Galaxies without dark matter halos?

Steve Willner wrote in message
...
In article ,
"greywolf42" writes:

{replacing 'invisible' snip}
===========================
Since we believe stars condense from gas, a 'newly-born' star will
begin
life with the motion of the gas cloud from which it condensed. Once
any
star has condensed, it will eventually (over tens of millions to
hundreds of
millions of years) be accelerated to the average motion of the other
*stars*
around it.

Is there any evidence for different motions between local star
formation regions and "the average motion of the other *stars*" near
them?

Yes. See Mihalas and Binney. Section 8-3.

And how did you calculate the acceleration time, assuming that
there is a difference at birth?

The above numbers are SWAG. They neatly illustrate the point. The
specifics require knowing gas type (mass, paramagnetic moment, diamagnetic
moment, etc) and magnetic field strength. So SWAG was good enough.

The point being that magnetic fields move non-ionized gas in the same
fashion as they move ionized gas.
===========================

Sorry about that. I answered for the wrong estimate (gas spinup instead
of
stellar relaxation). The estimate of tens of millions to hundreds of
millions of years I came up with by combining galactic (open) cluster
'evaporation' times and galactic disk rotation times. These should be
of
the proper order-of-magnitude. (Still SWAG).

The first estimate -- open cluster relaxation times -- is the right
approach, but you need to scale for the greater relative velocities
of cluster stars and their much smaller average distance from each
other.

I used them for the lower bound.

This will lengthen the time by a couple of orders of magnitude.

No -- because we think stars usually form in open clusters.

You might expect this result because disk stars and halo
stars have very different velocities despite 10 Gyr of time to relax.

They also have very different formation histories, and different
environments. (i.e. Halo stars have fewer nearby stars to interact with,
gravitationally).

I don't see that the galactic disk rotation time has anything to do
with the problem.

Stars move in the galaxy as individual stars, under the influence of
gravity. Stars move in open clusters as individual stars, under the
influence of gravity. Since the stars begin life as gas clouds -- under the
influence of the EM force -- once a star coalesces, it's motion shifts from
EM-driven to gravity-driven. If a gas cloud remained behind (and were not
blown away by UV of the new star) the star would begin to move differently
than it's parent gas cloud. This change 'has to' take place before the new
star makes a complete orbit of the galaxy.

greywolf42
ubi dubium ibi libertas

Steve Willner wrote in message
...
In article ,
"greywolf42" writes:


I believe mine is the standard definition. Dark matter 'is'
non-baryonic.

An ADS abstract search for the exact phrase "baryonic dark matter"
produced 45 papers in refereed journals published in 2000 and later.
You confuse people by using terminology in a non-standard way.

Since we're back onto the term 'dark matter,' do you agree that the
definition you are supporting has the following problems?
1) The constituents of 'dark matter' will change as technology changes.
2) The constituents of 'dark matter' change with distance from the observer
(things farther away are harder to observe).
3) The constituents of 'dark matter' change with changes in theory.
4) The constituents of 'dark matter' change with the level of effort have
available to direct at the objects.

So who is confusing the issue?

And anyway, your methodology is flawed. (I'd reject such an attempt in
Physics 111 lab.) Now had you searched for merely 'dark matter' what would
your results have been? And the net difference would not necessarily be the
number of papers that DON'T use the term 'baryonic dark matter'.

But let's put aside the terminology and go to the numbers. What do
you think the mass to light ratio is for:
a) a normal stellar population?
b) mass implied by galaxy rotation curves?
c) mass implied by the baryonic matter density given by cosmological
observations (especially WMAP)?
d) mass implied by non-baryonic dark matter?

I have no need to follow you onto this tangent on a tangent. The issue
under discussion *is* the definition of the term 'dark matter.' (What it
is, was, and should be.)

That is 'your mistake.' 'Dark matter' is not defined merely as matter
that
is not radiating in the visible (or radio) region. All baryonic matter
radiates in EM.

If you remove the 'not' from your second sentence, it will be
correct.

Again, you are incorrect.

The third sentence is correct but misleading. Quite a lot
of baryonic matter produces no _detectable_ EM radiation; see
examples in my previous post.

