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Galaxies without dark matter halos?



 
 
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  #21  
Old September 25th 03, 01:07 PM
Craig Markwardt
external usenet poster
 
Posts: n/a
Default Galaxies without dark matter halos?

greywolf42 writes:
Craig Markwardt wrote in message
...

[ ... snip for brevity ... ]
In particular,

(1) the mixing of the two data sets is reasonable, since it
increases the time baseline, thus increasing the sensitivity to
stellar accelerations; you have provided no factual basis to
claim that such combination would be incorrect or biased.


It is not the mixing of two data SETS but the mixing of different TYPES of
data: "Combines the high precision but shorter time scale NIRC/Keck data
with the lower precision but longer time scale SHARP/NTT data set."


Referring to the Eckart et al (2002) paper, you are apparently
erroneously presupposing that the NIRC/Keck and SHARP/NTT are of
different type, when clearly they are both imaging data, taken with
comparable near-infrared speckle cameras, on large telescopes.
Questions of precision are handled properly in the analysis.
Furthermore, you have not described how the act of combining two
different data sets would improperly bias the results of Eckart et al.



(2) 2D projection effects are a fact of our natural world. They
must be corrected for, in order to infer physical accelerations.


The observable universe is 3D. 2D 'projection' normally is the result of
theory or of limited representation. In this specific case, we have
'statistically corrections' to the 'observed accelerations' for 'theoretical
projection effects.' A 2D-3D correction would not be statistical. A theory
of some kind has been imposed onto the data. This makes the result
questionable.


Clearly an optical 2D image of the galactic center, made by an
earthbound telescope, is a limited representation of the 3D
environment there. Therefore, to make any reasonable statements about
the 3D accelerations of stars, one must account for the projection
effects. Some assumptions must be made: however, Eckart shows that
their results are robust over a wide range of assumptions.
Furthermore, you have not challenged any of the assumptions.


(3) Filtering of data can be a concern. However, whether or not
star "S8" is retained does not change the conclusion that there
is a supermassive compact object at Sgr A*.


It changes the conclusion, because it is contradictory data. S8 is moving
'too slowly' to allow for the supermassive object inferred by the 'primary'
object. That is why S8 was 'filtered.' That's why I find the study
'questionable.'


This is a fair point. However, considering that the galactic center
has a much higher density of stars than our solar neighborhood, it is
not unreasonable for a steller interaction to occur. One aberrant
star does not necessarily invalidate the others.


http://www.arxiv.org/abs/astro-ph/0210426

"Closest Star Seen Orbiting the Supermassive Black Hole at the
Centre of the
Milky Way", Oct 2002, Schodel et al.

.... snip for brevity ...

I care not what the authors say. One data point does not a definitive
study make. As there are other stars observed in the same region
that have been observed to contradict the conclusion. (We don't
get to 'pick' the data we like, and ignore the rest, in science.)


What you care about is irrelevant.


It is not that *I* care about data. It is the fact that one cannot pick and
choose data in the scientific method.


This is a fallacy. Of course scientists pick and choose the data.
Would you say that scientists should use their uncalibrated data?
Observations taken with the dome closed? taken when the sun was up?
Of course not. The point is that observations must be chosen on an
objective basis.

The Sch\"odel et al (2002) paper chooses the particular star "S2"
precisely because nearly one complete orbit is detectable, which is
untrue for any other known star near the galactic center. And your
pick-and-choose argument is further fallacious because many stars with
*some* or *marginal* curvature in their proper motions are discussed
by the same authors in Eckart et al (2002). And further, Genzel et al
(2000, 2003) treat the ensemble of *all* known stars in the galactic
center region. So the facts demonstrate quite the opposite: the
Genzel group has consistently examined a broad spectrum of evidence.


It is certainly a fact that once
measured, a Keplerian orbit determines the central object mass and
minimum mass density.


Even if you only have one star, and if you first assume a theory of the
forces (to 'call' it 'Keplerian') -- it is still not a 'fact.' Observations
are facts. Conclusions are not.

And even then, it is not even correct if other stars in the same region
contradict the conclusion. And that is why the conclusions are
questionable.


It is a fact that the measured trajectory of star "S2" is consistent
with a Keplerian ellipse. It is also a fact that the measured time
profile of the motion of star "S2" is consistent with Keplerian
motion. It is a fact that the central mass inferred from the
Keplerian solution is consistent with other mass determinations.
Finally, it is a fact that the position of the central attractor of
"S2" is consistent with Sgr A* to within the estimated 2 sigma errors.

How one interprets these facts is another matter. However, the
signatures of Keplerian orbits are very distinct. I am unaware of any
alternate interpretation -- by you, or another -- which is consistent
with the data.

No one piece of evidence will clinch or refute the conclusion that
there is a supermassive black hole associated with Sgr A*. However,
the S2 orbit appears to be an extremely strong piece of evidence in
favor.



[ ... ]
None of these address all the stars found in the Rieke's paper.
Specifically, instead of focusing on one or a few individual stars

selected
for their apparent support of one theory, one needs to address the

following
findings from Rieke and Rieke:

"The simplest model with a central black hole that dominates the

mass within
2 pc would have a velocity dispersion increasing as r^-1/2; clearly

there is
no indication of this trend in our data. A chi^2 analysis indicates

there is
a 95% probability that the stellar velocity dispersion for r 0.5

pc ... is
less than 120 km s^-1. Perhaps the most important result of this

letter is
that the upper limit of 120 km s^-1 ... is far below the velocities

observed
for gas in this region, which vary over +- 300 km s^-1. ..."

.... snip for brevity ...
The fact is
that the Genzel 2000 paper is directly relevant to the question of
stellar velocity dispersions in the galactic center region, for which
you directly asked for "follow-up papers."


No, it is not. And it is not a direct followup because it doesn't address
either the observations, or the stars addressed in Rieke. That it
contradicts Rieke does not mean that Rieke is 'wrong.' (It is likely that
one or the other are wrong -- or both) Nor does the fact that it is done
later mean it is a 'followup' study.


