black holes which then merge into an IMBH, which then accretes gas to become a SMBH. A problem for the accretion model has long been the lack of any evidence for IMBHs --- this could change that.

It was also thought that if IMBHs existed anywhere, they would be in globular clusters. There the stellar densities are high enough that you might expect stellar mass black holes to coalesce into an IMBH in a reasonable amount of time (for reference, a globular cluster packs about a million stars in the space between our Sun and the nearest star). The fact that there is an IMBH in the galaxy might mean that it's considerably easier to form IMBHs than previously believed.

Why are those two models competing? Couldn't it be that SMBH are formed via both direct collapse and accretion? Finding evidence for one should not rule out the other I would think.

Generally in astronomy, physical processes have so many orders of magnitude to work with that if you have several physically possible mechanisms, one will be many orders of magnitude more effective than the others and will completely dominate. It's fairly unusual for there to be some sort of "cosmic coincidence" that leads to two processes having almost exactly the same effectiveness. (Type Ia SN may be an exception to this --- they may be produced roughly equally by accretion from ordinary stars and WD-WD mergers.)

For the unfamiliar:

SN = supernova

WD = white-dwarf (a stellar remnant)

If stellar mass black holes are merging to form IMBHs, wouldn't we expect to detect such collision events frequently? Or have we not yet collected enough gravitational wave data to put bounds on those events?

Yes, globular clusters are expected to be prime locations for BH-BH mergers and gravitational wave events. The few GW detections we now have put some bounds on the BH-BH merger rate, but as far as I know, those bounds are consistent with the rates needed for IMBH formation.

> In the case of direct collapse you have a big cloud of gas that just collapses into a SMBH as the galaxy is forming. In the case of accretion, the SMBH starts off as a collection of stellar mass black holes which then merge into an IMBH, which then accretes gas to become a SMBH.

Could you clarify if SMBH is shorthand for "Stellar Mass Black Hole" or "SuperMassive Black Hole"?

Oh, sorry, that is pretty confusing. SMBH stands for "supermassive black hole." BH on its own usually refers to a stellar mass black hole. (Astronomers have lots of weird conventions.)

Like using scientific notation for 1000 and 100,000?

In those two cases scientific notation may not be very helpful. But consider that astronomers could very easily be comparing them to many other numbers with a huge range of magnitude, and it makes sense to just use it everywhere. Also scientific notation conveys the real precision where just "1000" could mean "1000 +-1” or "1000 +-1000", which by itself can be even more important than any other reason.

It's referring to the velocity dispersion of the cloud. If you imagine the cloud is rotating about a central axis, the parts of the cloud spinning toward us will have a higher relative velocity than those spinning away. The spread of these velocities is the velocity dispersion. A broad velocity width then indicates that parts of the cloud are moving rapidly compared to others, as one might observe if the cloud was being accreted by a black hole - the material closest the black hole would be travelling much faster than that at the edge of the cloud.

I thought black holes don't actually exist.

https://www.quora.com/In-a-paper-written-in-1939-Albert-Eins...

and

http://www.nature.com/news/stephen-hawking-there-are-no-blac...

Can anyone shed some light?

In a nutshell, from the first link, Einstein was wrong about the particular issue he wrote about. From the second, well, that was sort of a click-baity title (that Hawking himself used). He is not arguing against black holes, but rather is stating that the event horizon described by general relativity is inconsistent with quantum mechanics. Instead, there is an "apparent horizon." You might say that quantum mechanics makes the event horizon "fuzzy." Regardless, there are most definitely black holes.

Thank you!

> intermediate-mass black holes (IMBHs), which are formed by the runaway coalescence of stars in young compact star clusters

How exactly would this work to produce something with a mass of 10^5 solar masses? Isn't that a bit large for the combination of a couple stars?

You obviously need 10^5 solar masses to make something that large. Whether they come via a number of IMBH's collapsing into one SMBH or otherwise, I don't understand your question.

I was wondering where the 10^5 solar masses were coming from–certainly it's not just a couple of stars merging and creating a black hole.

Well, if you get the process started in a star cluster, you might end up with a pretty hefty BH.

Star clusters do lose energy, just very slowly. On a cosmic time scale, it's conceivable that this would lead in some cases to collapse and the formation of large BHs.

In globular clusters, dynamical interactions are fairly frequent. A consequence of this is that you end up with a phenomenon called "mass segregation" --- heavy stars sink to the center of the cluster, and lighter ones float to the top. As a consequence, if you get a bunch of stellar mass black holes form (and they remain bound to the globular cluster --- not necessarily guaranteed), they will sink to its center, where they will begin to interact with each other. Over time, they will kick out all lighter stars, removing energy, and shrinking their obrits, until gravitational radiation takes over and causes them to coalesce and all merge together into a single intermediate mass black hole.

In the Galaxy, a similar process could take place in a dense open cluster, but the stellar densities are so much lower that it's a lot harder to get the same dynamics. The presence of gas in an open cluster could contribute to the dynamical friction that leads to BH-BH mergers.

Ahh, I get it. Thanks for the detailed response!

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