And see my examples above (replaced from prior snip).

We could argue about black holes. But those are no longer 'baryonic.'

The standard cosmological picture counts black holes as baryonic, but
I suppose one could argue with it. However, if stars are baryonic
and black holes are not, baryon conservation is violated every time a
star turns into a black hole.

You said it -- I didn't.

Black holes are believed to be a small
fraction of the overall baryonic mass, so it doesn't matter to the
overall matter density picture whether they are counted as baryonic
or not.

That was exactly one of my original points. Before I followed down this
tangent of definitions.

The
'non-radiating' 'normal matter' is already included in those
galactic
dynamics and evolution theories.

References?

Mihalas and Binney, "Galactic Astronomy, Structure and Kinematics",
Second
Ed, 1981, 'Portrait of the Galaxy.' Gas-dynamics (and differing
possible
relaxation) of the proto galaxy. All of these possibilities include the
gas
(baryonic matter), vs. the 'dark population.' Note that Mihalas and
Binney
provide pure, 'gravity-only' dynamics. See section 8-3 for the overall
motions (and the use of gas and newborn stars).

I don't have this book handy and don't have time to go to the library
to check it. Perhaps someone else will comment. Anyway, the issue
will be moot if you give those mass to light figures I ask for above.

How could it be 'moot'? One has to look at the dynamics included in Mihalas
and Binney.

You do understand, don't you, that "halo stars" means local stars in
the Milky Way?

Yes.

How do you explain their velocities?

Because their velocities are also 'consistent with' zero dark matter.
The
halo stars have a (quasi)Maxwellian distribution. Which works with or
without 'dark matter.'

I don't think this can possibly be right.

Again, I'm using 'dark matter' as 'non-baryonic dark matter.'

The *shape* of the *disk* gas rotation curve in a disk galaxy is what
identifies the need for 'N-B dark matter' -- if we assume pure gravitation.
But the halo stars are all roughly random elliptical orbits. The 'shape'
and speed of their orbits is not significantly different (on average)
whether the disk galaxy has a spherical 'dark matter' halo or not.

Maybe you could have
another look at that Mihalas and Binney book. Don't they use the
virial theorem to derive the mass of the Milky from halo star
motions? What mass to light is implied?

On page 268, M&B determine the halo star mass estimates from observational
data. They note on p 269 that these estimates do not apply to the
'hypothetical dark population' remains undetermined by the evidence to that
point (motions of the visible halo stars).

In section 7.2, they discuss the addition of 'expected' dim star
populations. But expect the distribution to be similar to the visible
stars.

What about globular cluster velocities, come to think of it?

What about them? (Could you specify what you are asking about?)

What Milky Way mass does their motion imply? (Virial theorem again.)

Since the globular clusters are well beyond the disk of the galaxy, their
motion is affected only by the overall mass. Again, they may indicate
'undetected stars', or 'dim stars'. It is the *distribution* of that matter
that is needed in explaining the motion of disk gases and/or stars.

Can you produce a model without dark matter that is consistent with
all the observational data?

Yes.

We are all waiting with bated breath. Remember, though, "model"
means something with numbers, not hand waving. You need to match the
galaxy rotation curves, the velocity distributions of Milky Way stars
(halo and disk),

See Peratt's simulations. (p 230- 241 in 'The Big Bang Never Happened." --
especially figure 6.11). Also Astrophysics and Space science, 1983.

and the cosmological data.

Which 'cosmological data', specifically, are you interested in? (We're
talking 'galaxies without dark matter halos' here.)

Is there any evidence for different motions between local star
formation regions and "the average motion of the other *stars*" near
them?

Yes. See Mihalas and Binney. Section 8-3.

Sorry, no time. Perhaps you could summarize?

Sorry, no time.

Is there a difference
between the velocity of, say, stars in the Orion Nebula Cluster and
the Orion Nebula itself?

