In spiral galaxies, much of the mass of the galaxy is in spiral
strands.

Consider a star in one of the spiral strands. It is 'close', to other
stars in it's own spiral strand.

So the gravitational force on the star will be dominated by its own
spiral strand, rather than the stars in other spiral strands.

Moreover, according to the (visible) shape of the spiral strands, the
stars outside our star (in its own spiral strand), tend to be further
behind it, than the stars inside it.

So, this got me thinking about two things related to the observed
velocity distribution of stars in spiral galaxies:

1. A better model than a flat continous 2-D disk, is a 1-D 'spiral'
line. Does this change the expected rotational velocity by distance
from the centre compared to the Keplerian model?
2. Stars' velocities might be retarded according to the shape of their
own spiral, specifically by their own spiral's trailing tail.

Just some ideas. I'm too lazy right now to do the maths.
Roland

"Roland PJ" wrote in message
...


Roland,

Your post contains a lot of assertions. Let's examine these.

In spiral galaxies, much of the mass of the galaxy is in spiral
strands.

This happens not to be true. Spiral arms contain lots of luminous young
stars and dust, so they are highly visible, but their actual mass density is
not much larger than the density of the inter-arm disk.

Consider a star in one of the spiral strands. It is 'close', to other
stars in it's own spiral strand.

Yes, it is close to nearby stars. Nice catch.

So the gravitational force on the star will be dominated by its own
spiral strand, rather than the stars in other spiral strands.

Not really, in fact it is dominated by both the disk component inside its
own radius plus the spheroidal component inside its radius. All the mass
outside its position pretty much averages out to zero gravitational force,
for the same reasons that the mass of a uniform spherical hollow shell has
no net gravitational effect on a test mass located inside it.

Moreover, according to the (visible) shape of the spiral strands, the
stars outside our star (in its own spiral strand), tend to be further
behind it, than the stars inside it.

Yes, but this won't have any significant gravitational effect.

So, this got me thinking about two things related to the observed
velocity distribution of stars in spiral galaxies:

1. A better model than a flat continous 2-D disk, is a 1-D 'spiral'
line. Does this change the expected rotational velocity by distance
from the centre compared to the Keplerian model?

If this model were correct, yes the curves should look different from what
is observed. The fact is the observed curves fit a model of continuous
distribution of mass much better.

2. Stars' velocities might be retarded according to the shape of their
own spiral, specifically by their own spiral's trailing tail.

Just some ideas. I'm too lazy right now to do the maths.

Or the requisite reading?

Roland

Run, don't walk, to the nearest university and sign up for an astronomy
course.

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

On Jan 7, 4:08 pm, "Mike Dworetsky"
wrote:
"Roland PJ" wrote in message

...

Roland,

Your post contains a lot of assertions. Let's examine these.

In spiral galaxies, much of the mass of the galaxy is in spiral
strands.

This happens not to be true. Spiral arms contain lots of luminous young
stars and dust, so they are highly visible, but their actual mass density is
not much larger than the density of the inter-arm disk.

Thanks. Where can I find out more about the composition of the inter-
arm disk?

snip

So the gravitational force on the star will be dominated by its own
spiral strand, rather than the stars in other spiral strands.

Not really, in fact it is dominated by both the disk component inside its
own radius plus the spheroidal component inside its radius. All the mass
outside its position pretty much averages out to zero gravitational force,
for the same reasons that the mass of a uniform spherical hollow shell has
no net gravitational effect on a test mass located inside it.

Well, there are two steps of the argument here. Firstly, my assertion
that the mass is concentrated in 1-D strands. Then, mass is
concentrated in (close to) a 2-D disk. Finally, your leap to the
assertion that the mass of galaxies is 3-D spherically symmetrical.
The last just does't seem to be consistent with galaxy images I've
seen. Your asserting about a hollow sphere is true, but is it useful
for a spiral galaxy model?

snip

1. A better model than a flat continous 2-D disk, is a 1-D 'spiral'
line. Does this change the expected rotational velocity by distance
from the centre compared to the Keplerian model?

If this model were correct, yes the curves should look different from what
is observed. The fact is the observed curves fit a model of continuous
distribution of mass much better.

So what's all the buzz about the 'galaxy rotation problem' then? It
seems quite clear that galaxy rotation observations _don't_ fit a
model of continous distribution of mass.

snip

Or the requisite reading?