Interesting. Your original request was for "any followup papers," not
just for direct follow-ups. The papers being discussed certainly
follow up on issues connected to the Rieke & Rieke (1988 == RR) work.

Very well, let us examine the RR paper in more detail. They make
several disclaimers about their analysis, one of them being an
assumption about the core radius of the stellar distribution. More
recent results (Genzel 1996) have demonstrated that the actual core
radius (0.34 +/- 0.1 pc) is significantly broader than the
no-hidden-mass conclusion by RR.

Also, in RR's Table 3, the enclosed mass at 0.5 pc, 2.4 - 6 M_sun, is
consistent with the more recent central mass determinations by other
techniques. There is no heavy contradiction there.

McGinn et al (1989) failed to confirm the result of RR by examining
the integrated starlight near Sgr A*. McGinn found a velocity
dispersion increase with decreasing radius, as expected for a central
compact object. As they point out, "Small number statistics [ in the
Rieke & Rieke study ] may particularly mask a radial gradient in the
velocity dispersion of the bright stars, since only ~10 stars
contribute to its determination in each radius bin." [ Incidentally,
RR does not provide error bars for their measurements. ]

McGinn continues, "It is also possible that bright sources have a
truly different velocity dispersion." Indeed, when performing
flux-limited studies, one is highly susceptible to Malmquist-type
biases (i.e. the brightest objects are typically also the most
aberrant). RR's reliance on only the brightest stars, based on their
spatial resolution limitations, may lead to significant systematic
biases. More complete studies, such as Genzel et al (2000, 2003),
incorporate far more fainter stars, and are thus freer of Malmquist
biases. Of course the Genzel papers *include* the RR stars, plus a
whole lot more. Finally, it's worth noting that RR did not survey
stars within about 5" of Sgr A*, and this is precisely the region
where the velocity dispersion effect would be largest. Follow-on
studies such as the Genzel ones, cover this region more completely,
and are thus far more sensitive to the effect. And in fact, Genzel et
al (2000) find a strong central peak in the velocity dispersion.

Finally, rather ironic that while you criticise the Eckart paper for,
(1) combining data, (2) adjusting data, and (3) excluding stars, it is
true that RR are "guilty" of the same "offenses." Namely, velocity
data from previous studies was combined; they perform various
statistic adjustments to the data based on a number of theoretical
assumptions (eqn 1; Table 3); and they exclude at least one star.

In short, the study of Genzel (2000) is far more statistically
complete than Rieke & Rieke.


Excuse me, but on what do you base your claim of 'statistically complete'?
Genzel only used one star in one study, and half a dozen in another.


You must be mistaken. Genzel et al (2000) consider approximately 300
stars within 23 arcsec of Sgr A*, all of which have either measured
proper motions or radial velocities.

CM

References
Eckart, A. et al 2002, MNRAS, 331, 917
Gehz, A. M. et al 2000, Nature, 407, 349
Genzel, R. et al 2003, ApJ, 594, 812
Genzel, R. et al 2000, MNRAS, 317, 348
Genzel, R. et al 1996, ApJ, 472, 153
Ghez, A. M. et al 2000, Nature, 407, 349
McGinn, M. T. et al 1989, ApJ, 824, 840
Rieke, G. H. & Rieke, M. J. 1988, ApJL, 330, L33
  #22  
Old September 25th 03, 10:05 PM
greywolf42
external usenet poster
 
Posts: n/a
Default Galaxies without dark matter halos?

Joseph Lazio wrote in message
...
"g" == greywolf42 writes:


g LOL! We don't get to pick and choose our data, in science. It's
g quite funny to keep hearing that a 'newer' paper will allow us to
g ignore prior papers that are irritatingly at odds with popular
g theory. The correct approach is to evaluate the differences in the
g approach of the papers, to see which is right.

At the risk of stating the obvious to other readers in this newsgroup,
greywolf's last statement is implicit in the references to newer
papers.


LOL! It's not 'implicit.' You assumption is that 'newer' papers must be
more correct than 'older' papers.

To take just two examples that have appeared in the newsgroup
recently:

* The Galactic center: The IR observations have made enormous
progress over the past 5 years or so by using adaptive optics and
related techniques. I gather that subarcsecond imaging in the GC is
now nearly routine (if the weather cooperates of course! . Thus,
which observation of stars is more likely to be constraining for a
model: One at 2 arcsecond resolution or one at 0.2 arcsecond
resolution?


A complete strawman. Precision in resolution isn't the issue. Ignoring
contradictory data is the issue.

* Globular cluster ages: Globular cluster ages are determined from
stellar models and an HR diagram. A crucial aspect of this
determination is the distance to the globular cluster as it affects
our estimates of stellar luminosities. Recent Hipparcos distance
determinations, which are much higher in accuracy and precision than
previous estimates, have shown that globular clusters were a bit
more distant than thought.


It's not the higher precision, per se, that caused the change. The reason
for the change in globular cluster distances with Hipparcos was that
parallaxes for low-mettalicity halo stars were determined for the first
time.

In turn, that means the stars in them
are brighter and therefore younger. Thus, newer globular cluster
ages are more reliable because they are based on better distance
estimates (and I think improved stellar models as well).


It is because they are based on measurements, instead of just theory. The
low-metallicity simulations 'should' change to meet these new requirements
(I don't know if they've shifted, yet.)

greywolf42
ubi dubium ibi libertas
  #23  
Old September 25th 03, 10:05 PM
greywolf42
external usenet poster
 
Posts: n/a
Default Galaxies without dark matter halos?

Joseph Lazio wrote in message
...
"g" == greywolf42 writes:


g LOL! We don't get to pick and choose our data, in science. It's
g quite funny to keep hearing that a 'newer' paper will allow us to
g ignore prior papers that are irritatingly at odds with popular
g theory. The correct approach is to evaluate the differences in the
g approach of the papers, to see which is right.

At the risk of stating the obvious to other readers in this newsgroup,
greywolf's last statement is implicit in the references to newer
papers.


LOL! It's not 'implicit.' You assumption is that 'newer' papers must be
more correct than 'older' papers.