I don't know about that specific case. But I wouldn't expect it offhand.
The stars are far too new to have diverged much. However, I've given
references to observations to differences in gas motion and stellar motions
numerous times in this very thread (or it's progenitor in
sci.physics.research). Please be so kind as to read.

greywolf42
ubi dubium ibi libertas

Steve Willner wrote in message
...
In article ,
"greywolf42" writes:

{Another 'heroic' 'invisible' snip by Steve. I'll leave most of it out, but
there needs to be just a smidgen of context.}

=====================================
Now, we finally return to my request about 'plenty of late-type stars in
the
Galactic center region'.

Please provide a specific reference to back up your claim.

IRS 7 has been known to be a red supergiant since 1975 or so. Carr
et al. (2000 ApJ 530, 307) give a detailed abundance study.
=====================================

Supergiants are not late-type (i.e. cool, main sequence) stars. A red
supergiant is the remnant of an O or B star.

Supergiants come in both early and late types. "Late-type" is a
synonym for "cool." It does not imply main sequence. Your second
sentence above is corrrect.

"Late-type" is not simply a synonym for 'cool.' Do you have a reference for
your view? I'd look up mine, but the point is irrelevant to the issue at
hand. The point being that giant stars are fairly young in our galaxy --
since mainsequence F and later stars have not had time to become giants,
yet.

{returning to the whole point of my statement -- prior to Steve's quibbling
over the definition of 'late-type' stars.}
=====================================
Genzel et al. (1996 ApJ 472, 153) report spectra and radial
velocities for 223 stars near the Galactic center. About 200 of the
223 are late type stars.

Thanks, this looks promising... assuming that those 200 are really
late-type
main sequence and not confused with red giants.

Deguchi et al. (2002 PASJ 54, 61) studied SiO masers. (SiO masers
occur in the extended atmospheres of cool stars.)

Extended atmospheres are found in giants (red giants for cool extended
atmospheres). Red giants are not late-type main sequence stars.
Luminous
red giants are formed from luminous (O and B) main sequence stars.

All the above references (and several more) were found in a single
ADS search. I probably could have refined my keywords and found more
such papers, but these suffice to make Joe's (JL) point.

Thank you Steve, Genzel looks promising.
=====================================

Hence, we see that Joe's point is still lost.

greywolf42
ubi dubium ibi libertas

Craig Markwardt wrote in message
news

"greywolf42" writes:
...
[ Lazio : ]

{I'm going to replace some of the snipped context about what 'that' is.}
===========================================
I'm probably going to be sorry I asked, but why should O or B stars
have different velocities than any other stars?
gw:
g [...] But an 'O' or a 'B' star won't live long enough to match the
g general rotation of the later-type stars.
JL:
But stars with orbital periods of less than 100 years have been found
in the Galactic center. These orbital periods are so short that the
stars have no "memory" of the velocity of the gas from which they were
born.
===========================================
JL:
(Of course, that raises the interesting issue that we don't quite
understand how stars could form at the very center of the Galaxy.)

gw:
It could indicate a basic flaw in the observation. What's the
reference you are using?

I've recently seen a seminar talk, and there is published work,
demonstrating that a substantial number of high mass stars can be
deposited in the galactic center. The described mechanism involves
clusters that form outside the center, but sink towards it by
dynamical friction. (Zwart McMillan & Gerhard 2003). At the end of
this track, the individual stars in the (dissolving) cluster will have
no memory of their original velocity.

OK, they don't need to 'form' in the center. But can you describe this
'dynamical friction' any better? Seems that stars 'loitering' around the
center of a mass of halo and disk elliptical orbits will not shrink their
orbits. But tend to be 'ejected' from the region.

As to whether there is a basic flaw in the observation, it appears
unlikely.

It is not possible to come to such a conclusion without specifically
referencing the study. See my post to Craig Markwardt on the three studies
he proposed. I found several reasons to be suspicious of the data.
Primarily that it selects only stars that match the desired result -- and
ignores the rest. Which have been observed moving far more slowly, in the
same region (Rieke).

Some of the stars are observed to directly orbit the
galactic center compact object (i.e. likely black hole). Continuous
trajectories are consistent with Keplerian orbits around a single
central point mass. For one star, both peri- and apocenter have been
covered, with a pericenter distance of only 17 lt-hr (Sh\"odel et al
2002; Eckart et al 2002). They have the appropriate extinctions,
Doppler velocities, etc. These observations clearly tie the stars to
the immediate galactic center region.