Well, maybe I'm doing the reading _and_ asking questions. We are
allowed to think for ourselves, even if we're misguided. You haven't
actually made any concrete contribution in your response other to deny
the galaxy rotation anomaly exists, and to assert that galaxies enjoy
3-D spherical symmetry. I have to reply that my reading indicates you
are wrong on both accounts.

Can you back up your counter-assertions with any references that will
educate me further?

Thanks
Roland

"Roland PJ" wrote in message
...
On Jan 7, 4:08 pm, "Mike Dworetsky"
wrote:
"Roland PJ" wrote in message


...

Roland,

Your post contains a lot of assertions. Let's examine these.

In spiral galaxies, much of the mass of the galaxy is in spiral
strands.

This happens not to be true. Spiral arms contain lots of luminous young
stars and dust, so they are highly visible, but their actual mass
density is
not much larger than the density of the inter-arm disk.

Thanks. Where can I find out more about the composition of the inter-
arm disk?

An excellent standard resource for galactic dynamics is,
Galactic Dynamics by Binney and Tremaine.


snip

So the gravitational force on the star will be dominated by its own
spiral strand, rather than the stars in other spiral strands.

Not really, in fact it is dominated by both the disk component inside
its
own radius plus the spheroidal component inside its radius. All the
mass
outside its position pretty much averages out to zero gravitational
force,
for the same reasons that the mass of a uniform spherical hollow shell
has
no net gravitational effect on a test mass located inside it.

Well, there are two steps of the argument here. Firstly, my assertion
that the mass is concentrated in 1-D strands. Then, mass is
concentrated in (close to) a 2-D disk. Finally, your leap to the
assertion that the mass of galaxies is 3-D spherically symmetrical.
The last just does't seem to be consistent with galaxy images I've
seen. Your asserting about a hollow sphere is true, but is it useful
for a spiral galaxy model?

You've probably only 'seen' the visible matter in images.
That is, the areas where stars are shining brightly as
in the density waves producing burst of star formation and
comprising the spiral arms.


snip

1. A better model than a flat continous 2-D disk, is a 1-D 'spiral'
line. Does this change the expected rotational velocity by distance
from the centre compared to the Keplerian model?

If this model were correct, yes the curves should look different from
what
is observed. The fact is the observed curves fit a model of continuous
distribution of mass much better.

So what's all the buzz about the 'galaxy rotation problem' then? It
seems quite clear that galaxy rotation observations _don't_ fit a
model of continous distribution of mass.

They don't fit a model consisting of just the visible mass
plus the observed nebualae. They behave as though there
there is a significant amount of matter that we can't see.
Being density waves, the spiral arms aren't necessarily
diagnostic. Direct measurement of the velocity profile by
redshift observations is the way to go.


snip

Or the requisite reading?

Well, maybe I'm doing the reading _and_ asking questions. We are
allowed to think for ourselves, even if we're misguided. You haven't
actually made any concrete contribution in your response other to deny
the galaxy rotation anomaly exists, and to assert that galaxies enjoy
3-D spherical symmetry. I have to reply that my reading indicates you
are wrong on both accounts.

I don't believe that Mike D. ever stated that the rotation
"anomaly" as you call it doesn't exist. He simply pointed out
that the spiral features observed are not indicative of
significant mass concentration compared to the rest of the body
of the galaxy.


Can you back up your counter-assertions with any references that will
educate me further?

Thanks
Roland

On Jan 7, 6:41*am, Roland PJ wrote:
In spiral galaxies, much of the mass of the galaxy is in spiral
strands.

Consider a star in one of the spiral strands. It is 'close', to other
stars in it's own spiral strand.

So the gravitational force on the star will be dominated by its own
spiral strand, rather than the stars in other spiral strands.

Moreover, according to the (visible) shape of the spiral strands, the
stars outside our star (in its own spiral strand), tend to be further
behind it, than the stars inside it.

So, this got me thinking about two things related to the observed
velocity distribution of stars in spiral galaxies:

1. A better model than a flat continous 2-D disk, is a 1-D 'spiral'
line. Does this change the expected rotational velocity by distance
from the centre compared to the Keplerian model?
2. Stars' velocities might be retarded according to the shape of their
own spiral, specifically by their own spiral's trailing tail.

Just some ideas. I'm too lazy right now to do the maths.
Roland

What dominates is brownian, boiling.

Most stars move with similar speeds, and the direction of stars
in the arms is random but collective. Some stars flow out in the
arm, some stream of stars in. There is a distribution force
and dark matter. The distribution force is yet unknown.