To take just two examples that have appeared in the newsgroup
recently:

* The Galactic center: The IR observations have made enormous
progress over the past 5 years or so by using adaptive optics and
related techniques. I gather that subarcsecond imaging in the GC is
now nearly routine (if the weather cooperates of course! . Thus,
which observation of stars is more likely to be constraining for a
model: One at 2 arcsecond resolution or one at 0.2 arcsecond
resolution?


A complete strawman. Precision in resolution isn't the issue. Ignoring
contradictory data is the issue.

* Globular cluster ages: Globular cluster ages are determined from
stellar models and an HR diagram. A crucial aspect of this
determination is the distance to the globular cluster as it affects
our estimates of stellar luminosities. Recent Hipparcos distance
determinations, which are much higher in accuracy and precision than
previous estimates, have shown that globular clusters were a bit
more distant than thought.


It's not the higher precision, per se, that caused the change. The reason
for the change in globular cluster distances with Hipparcos was that
parallaxes for low-mettalicity halo stars were determined for the first
time.

In turn, that means the stars in them
are brighter and therefore younger. Thus, newer globular cluster
ages are more reliable because they are based on better distance
estimates (and I think improved stellar models as well).


It is because they are based on measurements, instead of just theory. The
low-metallicity simulations 'should' change to meet these new requirements
(I don't know if they've shifted, yet.)

greywolf42
ubi dubium ibi libertas
  #24  
Old September 25th 03, 10:08 PM
greywolf42
external usenet poster
 
Posts: n/a
Default Galaxies without dark matter halos?

Craig Markwardt wrote in message
...
greywolf42 writes:
Craig Markwardt wrote in message
...

[ ... snip for brevity ... ]



In particular,

(1) the mixing of the two data sets is reasonable, since it
increases the time baseline, thus increasing the sensitivity to
stellar accelerations; you have provided no factual basis to
claim that such combination would be incorrect or biased.


It is not the mixing of two data SETS but the mixing of different TYPES

of
data: "Combines the high precision but shorter time scale NIRC/Keck data
with the lower precision but longer time scale SHARP/NTT data set."


Referring to the Eckart et al (2002) paper, you are apparently
erroneously presupposing that the NIRC/Keck and SHARP/NTT are of
different type, when clearly they are both imaging data, taken with
comparable near-infrared speckle cameras, on large telescopes.


I wasn't presupposing. I was quoting from the paper (see quotation marks).
Apparently, you are the one presupposing.

Questions of precision are handled properly in the analysis.


How do you know? It's not described in the paper.

Furthermore, you have not described how the act of combining two
different data sets would improperly bias the results of Eckart et al.


There are several possibilities. But I would be speculating, because the
method of combing the two types of results is not described in the paper.

My point is that there were several questionable procedures in the paper.
This is one of those questionable situations.

(2) 2D projection effects are a fact of our natural world. They
must be corrected for, in order to infer physical accelerations.


The observable universe is 3D. 2D 'projection' normally is the result

of
theory or of limited representation. In this specific case, we have
'statistically corrections' to the 'observed accelerations' for

'theoretical
projection effects.' A 2D-3D correction would not be statistical. A

theory
of some kind has been imposed onto the data. This makes the result
questionable.


Clearly an optical 2D image of the galactic center, made by an
earthbound telescope, is a limited representation of the 3D
environment there. Therefore, to make any reasonable statements about
the 3D accelerations of stars, one must account for the projection
effects. Some assumptions must be made: however, Eckart shows that
their results are robust over a wide range of assumptions.
Furthermore, you have not challenged any of the assumptions.


True. However such theoretical adjustments would apply to all measurements.
Hence it would not impact the 'statistical' data that Eckart et all
described.


(3) Filtering of data can be a concern. However, whether or not
star "S8" is retained does not change the conclusion that there
is a supermassive compact object at Sgr A*.


It changes the conclusion, because it is contradictory data. S8 is

moving
'too slowly' to allow for the supermassive object inferred by the

'primary'
object. That is why S8 was 'filtered.' That's why I find the study
'questionable.'


This is a fair point. However, considering that the galactic center
has a much higher density of stars than our solar neighborhood, it is
not unreasonable for a steller interaction to occur. One aberrant
star does not necessarily invalidate the others.


I didn't claim absolute certainty.

{snip for brevity}

It is not that *I* care about data. It is the fact that one cannot pick

and
choose data in the scientific method.


This is a fallacy.


No, its the one of the fundamental requirements of the scientific method.

Of course scientists pick and choose the data.


People holding PhD's might do this. But they wouldn't be scientists. By
definition. It matters not what their job title is.

Would you say that scientists should use their uncalibrated data?


Good scientists don't use uncalibrated instruments.

Observations taken with the dome closed?
taken when the sun was up?
Of course not. The point is that observations must be chosen on an
objective basis.


Your straw man is specious. If you can't see an object, one obviously
cannot have an observation.

Science requirest that all observations be explained. Now, if one later
finds out that the power supply spiked during the measurement, or the
measurement can't be repeated -- then one can lower the weight of the
observation. But only for objective, physical failures of the measurment --
NEVER for theoretical preferences.

The Sch\"odel et al (2002) paper chooses the particular star "S2"
precisely because nearly one complete orbit is detectable, which is
untrue for any other known star near the galactic center.


That is indeed the theoretical interpretation. But 'S8' -- measured with
the same instruments -- contradicts the theoretical conclusions of the
paper.

And your
pick-and-choose argument is further fallacious because many stars with
*some* or *marginal* curvature in their proper motions are discussed
by the same authors in Eckart et al (2002).


Science requires them to use *all* stars that they measure. Not just one or
a selected 'many'.

And further, Genzel et al
(2000, 2003) treat the ensemble of *all* known stars in the galactic
center region. So the facts demonstrate quite the opposite: the
Genzel group has consistently examined a broad spectrum of evidence.


Since you snipped my reply to your earlier attempt to bring Genzel in to
justify Eckart, I'll replace this part of the snip here, instead of later.
==========================================
gw:
This paper is not the three provided, above, and under discussion. What is
the need for constantly proffering different studies? Can't you just
address the issue, above?