Yes. But only one star was used in that study. The other stars that
condradict this single observation (Rieke) are ignored.


Ekart et al 2002, MNRAS, 331, 917
Sh\"odel, R. et al 2002, 419, 694
Zwart, S., McMillan, S. & Gerhard, O. 2003, ApJ, 593, 352

greywolf42
ubi dubium ibi libertas

The first estimate -- open cluster relaxation times -- is the right
approach, but you need to scale for the greater relative velocities
of cluster stars and their much smaller average distance from each
other.

In article ,
"greywolf42" writes:
I used them for the lower bound.

A lower bound is nice, but we want an estimate of the true value. In
particular, is it less than the stellar lifetime or even the current
age of the Universe?

[lower densities and velocities]
This will lengthen the time by a couple of orders of magnitude.

No -- because we think stars usually form in open clusters.

This looks like a non-sequitur to me. Your picture has the stars
changing velocity from a "birth" velocity to a "spiral arm" velocity
in only 100 Myr. It looks to me as though the time scale for any
such change is much longer.

You might expect this result because disk stars and halo
stars have very different velocities despite 10 Gyr of time to relax.

They also have very different formation histories, and different
environments. (i.e. Halo stars have fewer nearby stars to interact with,
gravitationally).

I don't see that formation history matters, but the latter is a fair
point. Halo stars spend only a fraction of their lifetimes in the
disk. Nevertheless, their rotation velocities are about 170 km/s
different from the disk stars. If newborn disk stars can "catch up"
to the rest of the disk population in only 100 Myr, how come the halo
stars haven't caught up in the entire age of the Milky Way?

I don't see that the galactic disk rotation time has anything to do
with the problem.

Stars move in the galaxy as individual stars, under the influence of
gravity. Stars move in open clusters as individual stars, under the
influence of gravity. Since the stars begin life as gas clouds -- under the
influence of the EM force -- once a star coalesces, it's motion shifts from
EM-driven to gravity-driven. If a gas cloud remained behind (and were not
blown away by UV of the new star) the star would begin to move differently
than it's parent gas cloud. This change 'has to' take place before the new
star makes a complete orbit of the galaxy.

It's the 'has to' I don't see any justification for. I would have
said 'cannot'.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

"greywolf42" wrote in message ...
Since we're back onto the term 'dark matter,' do you agree that the
definition you are supporting has the following problems?

I have given you the definition used by working scientists. What you
or I think of its merits is irrelevant. There have been efforts in
the past to change the usage, but those efforts have not succeeded.

But let's put aside the terminology and go to the numbers. What do
you think the mass to light ratio is for:
a) a normal stellar population?
b) mass implied by galaxy rotation curves?
c) mass implied by the baryonic matter density given by cosmological
observations (especially WMAP)?
d) mass implied by non-baryonic dark matter?

I have no need to follow you onto this tangent on a tangent.

If you think this is a "tangent," how would you like to specify the
amount of dark matter? Science requires numbers.

[halo stars]
How do you explain their velocities?

Because their velocities are also 'consistent with' zero dark matter.
The
halo stars have a (quasi)Maxwellian distribution. Which works with or
without 'dark matter.'

What disk mass is implied by the magnitude of the halo stars' motions?
What disk mass is implied by visible stars? Are the two equal?

The *shape* of the *disk* gas rotation curve in a disk galaxy is what
identifies the need for 'N-B dark matter' -- if we assume pure gravitation.

Actually it's the magnitude of the velocity plus the distance from the
galaxy's center. This is just as the velocity of Pluto together with
the semi-major axis of Pluto's orbit determines the solar system mass
interior to Pluto's orbit. Of course the same calculation works for
all the planets, and all planets give essentially the same mass
because most of the solar system's mass is concentrated in the Sun.
The result for galaxies is different.

But the halo stars are all roughly random elliptical orbits. The 'shape'
and speed of their orbits is not significantly different (on average)
whether the disk galaxy has a spherical 'dark matter' halo or not.

