Lack of runaway black holes hints at dark matter density

In article ,
Yousuf Khan writes:
Although the study's authors don't dare to challenge the orthodoxy of
Dark Matter belief, reading between the lines, one can say that this may
actually be the falsification of Dark Matter's existence.

That is a bizarre conclusion to draw. The paper is published in
_Monthly Notices of the Royal Astronomical Society_, but there's a
preprint at http://lanl.arxiv.org/abs/1002.0553

What the paper shows is that if the dark matter density is high
enough, there will be runaway accretion onto the black hole. The
limiting density for the most massive black holes (5E9 solar masses)
is around 250 solar masses per cubic parsec. Typical dark matter
models have maximum densities around 1 solar mass per cubic parsec.
This doesn't look like much of a problem to me.

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Greg Neill wrote:
Escape velocity is irrelevant for objects approaching
from outside the system, since conservation of energy
dictates a symmetrical energy profile: they gain
kinetic energy (speed) as they fall in and shed it as
they rise back out of the gravitational well. Any body
that has enough speed to approach the well from a long-
way-off has enough energy to leave to the same long-way-
off distance, barring direct impact.

Only if the obects can shed energy by another mechanism
(friction, impact, etc.) can they be captured.


Yes, true, some objects will just enter and leave the system. But others
will have their trajectories bent enough to go into orbit around the
blackhole. This in turn will make it part of the overall blackhole's
system mass, which will make the escape velocity shell around the
blackhole increase some more. The blackhole doesn't have to absolutely
capture the Dark Matter inside its event horizon, if the DM just enters
into orbit around is enough. As I said a 5 billion Msun blackhole is as
big as some dwarf galaxies. Just like a dwarf galaxy might have DM
orbiting around it in a shell, a similar massed BH should have an equal
amount of DM orbiting it. The combined weight of the DM and BH will in
turn encompass more DM, and so on. If the DM density is high enough, of
course.

Yousuf Khan

Steve Willner wrote:
What the paper shows is that if the dark matter density is high
enough, there will be runaway accretion onto the black hole. The
limiting density for the most massive black holes (5E9 solar masses)
is around 250 solar masses per cubic parsec. Typical dark matter
models have maximum densities around 1 solar mass per cubic parsec.
This doesn't look like much of a problem to me.

Which is basically the point I was trying to make. If DM density was
much higher, then there would've been a runaway accretion process. So
this sets an upper limit on the density of DM.

Yousuf Khan

Yousuf Khan wrote:
Greg Neill wrote:
Escape velocity is irrelevant for objects approaching
from outside the system, since conservation of energy
dictates a symmetrical energy profile: they gain
kinetic energy (speed) as they fall in and shed it as
they rise back out of the gravitational well. Any body
that has enough speed to approach the well from a long-
way-off has enough energy to leave to the same long-way-
off distance, barring direct impact.

Only if the obects can shed energy by another mechanism
(friction, impact, etc.) can they be captured.


Yes, true, some objects will just enter and leave the system. But others
will have their trajectories bent enough to go into orbit around the
blackhole.

Only objects that get tied up in the highly relativistic
curved space very close the black hole can be so captured,
and then only very particular trajectories. Otherwise,
they're stuck with the same old Newtonian type trajectories:
hyperbolic or parabolic. The net effect is that the black
hole represents a very slightly larger "target" than its
event horizon.

Dear Yousuf Khan:

On Apr 1, 3:03*pm, Yousuf Khan wrote:
Greg Neill wrote:
Escape velocity is irrelevant for objects approaching
from outside the system, since conservation of energy
dictates a symmetrical energy profile: they gain
kinetic energy (speed) as they fall in and shed it as
they rise back out of the gravitational well. *Any body
that has enough speed to approach the well from a long-
way-off has enough energy to leave to the same long-way-
off distance, barring direct impact.

Only if the obects can shed energy by another
mechanism (friction, impact, etc.) can they be
captured.

Yes, true, some objects will just enter and leave the
system. But others will have their trajectories bent
enough to go into orbit around the blackhole.

If Dark Matter is anywhere associated with a galaxy containing this
supermassive black hole, it is "in orbit" around it.

This in turn will make it part of the overall
blackhole's system mass, which will make the
escape velocity shell around the blackhole
increase some more.

Not if its Dark Matter, it won't.

The blackhole doesn't have to absolutely capture
the Dark Matter inside its event horizon, if the DM
just enters into orbit around is enough.

It *can't* work like this.

As I said a 5 billion Msun blackhole is as big as
some dwarf galaxies. Just like a dwarf galaxy might
have DM orbiting around it in a shell, a similar
massed BH should have an equal amount of DM
orbiting it.

No "should have"... "might have".

The combined weight of the DM and BH will in turn
encompass more DM, and so on. If the DM density
is high enough, of course.

No, only if the Dark Matter is suddenly not Dark, but has friction,
will this occur.

David A. Smith