CM:
"What I need is also irrelevant. It is ironic that you are carefully
'picking and choosing' the studies that you will discuss."

gw:
"I'm addressing the three papers first proffered as evidence. I consider
all three papers lacking at a very basic, fundamental level. I have no
desire to discuss other papers until these three are 'disposed of.'"

" I am trying to make a point, here. I am trying to point out to you the
slender theoretical threads upon which you are hanging your arguments. And
you keep changing the focus of your arguments."

"I'm attempting to induce at least a minor acknowledgement of doubt inherent
in such arcane, theory-filled and data-selected studies. The conclusions of
such studies are not 'facts.'"
==========================================

It is certainly a fact that once
measured, a Keplerian orbit determines the central object mass and
minimum mass density.


Even if you only have one star, and if you first assume a theory of the
forces (to 'call' it 'Keplerian') -- it is still not a 'fact.'

Observations
are facts. Conclusions are not.

And even then, it is not even correct if other stars in the same region
contradict the conclusion. And that is why the conclusions are
questionable.


It is a fact that the measured trajectory of star "S2" is consistent
with a Keplerian ellipse. It is also a fact that the measured time
profile of the motion of star "S2" is consistent with Keplerian
motion. It is a fact that the central mass inferred from the
Keplerian solution is consistent with other mass determinations.
Finally, it is a fact that the position of the central attractor of
"S2" is consistent with Sgr A* to within the estimated 2 sigma errors.


The above are all theoretical conclusions. Not bad ones, necessarily. But
not facts.

How one interprets these facts is another matter. However, the
signatures of Keplerian orbits are very distinct. I am unaware of any
alternate interpretation -- by you, or another -- which is consistent
with the data.


Such is not needed to identify the study as questionable.

No one piece of evidence will clinch or refute the conclusion that
there is a supermassive black hole associated with Sgr A*. However,
the S2 orbit appears to be an extremely strong piece of evidence in
favor.


And the orbit of S8 a piece of evidence contradicting it.


[ ... ]
None of these address all the stars found in the Rieke's paper.
Specifically, instead of focusing on one or a few individual
stars selected for their apparent support of one theory,
one needs to address the following findings from
Rieke and Rieke:

"The simplest model with a central black hole that dominates the
mass within 2 pc would have a velocity dispersion increasing
as r^-1/2; clearly there is no indication of this trend in our

data.
A chi^2 analysis indicates there is a 95% probability that the
stellar velocity dispersion for r 0.5 pc ... is less than
120 km s^-1. Perhaps the most important result of this letter

is
that the upper limit of 120 km s^-1 ... is far below the
velocities observed for gas in this region, which vary
over +- 300 km s^-1. ..."


... snip for brevity ...


{Snip replaced, above}

The fact is
that the Genzel 2000 paper is directly relevant to the question of
stellar velocity dispersions in the galactic center region, for which
you directly asked for "follow-up papers."


No, it is not. And it is not a direct followup because it doesn't

address
either the observations, or the stars addressed in Rieke. That it
contradicts Rieke does not mean that Rieke is 'wrong.' (It is likely

that
one or the other are wrong -- or both) Nor does the fact that it is

done
later mean it is a 'followup' study.


Interesting. Your original request was for "any followup papers," not
just for direct follow-ups. The papers being discussed certainly
follow up on issues connected to the Rieke & Rieke (1988 == RR) work.


Not unless they address R&R.

Very well, let us examine the RR paper in more detail. They make
several disclaimers about their analysis, one of them being an
assumption about the core radius of the stellar distribution. More
recent results (Genzel 1996) have demonstrated that the actual core
radius (0.34 +/- 0.1 pc) is significantly broader than the
no-hidden-mass conclusion by RR.


Again, you are assuming that a later paper invalidates the result of a prior
paper, simply because it is later. (i.e. that Genzel reaches different
conclusions than RR does not, per se, invalidate either paper.)

But you were unclear about whether your think RR's 'core radius' is an
assumption (your second sentence) or a conclusion (your final sentence).

Also, in RR's Table 3, the enclosed mass at 0.5 pc, 2.4 - 6 M_sun, is
consistent with the more recent central mass determinations by other
techniques. There is no heavy contradiction there.


I agree that apparent enclosed mass is not a point of significant
contention.

McGinn et al (1989) failed to confirm the result of RR by examining
the integrated starlight near Sgr A*. McGinn found a velocity
dispersion increase with decreasing radius, as expected for a central
compact object.


Now McGinn is a followup study to Rieke and Rieke! Had you presented McGinn
before Eckhart, etc, we would have avoided a lot of effort.

As they point out, "Small number statistics [ in the
Rieke & Rieke study ] may particularly mask a radial gradient in the
velocity dispersion of the bright stars, since only ~10 stars
contribute to its determination in each radius bin." [ Incidentally,
RR does not provide error bars for their measurements. ]


True, but specious. There are no 'error bars' on the *data* given in Table
1. However, the errors are discussed in detail in section III. And there
are error bars are provided in Table 3 (conclusions).

McGinn continues, "It is also possible that bright sources have a
truly different velocity dispersion."


That, indeed, was one of the conclusions of R&R's paper. That gas and stars
move differently.

It was also one of my early points. That O and B stars do not necessarily
move like later-type stars.

Indeed, when performing
flux-limited studies, one is highly susceptible to Malmquist-type
biases (i.e. the brightest objects are typically also the most
aberrant). RR's reliance on only the brightest stars, based on their
spatial resolution limitations, may lead to significant systematic
biases.


However, the point then would be to explain why brighter stars move
differently than dimmer stars. Merely noting that this *MAY* be a
difference between M&M and McGinn is not sufficient. However, since McGinn
used 'integrated starlight' instead of specific stars (as in MM), there is
yet another difference in analysis to complicate the issue.

More complete studies, such as Genzel et al (2000, 2003),
incorporate far more fainter stars, and are thus freer of Malmquist
biases.


Not if they're fundamentally the same (i.e. O and B stars instead of just
O).

Of course the Genzel papers *include* the RR stars, plus a
whole lot more.