On page 268, M&B determine the halo star mass estimates from observational
data. They note on p 269 that these estimates do not apply to the
'hypothetical dark population' remains undetermined by the evidence to that
point (motions of the visible halo stars).

The point is to determine the _disk_ mass from the motion of halo
stars.

What about globular cluster velocities, come to think of it?

What about them? (Could you specify what you are asking about?)

What Milky Way mass does their motion imply? (Virial theorem again.)

Since the globular clusters are well beyond the disk of the galaxy, their
motion is affected only by the overall mass.

Exactly. So how much mass is that? And what is the mass known from
visible stars? (Use infrared measurements to get around dust
extinction.) Do the two masses agree?

--
Steve Willner Phone 617-495-7123

Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

Steve Willner wrote in message
...
The first estimate -- open cluster relaxation times -- is the right
approach, but you need to scale for the greater relative velocities
of cluster stars and their much smaller average distance from each
other.

In article ,
"greywolf42" writes:
I used them for the lower bound.

A lower bound is nice, but we want an estimate of the true value. In
particular, is it less than the stellar lifetime or even the current
age of the Universe?

That's why I gave *both* a lower and an upper bound. There is no *single*
value, as stars form under varying conditions.


[lower densities and velocities]
This will lengthen the time by a couple of orders of magnitude.

No -- because we think stars usually form in open clusters.

This looks like a non-sequitur to me. Your picture has the stars
changing velocity from a "birth" velocity to a "spiral arm" velocity
in only 100 Myr. It looks to me as though the time scale for any
such change is much longer.

What time scale to you favor -- and why?

But the stars don't change their velocity right away. They change their
*orbit*. According to studies that actually measure stellar motions and gas
motions, the stellar motions are found to be smaller than the gas motions.
Hence, a newly-formed star will be moving *too fast* for a circular orbit at
the radius at which it is formed. This will put the star into an elliptical
orbit (not the circular one that is commonly assumed). By the time the star
has performed a full circuit of the galaxy, it will have had more
opportunity to 'normalize' it's motion.

You might expect this result because disk stars and halo
stars have very different velocities despite 10 Gyr of time to relax.

They also have very different formation histories, and different
environments. (i.e. Halo stars have fewer nearby stars to interact with,
gravitationally).

I don't see that formation history matters, but the latter is a fair
point. Halo stars spend only a fraction of their lifetimes in the
disk. Nevertheless, their rotation velocities are about 170 km/s
different from the disk stars. If newborn disk stars can "catch up"
to the rest of the disk population in only 100 Myr, how come the halo
stars haven't caught up in the entire age of the Milky Way?

1) Well, the passages through the disk will only be around 1% of the orbit
of a halo star. Hence, 100 MY / .01 = 10 BY. Which is roughly the age of
the galaxy.

2) The stars that halo stars encounter have random velocities. Whereas the
stars that the disk stars encounter all tend to be moving the same.

I don't see that the galactic disk rotation time has anything to do
with the problem.

Stars move in the galaxy as individual stars, under the influence of
gravity. Stars move in open clusters as individual stars, under the
influence of gravity. Since the stars begin life as gas clouds -- under
the
influence of the EM force -- once a star coalesces, it's motion shifts
from
EM-driven to gravity-driven. If a gas cloud remained behind (and were
not
blown away by UV of the new star) the star would begin to move
differently
than it's parent gas cloud. This change 'has to' take place before the
new
star makes a complete orbit of the galaxy.

It's the 'has to' I don't see any justification for. I would have
said 'cannot'.

Why do you say 'cannot?' What it the time frame you favor. Start with a
star moving at 2 x stellar orbital velocity. How long before it slows to
stellar orbital velocity, while interacting with stars already at stellar
orbital velocity?

greywolf42
ubi dubium ibi libertas

Steve Willner wrote in message
om...
"greywolf42" wrote in message
...

{dodging and weaving, Steve performs random, invisible snips to avoid any
substantive issue}
==================================
[greywolf42:]
I believe mine is the standard definition. Dark matter 'is' non-baryonic.

