
An intermediate-mass black hole candidate in the Milky Way - Jerry2
https://www.nature.com/articles/s41550-017-0224-z
======
antognini
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.

~~~
biggc
> 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"?

~~~
antognini
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.)

~~~
Pxtl
Like using scientific notation for 1000 and 100,000?

~~~
infogulch
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.

------
hrgeek
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.

------
carapace
I thought black holes don't actually exist.

[https://www.quora.com/In-a-paper-written-in-1939-Albert-
Eins...](https://www.quora.com/In-a-paper-written-in-1939-Albert-Einstein-
rejected-the-notion-of-Black-Holes-Schwarzschild%E2%80%99s-singularities-Why)

and

[http://www.nature.com/news/stephen-hawking-there-are-no-
blac...](http://www.nature.com/news/stephen-hawking-there-are-no-black-
holes-1.14583)

Can anyone shed some light?

~~~
goodcanadian
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.

~~~
carapace
Thank you!

------
saagarjha
> 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?

~~~
antognini
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.

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