It would be interesting to see the measurements Genzel obtains for the RR
stars. Since 43 stars is not a statistically insignificant sample. (It's
far larger than the sample of 1 star or 6 that you tout for Eckart).

Finally, it's worth noting that RR did not survey
stars within about 5" of Sgr A*, and this is precisely the region
where the velocity dispersion effect would be largest.


And regions within .0005" of SgrA* would be even stronger. So what? R&R
evaluated motions from .36pc to 6 pc. And their data was suffienctly
precise to measure the predicted dependence -- but it was not found.

Follow-on
studies such as the Genzel ones, cover this region more completely,
and are thus far more sensitive to the effect. And in fact, Genzel et
al (2000) find a strong central peak in the velocity dispersion.


And a followup study would compare the results to MM. And explain the
difference. Merely noting that more stars were measured does not
necessarily explain the difference in conclusion. Becuase MM had quite
sufficient measurment numbers and precision to see the predicted effect --
and didn't.

Finally, rather ironic that while you criticise the Eckart paper for,
(1) combining data, (2) adjusting data, and (3) excluding stars, it is
true that RR are "guilty" of the same "offenses." Namely, velocity
data from previous studies was combined;


I did not criticize Eckart for combing data, per se, but for combining
different TYPES of data. R&R do not combine different types of data.

they perform various
statistic adjustments to the data based on a number of theoretical
assumptions (eqn 1; Table 3);


R&R do not adjust DATA. They adjust their CONCLUSION of mass estimates.
And they do this for two assumptions to choose between the two conclusions
(dominant central mass and self-gravitating mass). They represent the
"extremes the actual mass distribution may take." Eckart actually 'adjusts'
the data.

and they exclude at least one star.


R&R do not exclude any star from the data or the statistical analysis. What
they do is examine the probability that one of the stars is an interloper
(prob 0.4) and then assume that they have actually caught 2 interlopers.
They then examine the effect on the results of being 'tricked' by
interlopers. (No net effect.)

In short, the study of Genzel (2000) is far more statistically
complete than Rieke & Rieke.


Excuse me, but on what do you base your claim of 'statistically

complete'?
Genzel only used one star in one study, and half a dozen in another.


You must be mistaken. Genzel et al (2000) consider approximately 300
stars within 23 arcsec of Sgr A*, all of which have either measured
proper motions or radial velocities.


Sorry, I didn't notice that you had again gone back to Genzel -- since our
discussions were on Eckart.

References
Eckart, A. et al 2002, MNRAS, 331, 917
Gehz, A. M. et al 2000, Nature, 407, 349
Genzel, R. et al 2003, ApJ, 594, 812
Genzel, R. et al 2000, MNRAS, 317, 348
Genzel, R. et al 1996, ApJ, 472, 153
Ghez, A. M. et al 2000, Nature, 407, 349
McGinn, M. T. et al 1989, ApJ, 824, 840
Rieke, G. H. & Rieke, M. J. 1988, ApJL, 330, L33


greywolf42
ubi dubium ibi libertas
  #25  
Old September 25th 03, 10:08 PM
greywolf42
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Posts: n/a
Default Galaxies without dark matter halos?

Craig Markwardt wrote in message
...
greywolf42 writes:
Craig Markwardt wrote in message
...

[ ... snip for brevity ... ]



In particular,

(1) the mixing of the two data sets is reasonable, since it
increases the time baseline, thus increasing the sensitivity to
stellar accelerations; you have provided no factual basis to
claim that such combination would be incorrect or biased.


It is not the mixing of two data SETS but the mixing of different TYPES

of
data: "Combines the high precision but shorter time scale NIRC/Keck data
with the lower precision but longer time scale SHARP/NTT data set."


Referring to the Eckart et al (2002) paper, you are apparently
erroneously presupposing that the NIRC/Keck and SHARP/NTT are of
different type, when clearly they are both imaging data, taken with
comparable near-infrared speckle cameras, on large telescopes.


I wasn't presupposing. I was quoting from the paper (see quotation marks).
Apparently, you are the one presupposing.

Questions of precision are handled properly in the analysis.


How do you know? It's not described in the paper.

Furthermore, you have not described how the act of combining two
different data sets would improperly bias the results of Eckart et al.


There are several possibilities. But I would be speculating, because the
method of combing the two types of results is not described in the paper.

My point is that there were several questionable procedures in the paper.
This is one of those questionable situations.

(2) 2D projection effects are a fact of our natural world. They
must be corrected for, in order to infer physical accelerations.


The observable universe is 3D. 2D 'projection' normally is the result

of
theory or of limited representation. In this specific case, we have
'statistically corrections' to the 'observed accelerations' for

'theoretical
projection effects.' A 2D-3D correction would not be statistical. A

theory
of some kind has been imposed onto the data. This makes the result
questionable.


Clearly an optical 2D image of the galactic center, made by an
earthbound telescope, is a limited representation of the 3D
environment there. Therefore, to make any reasonable statements about
the 3D accelerations of stars, one must account for the projection
effects. Some assumptions must be made: however, Eckart shows that
their results are robust over a wide range of assumptions.
Furthermore, you have not challenged any of the assumptions.


True. However such theoretical adjustments would apply to all measurements.
Hence it would not impact the 'statistical' data that Eckart et all
described.


(3) Filtering of data can be a concern. However, whether or not
star "S8" is retained does not change the conclusion that there
is a supermassive compact object at Sgr A*.


It changes the conclusion, because it is contradictory data. S8 is

moving
'too slowly' to allow for the supermassive object inferred by the

'primary'
object. That is why S8 was 'filtered.' That's why I find the study
'questionable.'


This is a fair point. However, considering that the galactic center
has a much higher density of stars than our solar neighborhood, it is
not unreasonable for a steller interaction to occur. One aberrant
star does not necessarily invalidate the others.


I didn't claim absolute certainty.

{snip for brevity}

It is not that *I* care about data. It is the fact that one cannot pick

and
choose data in the scientific method.


This is a fallacy.


No, its the one of the fundamental requirements of the scientific method.