[Steve:]
An ADS abstract search for the exact phrase "baryonic dark matter" produced
45 papers in refereed journals published in 2000 and later. You confuse
people by using terminology in a non-standard way.
==================================

Since we're back onto the term 'dark matter,' do you agree that the
definition you are supporting has the following problems?

{another invisible snip by Steve -- refusing to answer the question}
==================================
1) The constituents of 'dark matter' will change as technology changes.
2) The constituents of 'dark matter' change with distance from the observer
(things farther away are harder to observe).
3) The constituents of 'dark matter' change with changes in theory.
4) The constituents of 'dark matter' change with the level of effort have
available to direct at the objects.

So who is confusing the issue?
==================================

I have given you the definition used by working scientists.

You disagree with a poster on sci.physics.research. However, this doesn't
prove either of us 'right.'

What you
or I think of its merits is irrelevant. There have been efforts in
the past to change the usage, but those efforts have not succeeded.

Since I'm not the first to attempt to change the usage, why should you be so
heavy-handed about keeping a confusing "common usage" -- when your argument
is solely based on avoiding 'confusion?'

{Another 'invisible' snip by Steve -- attempting to bury his prior error in
'research'}
============================
[greywolf42:]
And anyway, your methodology is flawed. (I'd reject such an attempt in
Physics 111 lab.) Now had you searched for merely 'dark matter' what would
your results have been? And the net difference would not necessarily be the
number of papers that DON'T use the term 'baryonic dark matter'.
============================

But let's put aside the terminology and go to the numbers. What do
you think the mass to light ratio is for:
a) a normal stellar population?
b) mass implied by galaxy rotation curves?
c) mass implied by the baryonic matter density given by cosmological
observations (especially WMAP)?
d) mass implied by non-baryonic dark matter?

I have no need to follow you onto this tangent on a tangent.

{Surprise! (NOT). Another 'invisible' snip of the rest of the paragraph.}
=========================
[greywolf42:]
The issue under discussion *is* the definition of the term 'dark matter.'
(What it is, was, and should be.)
=========================

If you think this is a "tangent," how would you like to specify the
amount of dark matter? Science requires numbers.

Numbers are meaninless unless we agree what we are quantifying. Hence, the
definition must come first.

{Another massive snip by Steve. But I won't put them all back in. In sum,
Steve lost arguments on baryonic matter interactions with EM, black holes,
the inclusion of 'non-radiating' 'normal matter' in current galactic
dynamics and evolution theories (with references).}


[halo stars]
How do you explain their velocities?

Because their velocities are also 'consistent with' zero dark
matter.
The halo stars have a (quasi)Maxwellian distribution. Which
works with or without 'dark matter.'

{Aother 'invisible' snip by Steve.}
=========================
[Steve:]
I don't think this can possibly be right.

[greywolf42:]
Again, I'm using 'dark matter' as 'non-baryonic dark matter.'
=========================

Now we see why Steve found it 'convenient' to jump into the middle of a
prior Q&A.

What disk mass is implied by the magnitude of the halo stars' motions?
What disk mass is implied by visible stars? Are the two equal?

Poor phrasing of the questions. I presume you meant 'What galactic mass is
implied by the magnitude of the halo stars' motions,' and 'what galactic
mass is implied by the motions of visible disk stars?'

What theory would you care to apply to the visible disk star motions?

The *shape* of the *disk* gas rotation curve in a disk galaxy is what
identifies the need for 'N-B dark matter' -- if we assume pure
gravitation.

Actually it's the magnitude of the velocity plus the distance from the
galaxy's center.

The 'shape' of the curve does not match the distribution of the observed,
luminous stellar bodies. If the shape of the (gas) velocity distribution
were not 'wrong', then we'd simply identify additional 'dim' stars -- by
skewing our presumed stellar mass function toward low-mass (low-luminosity)
stars.

This is just as the velocity of Pluto together with
the semi-major axis of Pluto's orbit determines the solar system mass
interior to Pluto's orbit. Of course the same calculation works for
all the planets, and all planets give essentially the same mass
because most of the solar system's mass is concentrated in the Sun.
The result for galaxies is different.