On Jan 7, 4:55 pm, Sam Wormley wrote:
Roland PJ wrote:
In spiral galaxies, much of the mass of the galaxy is in spiral
strands.

No.... spirals arm show up as areas of recent star formation
due to density waves.

Thanks, Sam

I quote from

http://www.ras.ucalgary.ca/CGPS/where/plan/

"Most gas, dust, and newly forming stars occur in a set of spiral-
shaped arms"

What does that leave over for the 'space' between the spirals?

I am aware of the deduction of dominant 'dark matter', and, of course
if dark matter dominates 80:20 then there will be limited actual mass
distribution variance between the spiral arms and the voids between.

However, I'm trying to avoid a circular analysis he galaxy rotation
anomalies - dark matter - almost uniform mass distribution even
though we can't see most of it.

I'm trying going back to first principles. Let's ignore the purported
presence of dark matter, and try to invent other reasons that the
rotation curves might be unexpectedly flat.

Hope that clarifies
Roland

On Jan 7, 5:13 pm, "Greg Neill" wrote:
"Roland PJ" wrote in message


You've probably only 'seen' the visible matter in images.
That is, the areas where stars are shining brightly as
in the density waves producing burst of star formation and
comprising the spiral arms.

Thanks, Greg, and for the reference. You and Sam have used the term
'density waves'. Does this have any special significance other than
the perceived mass density varies between the spiral strands and the
space in between them?


I don't believe that Mike D. ever stated that the rotation
"anomaly" as you call it doesn't exist. He simply pointed out
that the spiral features observed are not indicative of
significant mass concentration compared to the rest of the body
of the galaxy.

Yeah, I was mainly irritated by his supercilous tone

Thanks again
Roland

"Roland PJ" wrote in message
...
On Jan 7, 5:13 pm, "Greg Neill" wrote:
"Roland PJ" wrote in message


You've probably only 'seen' the visible matter in images.
That is, the areas where stars are shining brightly as
in the density waves producing burst of star formation and
comprising the spiral arms.

Thanks, Greg, and for the reference. You and Sam have used the term
'density waves'. Does this have any special significance other than
the perceived mass density varies between the spiral strands and the
space in between them?

A density wave in a galaxy is analogous to a sound
wave traveling through a rarified gas. As it
passes through, the slight compression triggers
some clouds of interstellar hydrogen and dust to
begin to collapse kicking off increased star
formation in the wake of the wave.

"Roland PJ" wrote in message
...
On Jan 7, 4:08 pm, "Mike Dworetsky"
wrote:
"Roland PJ" wrote in message

...

Roland,

Your post contains a lot of assertions. Let's examine these.

In spiral galaxies, much of the mass of the galaxy is in spiral
strands.

This happens not to be true. Spiral arms contain lots of luminous young
stars and dust, so they are highly visible, but their actual mass density
is
not much larger than the density of the inter-arm disk.

Thanks. Where can I find out more about the composition of the inter-
arm disk?


Greg Neill beat me to suggesting the textbook by Binney & Tremaine as a
source.

snip

So the gravitational force on the star will be dominated by its own
spiral strand, rather than the stars in other spiral strands.

Not really, in fact it is dominated by both the disk component inside its
own radius plus the spheroidal component inside its radius. All the mass
outside its position pretty much averages out to zero gravitational
force,
for the same reasons that the mass of a uniform spherical hollow shell
has
no net gravitational effect on a test mass located inside it.

Well, there are two steps of the argument here. Firstly, my assertion
that the mass is concentrated in 1-D strands. Then, mass is
concentrated in (close to) a 2-D disk. Finally, your leap to the
assertion that the mass of galaxies is 3-D spherically symmetrical.
The last just does't seem to be consistent with galaxy images I've
seen. Your asserting about a hollow sphere is true, but is it useful
for a spiral galaxy model?

The halo of our Galaxy contains a lot of mass and is very nearly spherically
symmetric. The massive Galactic Bulge is radially symmetric in the plane of
the Galaxy and shaped like a flattened sphere (oblate spheroid). Hence its
effect, to first order, is equivalent to the entire mass acting as if
located at the centre (not quite right, as there will be various non-radial
accelerations, but close enough given how far away it is). Just as being
interior to a sphere implies no accelerations due to matter in an exterior
shell, being interior to a ring of matter in the same plane results in no
net force. Hence the forces on a star orbiting the galaxy in the plane of
the disk are largely due to the mass interior to the star, and matter
outside produces very little force.

snip

1. A better model than a flat continous 2-D disk, is a 1-D 'spiral'
line. Does this change the expected rotational velocity by distance
from the centre compared to the Keplerian model?

