Joseph Lazio wrote in message
...
"g" == greywolf42 writes:
g Joseph Lazio wrote in message
g ...
"TS" == Thomas Smid writes:
TS It has never been the case that all galaxies show evidence for
TS dark matter halos (...).
While true, I think it is still the case that the number of
galaxies requiring dark matter to explain their rotation curves
vastly outnumbers those that do not.
g The fact of existence of a significant number of galaxies without
g dark matter halos (...) would add another layer of 'ad hoc' to the
g dark matter 'ad hoc' postulate.
It would be nice to have some numbers associated with this. One might
expect a few "exceptions to prove the rule."
?? Exceptions *never* prove the rule. Especially in science, one exception
disproves the rule.
The point is simple. You can't have a valid scientific theory that allows
you to arbitrarily add and remove 'dark matter' from galaxies as needed --
just to match observation and 'save' a theory. Such an approach is not
disprovable.
I'm not aware that a
"significant" number of galaxies do not require a dark halo to explain
their rotation curve. References?
Vega-Beltran is the most recent. However, your claim is ".. the number of
galaxies requiring dark matter to explain their rotation curves vastly
outnumbers those that do not." What would you 'a priori' consider a
'significant' number of galaxies?
You may also wish to review the
discussion between Thomas Smid and myself. Smid pointed to the work
of Vega-Beltran. I then commented that much of Vega-Beltran's work
concerns comparisons between stellar and *ionized* gas motions. He
shows reasonable agreement between the two, at least in the outer
portions of galaxies, where a dark halo requirement is most
stringent.
I disagree with your characterization of your discussion with Smid. It's
true that you made several similar assertions. But those assertions were
not backed up by Smid -- or by the work of Vega-Beltran.
Moreover, one would think that ionized gas would be more
likely to be affected by magnetic fields than neutral gas.
Neutral gas is affected by magnetic fields in the same manner as ionized
gas. It just accelerates more slowly. Gas may be paramagnetic or
diamagnetic. Gas has a magnetic moment. The neutral gas will not
accelerate as fast as ionized gas will -- but it will reach the same final
velocities relative to the magnetic field.
In the outer reaches of galaxies, rotation curves are based on gas
velocities because there are no (or so few) stars from which to
obtain stellar velocities. However, I think it is the case that
within the optical disk, gas and stellar velocities match fairly
well.
g You have no basis for this claim. There is not a single paper
g (...) for galactic rotation curves that does not depend on gas
g velocities or O and B stars. The O and B stars by necessity retain
g the initial velocities of the gas clouds which created them (...).
Admittedly, this is not my area, but I'm astounded. Nobody's ever
attempted to measure the stellar velocity dispersion of a spiral disk?
That's a pretty silly strawman. Of course people have attempted to measure
stellar velocity distribution. But it's very difficult (time consuming) to
do this on a star-by-star basis. (Think about how many stars there are in a
galaxy.)
This is done all the time for bulges and elliptical galaxies.
Those use GAS when they can get it. It's only when gas is not available
that one resorts to counting individual stars.
Are
spiral disks really that faint that nobody's tried it even with nearby
galaxies?
*Read* my statement. They use the very luminous 'O' and 'B' stars when they
count. There aren't as many, and they are easy to pick out. Individual
stars dimmer than 'B' are difficult to pick out of the background star fog
in even nearby galaxies.
Moreover, as a counter-argument I'll point out the case of the
Galactic center (i.e., center of the Milky Way Galaxy). For many
years, people measured gas velocities that seemed to indicate a
large dark mass (i.e., a supermassive black hole). A key
uncertainty was whether the gas velocities were being affected by
non-gravitational forces. There is no shortage of
non-gravitational forces in the Galactic center, either, strong
magnetic fields, stellar winds, etc.
When stellar velocities became available, they matched the gas
velocities.
g Reference, please. I think you'll find that there are either NO
g stars or O and B stars.
g I'd be happy to be disproved. I've been asking for any paper to
g the contrary for years.
Where to start? There's a huge literature on the Galactic center.
I suggest looking at the ADS. Authors to search on include
K. Sellgren, R. Genzel, A. Ghez, J. Zhao, and P. Ho. The last two
have been more involved in determining the gas motions, while the
first three have been more involved in determining stellar motions.
I'm not asking for general stratgies, but for specific references. I've
done the usual searches. The point is, I believe that there aren't any
stellar motion studies that don't either measure gas motions alone, or
measure gas plus a few O and B stars.
A good place to start might be Genzel et al. (1997, URL:
http://adsabs.harvard.edu/cgi-bin/np...NRAS.291..219G
)
as they show a plot of the mass distribution in the Galactic center as
inferred from both stellar and gas motions (Figure 8). They also have
a summary of a number of observations in Section 1. Also note that
much of the stellar work they summarize or reference concerns
late-type stars.
You're going to have to start reading abstracts a bit better (or perhaps you
missed the fact that 'likely' early-type stars of 15-20 solar masses are 'O'
and 'B' stars). Here is the abstract (the paper itself is not available on
the website, but doesn't appear to be needed in this case):
"We discuss constraints on the properties and nature of the dark mass
concentration at the core of the Milky Way. We present 0.15-arcsec
astrometric K-band maps in five epochs beween 1992 and 1996. From these we
derive imposed stellar proper motions within 3 arcsec of the compact radio
source SgrA* whose infrared counterpart may have been detected, for the
first time, in a deep image in 1996 June. We also report
lambda/Deltalambda~35 speckle spectroscopy and show that several of the
SgrA* (infrared) cluster members are likely early-type stars of mass ~15 to
20 Msolar. All available checks, including a first comparison with
high-resolution maps that are now becoming available from other groups,
support our previous conclusion that there are several fast-moving stars
(=10^3 km s^-1) in the immediate vicinity (0.01 pc) of SgrA*. From the
stellar radial and proper motion data, we infer that a dark mass of 2.61
(+/-0.15_stat)(+/-0.35_stat+sys)x10^6 Msolar must reside within about one
light-week of the compact radio source. Its density must be 2.2x10^12 Msolar
pc^-3 or greater. There is no stable configuration of normal stars, stellar
remnants or substellar entities at that density. From an equipartition
argument we infer
that at least 5 per cent of the dark mass (=10^5 Msolar) is associated with
the compact radio source SgrA* itself and is concentrated on a scale of less
than 15 times the Schwarzschild radius of a 2.6x10^6-Msolar black hole. The
corresponding density is 3x10^20 Msolar pc^-3 or greater. If one accepts
these arguments it is hard to escape the conclusion that there must be a
massive black hole at the core of the Milky Way."
1) "Compact radio source SgrA" indicates a *gas* measurement.
2) They 'may' have found an infrared signature of some stars.
3) However, they can only 'infer' that these are likely early-type stars
15-20 Msolar (i.e. 'O' and 'B' type stars).
4) There are 'several' fast-moving stars in the region. (They did not say
that they are *all* moving this fast.)
5) Using *theory* they then state that "there is no stable configuration of
normal stars, stellar remnants, or substellar entities at that density."
They ignore gas in this latter statement.
So, you'll note that my basic point is very well reinforced by your own
reference. We look at gas first, then -- sometimes, with difficulty --
manage to get a few very young 'O' and 'B' (recently condensed from gas) .
I'm still waiting