Of course scientists pick and choose the data.


People holding PhD's might do this. But they wouldn't be scientists. By
definition. It matters not what their job title is.

Would you say that scientists should use their uncalibrated data?


Good scientists don't use uncalibrated instruments.

Observations taken with the dome closed?
taken when the sun was up?
Of course not. The point is that observations must be chosen on an
objective basis.


Your straw man is specious. If you can't see an object, one obviously
cannot have an observation.

Science requirest that all observations be explained. Now, if one later
finds out that the power supply spiked during the measurement, or the
measurement can't be repeated -- then one can lower the weight of the
observation. But only for objective, physical failures of the measurment --
NEVER for theoretical preferences.

The Sch\"odel et al (2002) paper chooses the particular star "S2"
precisely because nearly one complete orbit is detectable, which is
untrue for any other known star near the galactic center.


That is indeed the theoretical interpretation. But 'S8' -- measured with
the same instruments -- contradicts the theoretical conclusions of the
paper.

And your
pick-and-choose argument is further fallacious because many stars with
*some* or *marginal* curvature in their proper motions are discussed
by the same authors in Eckart et al (2002).


Science requires them to use *all* stars that they measure. Not just one or
a selected 'many'.

And further, Genzel et al
(2000, 2003) treat the ensemble of *all* known stars in the galactic
center region. So the facts demonstrate quite the opposite: the
Genzel group has consistently examined a broad spectrum of evidence.


Since you snipped my reply to your earlier attempt to bring Genzel in to
justify Eckart, I'll replace this part of the snip here, instead of later.
==========================================
gw:
This paper is not the three provided, above, and under discussion. What is
the need for constantly proffering different studies? Can't you just
address the issue, above?

CM:
"What I need is also irrelevant. It is ironic that you are carefully
'picking and choosing' the studies that you will discuss."

gw:
"I'm addressing the three papers first proffered as evidence. I consider
all three papers lacking at a very basic, fundamental level. I have no
desire to discuss other papers until these three are 'disposed of.'"

" I am trying to make a point, here. I am trying to point out to you the
slender theoretical threads upon which you are hanging your arguments. And
you keep changing the focus of your arguments."

"I'm attempting to induce at least a minor acknowledgement of doubt inherent
in such arcane, theory-filled and data-selected studies. The conclusions of
such studies are not 'facts.'"
==========================================

It is certainly a fact that once
measured, a Keplerian orbit determines the central object mass and
minimum mass density.


Even if you only have one star, and if you first assume a theory of the
forces (to 'call' it 'Keplerian') -- it is still not a 'fact.'

Observations
are facts. Conclusions are not.

And even then, it is not even correct if other stars in the same region
contradict the conclusion. And that is why the conclusions are
questionable.


It is a fact that the measured trajectory of star "S2" is consistent
with a Keplerian ellipse. It is also a fact that the measured time
profile of the motion of star "S2" is consistent with Keplerian
motion. It is a fact that the central mass inferred from the
Keplerian solution is consistent with other mass determinations.
Finally, it is a fact that the position of the central attractor of
"S2" is consistent with Sgr A* to within the estimated 2 sigma errors.


The above are all theoretical conclusions. Not bad ones, necessarily. But
not facts.

How one interprets these facts is another matter. However, the
signatures of Keplerian orbits are very distinct. I am unaware of any
alternate interpretation -- by you, or another -- which is consistent
with the data.


Such is not needed to identify the study as questionable.

No one piece of evidence will clinch or refute the conclusion that
there is a supermassive black hole associated with Sgr A*. However,
the S2 orbit appears to be an extremely strong piece of evidence in
favor.


And the orbit of S8 a piece of evidence contradicting it.


[ ... ]
None of these address all the stars found in the Rieke's paper.
Specifically, instead of focusing on one or a few individual
stars selected for their apparent support of one theory,
one needs to address the following findings from
Rieke and Rieke:

"The simplest model with a central black hole that dominates the
mass within 2 pc would have a velocity dispersion increasing
as r^-1/2; clearly there is no indication of this trend in our

data.
A chi^2 analysis indicates there is a 95% probability that the
stellar velocity dispersion for r 0.5 pc ... is less than
120 km s^-1. Perhaps the most important result of this letter

is
that the upper limit of 120 km s^-1 ... is far below the
velocities observed for gas in this region, which vary
over +- 300 km s^-1. ..."


... snip for brevity ...


{Snip replaced, above}

The fact is
that the Genzel 2000 paper is directly relevant to the question of
stellar velocity dispersions in the galactic center region, for which
you directly asked for "follow-up papers."


No, it is not. And it is not a direct followup because it doesn't

address
either the observations, or the stars addressed in Rieke. That it
contradicts Rieke does not mean that Rieke is 'wrong.' (It is likely

that
one or the other are wrong -- or both) Nor does the fact that it is

done
later mean it is a 'followup' study.


Interesting. Your original request was for "any followup papers," not
just for direct follow-ups. The papers being discussed certainly
follow up on issues connected to the Rieke & Rieke (1988 == RR) work.


Not unless they address R&R.

Very well, let us examine the RR paper in more detail. They make
several disclaimers about their analysis, one of them being an
assumption about the core radius of the stellar distribution. More
recent results (Genzel 1996) have demonstrated that the actual core
radius (0.34 +/- 0.1 pc) is significantly broader than the
no-hidden-mass conclusion by RR.


Again, you are assuming that a later paper invalidates the result of a prior
paper, simply because it is later. (i.e. that Genzel reaches different
conclusions than RR does not, per se, invalidate either paper.)

But you were unclear about whether your think RR's 'core radius' is an
assumption (your second sentence) or a conclusion (your final sentence).

Also, in RR's Table 3, the enclosed mass at 0.5 pc, 2.4 - 6 M_sun, is
consistent with the more recent central mass determinations by other
techniques. There is no heavy contradiction there.


I agree that apparent enclosed mass is not a point of significant
contention.

McGinn et al (1989) failed to confirm the result of RR by examining
the integrated starlight near Sgr A*. McGinn found a velocity
dispersion increase with decreasing radius, as expected for a central
compact object.