If this model were correct, yes the curves should look different from
what
is observed. The fact is the observed curves fit a model of continuous
distribution of mass much better.

So what's all the buzz about the 'galaxy rotation problem' then? It
seems quite clear that galaxy rotation observations _don't_ fit a
model of continous distribution of mass.

Well, they do, but the problem is that we don't see any matter that could
produce the flat rotation curves. You need to assume much more mass than is
observed (adding up all the stars, etc) to get a sensible fit to the
rotation curves. This is where dark matter begins to come in.


snip

Or the requisite reading?

Well, maybe I'm doing the reading _and_ asking questions. We are
allowed to think for ourselves, even if we're misguided. You haven't
actually made any concrete contribution in your response other to deny
the galaxy rotation anomaly exists, and to assert that galaxies enjoy
3-D spherical symmetry. I have to reply that my reading indicates you
are wrong on both accounts.

No, I didn't say this. You never actually mentioned the rotation anomaly,
or maybe you thought you did but didn't phrase it in a recognizable form.

All I asserted was that effectively, all the matter in a spiral galaxy with
a halo acts on a star orbiting in the disk as if the mass was concentrated
at the centre, and none of the mass outside the orbit has any effect. (This
was to counter the idea that stars along a spiral arm pulled in both
directions and should produce observable effects in the rotation curve.)
This is pretty close to what results if you have spherical symmetry. (But
not identical).

Can you back up your counter-assertions with any references that will
educate me further?

Binney and Tremaine if the maths is not a problem for you. They also do the
descriptions very well.

In fact, I was not being facetious in suggesting an astronomy course, as you
sound interested and enthusiastic, but not very clear about the actual
observations and current models. So this might be just what you need--or
you could do a "distance-learning" course if physical attendance is a
problem.

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

"Roland PJ" wrote in message
...
On Jan 7, 4:55 pm, Sam Wormley wrote:
Roland PJ wrote:
In spiral galaxies, much of the mass of the galaxy is in spiral
strands.

No.... spirals arm show up as areas of recent star formation
due to density waves.

Thanks, Sam

I quote from

http://www.ras.ucalgary.ca/CGPS/where/plan/

"Most gas, dust, and newly forming stars occur in a set of spiral-
shaped arms"

What does that leave over for the 'space' between the spirals?

The spiral arms are highly visible, but don't have a mass density a lot
greater than the inter-arm regions, which contain mostly older disk stars.
Their mass-to-light ratios are higher, that is mainly why we don't see them
as obviously as the spiral arm population. Since arms are just regions with
star formation, in fact they also contain lots of these older disk stars
too.

I am aware of the deduction of dominant 'dark matter', and, of course
if dark matter dominates 80:20 then there will be limited actual mass
distribution variance between the spiral arms and the voids between.

However, I'm trying to avoid a circular analysis he galaxy rotation
anomalies - dark matter - almost uniform mass distribution even
though we can't see most of it.

Here is where an astronomy course would help. It is a remarkable thing that
dark matter in our Galaxy is largely if not entirely in the halo, and not
(apparently) in the Galactic disk. [We know this from doing a detailed
census of the disk's stellar, dust, and gas content, calculating the
vertical motions expected from this mass distribution for various groups of
stars, and comparing with observations.] Dark matter does not seem to have
any interaction other than gravitational with normal or baryonic matter.
Hence whenever the Galaxy merged with other smaller galaxies the dark matter
formed a halo but the gas and dust soon [cosmically speaking] settled into a
plane through dissipative forces (physical collisions of clouds, etc).

This means that the rotation curve (to first approximation, a constant
orbital speed with distance from the centre once you are out of the central
regions) implies a massive galactic halo, even though the stars seen there
are not nearly enough to produce these effects. The disk, by contrast, has
less effect on the rotation speed.

I'm trying going back to first principles. Let's ignore the purported
presence of dark matter, and try to invent other reasons that the
rotation curves might be unexpectedly flat.

It wouldn't surprise you to know that astronomers tried to do exactly what
you suggest when the rotation curves were first discovered and could not
come up with anything convincing other than dark matter. They were
following the old Sherlock Holmes dictum: When you have eliminated the
impossible, whatever remains, however improbable, is the truth."

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)