That may be because gravity is not the only force in the universe, and the
motions of gas may be driven by EM forces instead of gravitational ones.

But the halo stars are all roughly random elliptical orbits. The
'shape'
and speed of their orbits is not significantly different (on average)
whether the disk galaxy has a spherical 'dark matter' halo or not.


{Another 'invisible' snip by Steve.}
==============================
[Steve:]
Maybe you could have
another look at that Mihalas and Binney book. Don't they use the
virial theorem to derive the mass of the Milky from halo star
motions? What mass to light is implied?
==============================

On page 268, M&B determine the halo star mass estimates from
observational
data. They note on p 269 that these estimates do not apply to the
'hypothetical dark population' remains undetermined by the evidence to
that
point (motions of the visible halo stars).

{Steve 'invisibly' snips part of my answer to his question}
=========================
In section 7.2, they discuss the addition of 'expected' dim star
populations. But expect the distribution to be similar to the visible
stars.
=========================

The point is to determine the _disk_ mass from the motion of halo
stars.

The point is to determine the galactic mass *distribution* from both halo
and disk stellar and gas motions.


What about globular cluster velocities, come to think of it?

What about them? (Could you specify what you are asking about?)

What Milky Way mass does their motion imply? (Virial theorem again.)

Since the globular clusters are well beyond the disk of the galaxy,
their
motion is affected only by the overall mass.

{Again, putting back the rest of my answer, 'invisibly' snipped by Steve:}
=============================
Again, they may indicate
'undetected stars', or 'dim stars'. It is the *distribution* of that matter
that is needed in explaining the motion of disk gases and/or stars.
=============================

Exactly. So how much mass is that? And what is the mass known from
visible stars? (Use infrared measurements to get around dust
extinction.) Do the two masses agree?

Again, Steve simply *will not* address the issue of mass distribution.
Total mass is nice, but it's the distribution that causes the 'need' for
'dark matter' that is not distributed like highly luminous stars.

{More 'invisible' snipping evasions by Steve. I give up.}

Steve, I find you are a cowardly waste of time. You invisibly snip whatever
you want to avoid. You refuse to take a stand on your own primary issues.
Bye in this thread.

greywolf42
ubi dubium ibi libertas

[About the picture that stars form at some velocity and then over
time accelerate to the the local velocity of other stars. The
question is what is the timescale for this alleged process.]

In article ,
"greywolf42" writes:
That's why I gave *both* a lower and an upper bound. There is no *single*
value, as stars form under varying conditions.

Sorry, I didn't see a calculation of either one. You gave a
"scientific wild ass guess" based on open cluster relaxation times --
a guess that appears wildly wrong to me because it fails to correct
for the different densities and velocities in galaxy disks versus
open clusters.

What time scale to you favor -- and why?

As I wrote in the previous message, something quite a lot longer than
open cluster relaxation times because of the lower densities and
velocities in stellar populations outside open clusters.

But the stars don't change their velocity right away. They change their
*orbit*. According to studies that actually measure stellar motions and gas
motions, the stellar motions are found to be smaller than the gas motions.

Could we have a reference to such a study in the Milky Way? I asked
before about the Orion Nebula but didn't phrase the question very
well. Does the nebula have a different velocity than the stellar
population in its vicinity?

Hence, a newly-formed star will be moving *too fast* for a circular orbit at
the radius at which it is formed. This will put the star into an elliptical
orbit (not the circular one that is commonly assumed). By the time the star
has performed a full circuit of the galaxy, it will have had more
opportunity to 'normalize' it's motion.

This is correct as to the celestial mechanics. _If_ a star is
initially moving faster than the local circular velocity, it will
indeed be in an elliptical orbit. The question is the time scale for
this orbit to "relax" to the velocity of other stars.

You might expect this result because disk stars and halo
stars have very different velocities despite 10 Gyr of time to relax.

They also have very different formation histories, and different
environments. (i.e. Halo stars have fewer nearby stars to interact with,
gravitationally).

I don't see that formation history matters, but the latter is a fair
point. Halo stars spend only a fraction of their lifetimes in the
disk. Nevertheless, their rotation velocities are about 170 km/s
different from the disk stars. If newborn disk stars can "catch up"
to the rest of the disk population in only 100 Myr, how come the halo
stars haven't caught up in the entire age of the Milky Way?