Now McGinn is a followup study to Rieke and Rieke! Had you presented McGinn
before Eckhart, etc, we would have avoided a lot of effort.

As they point out, "Small number statistics [ in the
Rieke & Rieke study ] may particularly mask a radial gradient in the
velocity dispersion of the bright stars, since only ~10 stars
contribute to its determination in each radius bin." [ Incidentally,
RR does not provide error bars for their measurements. ]


True, but specious. There are no 'error bars' on the *data* given in Table
1. However, the errors are discussed in detail in section III. And there
are error bars are provided in Table 3 (conclusions).

McGinn continues, "It is also possible that bright sources have a
truly different velocity dispersion."


That, indeed, was one of the conclusions of R&R's paper. That gas and stars
move differently.

It was also one of my early points. That O and B stars do not necessarily
move like later-type stars.

Indeed, when performing
flux-limited studies, one is highly susceptible to Malmquist-type
biases (i.e. the brightest objects are typically also the most
aberrant). RR's reliance on only the brightest stars, based on their
spatial resolution limitations, may lead to significant systematic
biases.


However, the point then would be to explain why brighter stars move
differently than dimmer stars. Merely noting that this *MAY* be a
difference between M&M and McGinn is not sufficient. However, since McGinn
used 'integrated starlight' instead of specific stars (as in MM), there is
yet another difference in analysis to complicate the issue.

More complete studies, such as Genzel et al (2000, 2003),
incorporate far more fainter stars, and are thus freer of Malmquist
biases.


Not if they're fundamentally the same (i.e. O and B stars instead of just
O).

Of course the Genzel papers *include* the RR stars, plus a
whole lot more.


It would be interesting to see the measurements Genzel obtains for the RR
stars. Since 43 stars is not a statistically insignificant sample. (It's
far larger than the sample of 1 star or 6 that you tout for Eckart).

Finally, it's worth noting that RR did not survey
stars within about 5" of Sgr A*, and this is precisely the region
where the velocity dispersion effect would be largest.


And regions within .0005" of SgrA* would be even stronger. So what? R&R
evaluated motions from .36pc to 6 pc. And their data was suffienctly
precise to measure the predicted dependence -- but it was not found.

Follow-on
studies such as the Genzel ones, cover this region more completely,
and are thus far more sensitive to the effect. And in fact, Genzel et
al (2000) find a strong central peak in the velocity dispersion.


And a followup study would compare the results to MM. And explain the
difference. Merely noting that more stars were measured does not
necessarily explain the difference in conclusion. Becuase MM had quite
sufficient measurment numbers and precision to see the predicted effect --
and didn't.

Finally, rather ironic that while you criticise the Eckart paper for,
(1) combining data, (2) adjusting data, and (3) excluding stars, it is
true that RR are "guilty" of the same "offenses." Namely, velocity
data from previous studies was combined;


I did not criticize Eckart for combing data, per se, but for combining
different TYPES of data. R&R do not combine different types of data.

they perform various
statistic adjustments to the data based on a number of theoretical
assumptions (eqn 1; Table 3);


R&R do not adjust DATA. They adjust their CONCLUSION of mass estimates.
And they do this for two assumptions to choose between the two conclusions
(dominant central mass and self-gravitating mass). They represent the
"extremes the actual mass distribution may take." Eckart actually 'adjusts'
the data.

and they exclude at least one star.


R&R do not exclude any star from the data or the statistical analysis. What
they do is examine the probability that one of the stars is an interloper
(prob 0.4) and then assume that they have actually caught 2 interlopers.
They then examine the effect on the results of being 'tricked' by
interlopers. (No net effect.)

In short, the study of Genzel (2000) is far more statistically
complete than Rieke & Rieke.


Excuse me, but on what do you base your claim of 'statistically

complete'?
Genzel only used one star in one study, and half a dozen in another.


You must be mistaken. Genzel et al (2000) consider approximately 300
stars within 23 arcsec of Sgr A*, all of which have either measured
proper motions or radial velocities.


Sorry, I didn't notice that you had again gone back to Genzel -- since our
discussions were on Eckart.

References
Eckart, A. et al 2002, MNRAS, 331, 917
Gehz, A. M. et al 2000, Nature, 407, 349
Genzel, R. et al 2003, ApJ, 594, 812
Genzel, R. et al 2000, MNRAS, 317, 348
Genzel, R. et al 1996, ApJ, 472, 153
Ghez, A. M. et al 2000, Nature, 407, 349
McGinn, M. T. et al 1989, ApJ, 824, 840
Rieke, G. H. & Rieke, M. J. 1988, ApJL, 330, L33


greywolf42
ubi dubium ibi libertas
  #26  
Old October 5th 03, 04:09 PM
greywolf42
external usenet poster
 
Posts: n/a
Default Galaxies without dark matter halos?

Joseph Lazio wrote in message
...
"g" == greywolf42 writes:


g Craig Markwardt wrote in message
g ...

(2) 2D projection effects are a fact of our natural world. They
must be corrected for, in order to infer physical accelerations.


g The observable universe is 3D. 2D 'projection' normally is the
g result of theory or of limited representation. In this specific
g case, we have 'statistically corrections' to the 'observed
g accelerations' for 'theoretical projection effects.' A 2D-3D
g correction would not be statistical. A theory of some kind has
g been imposed onto the data. This makes the result questionable.

I think we will all agree that the Universe is 3-D. At the risk of
stating the obvious, the sky is not 3-D. If one watches an object in
orbit and sees that it describes an elliptical path, does this mean
that the orbit is intrinsically elliptical or that the orbit is
circular and inclined to our line of sight?


I thought all orbits were elliptical, since Kepler. (Sorry, couldn't
resist.)

The inclination of the orbit is one of those things that it is very
difficult to determine in astronomy. We can't tell -- just by looking --
whether we're looking nearly edge-on or straight down onto the plane of the
orbit.

(It's a similar argument to why don't all spiral galaxies look
circular? Because their disks are inclined to our line of sight.)