1) Well, the passages through the disk will only be around 1% of the orbit
of a halo star. Hence, 100 MY / .01 = 10 BY. Which is roughly the age of
the galaxy.

How do you get a fraction as small as 1%?

Even if we believe your 1%, an initial velocity difference should
have decayed by roughly a factor of e (2.718...) in 10 Gyr. Thus
today's 170 km/s difference would have been about 460 km/s at birth.
Is that reasonable? Do you think the disk was moving faster back
then, or was the halo moving at 430 km/s "backwards?"

2) The stars that halo stars encounter have random velocities. Whereas the
stars that the disk stars encounter all tend to be moving the same.

This I don't understand. The stars that both disk and halo stars
encounter are the same stars: the ones in the disk. Why should the
disk stars have an effect on newly-formed stars but not on the halo
stars?

Start with a
star moving at 2 x stellar orbital velocity. How long before it slows to
stellar orbital velocity, while interacting with stars already at stellar
orbital velocity?

You are the one arguing for a non-standard picture. It seems to me
you need to demonstrate two things: 1) evidence that stars initially
form at some velocity other than the local velocity of older stars,
and 2) that this excess velocity can be lost in less time than the
age of the Galaxy.

Both propositions look highly dubious to me.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

In article ,
"greywolf42" writes:
The issue under discussion *is* the definition of the term 'dark matter.'
(What it is, was, and should be.)

I thought the issue was science. Silly me. I have no interest in
discussing semantics other than as necessary to clarify exactly what
you are claiming.

Numbers are meaninless unless we agree what we are quantifying. Hence, the
definition must come first.

The question, at least the one that interests me, is whether mass
measured by dynamical means is more than the mass estimated from
stellar light and by how much in different populations (galaxies).

Poor phrasing of the questions. I presume you meant 'What galactic mass is
implied by the magnitude of the halo stars' motions,' and 'what galactic
mass is implied by the motions of visible disk stars?'

Yes. Also the globular clusters. (Most people capitalize Galactic
when referring to the Milky Way. I don't care whether you do or not,
but I want to make sure you understand that the questions refer
specifically to the Milky Way, not other galaxies.)

What theory would you care to apply to the visible disk star motions?

Newtonian gravity will do.

The *shape* of the *disk* gas rotation curve in a disk galaxy is what
identifies the need for 'N-B dark matter' -- if we assume pure
gravitation.

Actually it's the magnitude of the velocity plus the distance from the
galaxy's center.

The 'shape' of the curve does not match the distribution of the observed,
luminous stellar bodies.

Perhaps I'm wrong, but I think this not the most important evidence
effect except at very large distances.

If the shape of the (gas) velocity distribution
were not 'wrong', then we'd simply identify additional 'dim' stars -- by
skewing our presumed stellar mass function toward low-mass (low-luminosity)
stars.

This is why I was asking about mass to light ratios. It's necessary
to be quantitative. How much excess mass would be required?

Suppose there's a population of cold white dwarfs. Its distribution
wouldn't necessarily follow the distribution of visible stars. Why
couldn't such a population explain the observed rotation curves?

This is just as the velocity of Pluto together with
the semi-major axis of Pluto's orbit determines the solar system mass
interior to Pluto's orbit. Of course the same calculation works for
all the planets, and all planets give essentially the same mass
because most of the solar system's mass is concentrated in the Sun.
The result for galaxies is different.

That may be because gravity is not the only force in the universe, and the
motions of gas may be driven by EM forces instead of gravitational ones.

"May be," I suppose. Evidence appears to be a bit thin.

Again, Steve simply *will not* address the issue of mass distribution.
Total mass is nice, but it's the distribution that causes the 'need' for
'dark matter' that is not distributed like highly luminous stars.

Certainly the distribution indicates the need for some gravitating
material not distributed the same as the luminous stars. You were
arguing that the velocity distribution is spurious. _Even if so_,
you still have to explain the total mass.

You will never get anywhere until you can put numbers in.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)