However, we expect all disk galaxies to be roughly circular. Whereas
stellar orbits are expected to be elliptical -- sometimes highly so.

g This merely repeats the logical fallacy that if it is 'old' it must
g be wrong. Scientific observations are NEVER 'outdated'. An
g observation may later be found to be erroneous -- due to a specific
g finding of a flaw used in the instruments or methods. At which
g point the study or experiment is superseded. But it is never
g superseded simply by having somebody else use different methods
g that come to different conclusions.

Their study used an imager with ~1 arcsec seeing, and did not
resolve any stars within 1-2 arcsec of Sgr A.


g And this statement is relevant, how?

Because of a reason you don't cite above. Scientific observations can
be superseded by improvements in technology.


But I *did* discuss this earlier in the thread (you snipped it). And I
pointed out that such a view was incorrect and unscientific.

That doesn't mean the
old observations were "wrong," they are just not as useful. If you
are trying to study the motions of stars in the central region of the
Galaxy, angular resolution is vital, the more the better. At 1 arcsec
resolution one has to worry about confusion (two stars being so close
together that they appear as one). Moreover, 1 arcsec at the distance
of the Galactic center corresponds to about 0.04 pc. Current
observations can probe well within this region while older ones,
because of their limited angular resolution, could not.


Later publication still does not invalidate prior observation. Only if you
can explain -- from fundamental causation -- why earlier observations were
'wrong' is the prior work invalidated.

greywolf42
ubi dubium ibi libertas
  #27  
Old October 5th 03, 04:09 PM
greywolf42
external usenet poster
 
Posts: n/a
Default Galaxies without dark matter halos?

Joseph Lazio wrote in message
...
"g" == greywolf42 writes:


g Craig Markwardt wrote in message
g ...

(2) 2D projection effects are a fact of our natural world. They
must be corrected for, in order to infer physical accelerations.


g The observable universe is 3D. 2D 'projection' normally is the
g result of theory or of limited representation. In this specific
g case, we have 'statistically corrections' to the 'observed
g accelerations' for 'theoretical projection effects.' A 2D-3D
g correction would not be statistical. A theory of some kind has
g been imposed onto the data. This makes the result questionable.

I think we will all agree that the Universe is 3-D. At the risk of
stating the obvious, the sky is not 3-D. If one watches an object in
orbit and sees that it describes an elliptical path, does this mean
that the orbit is intrinsically elliptical or that the orbit is
circular and inclined to our line of sight?


I thought all orbits were elliptical, since Kepler. (Sorry, couldn't
resist.)

The inclination of the orbit is one of those things that it is very
difficult to determine in astronomy. We can't tell -- just by looking --
whether we're looking nearly edge-on or straight down onto the plane of the
orbit.

(It's a similar argument to why don't all spiral galaxies look
circular? Because their disks are inclined to our line of sight.)


However, we expect all disk galaxies to be roughly circular. Whereas
stellar orbits are expected to be elliptical -- sometimes highly so.

g This merely repeats the logical fallacy that if it is 'old' it must
g be wrong. Scientific observations are NEVER 'outdated'. An
g observation may later be found to be erroneous -- due to a specific
g finding of a flaw used in the instruments or methods. At which
g point the study or experiment is superseded. But it is never
g superseded simply by having somebody else use different methods
g that come to different conclusions.

Their study used an imager with ~1 arcsec seeing, and did not
resolve any stars within 1-2 arcsec of Sgr A.


g And this statement is relevant, how?

Because of a reason you don't cite above. Scientific observations can
be superseded by improvements in technology.


But I *did* discuss this earlier in the thread (you snipped it). And I
pointed out that such a view was incorrect and unscientific.

That doesn't mean the
old observations were "wrong," they are just not as useful. If you
are trying to study the motions of stars in the central region of the
Galaxy, angular resolution is vital, the more the better. At 1 arcsec
resolution one has to worry about confusion (two stars being so close
together that they appear as one). Moreover, 1 arcsec at the distance
of the Galactic center corresponds to about 0.04 pc. Current
observations can probe well within this region while older ones,
because of their limited angular resolution, could not.


Later publication still does not invalidate prior observation. Only if you
can explain -- from fundamental causation -- why earlier observations were
'wrong' is the prior work invalidated.

greywolf42
ubi dubium ibi libertas
  #28  
Old October 6th 03, 07:04 PM
John Chandler
external usenet poster
 
Posts: n/a
Default Galaxies without dark matter halos?

greywolf42 wrote:
: I thought all orbits were elliptical, since Kepler. (Sorry, couldn't
: resist.)

: The inclination of the orbit is one of those things that it is very
: difficult to determine in astronomy. We can't tell -- just by looking --
: whether we're looking nearly edge-on or straight down onto the plane of the
: orbit.

Actually, Kepler gave us more than the elliptical shape of orbits.
He also discovered that the body treated as "at rest" occupies one
focus of the ellipse and that the angular velocity varies inversely
with the radial distance. Using those two additional properties of
orbits allows us to tell what the inclination is.

--
John F. Chandler
  #29  
Old October 6th 03, 07:04 PM
John Chandler
external usenet poster
 
Posts: n/a
Default Galaxies without dark matter halos?

greywolf42 wrote:
: I thought all orbits were elliptical, since Kepler. (Sorry, couldn't
: resist.)

: The inclination of the orbit is one of those things that it is very
: difficult to determine in astronomy. We can't tell -- just by looking --
: whether we're looking nearly edge-on or straight down onto the plane of the
: orbit.

Actually, Kepler gave us more than the elliptical shape of orbits.
He also discovered that the body treated as "at rest" occupies one
focus of the ellipse and that the angular velocity varies inversely
with the radial distance. Using those two additional properties of
orbits allows us to tell what the inclination is.

--
John F. Chandler
  #30  
Old October 7th 03, 08:51 PM
greywolf42
external usenet poster
 
Posts: n/a
Default Galaxies without dark matter halos?

John Chandler wrote in message
...
greywolf42 wrote:



: The inclination of the orbit is one of those things that it is very
: difficult to determine in astronomy. We can't tell -- just by
: looking -- whether we're looking nearly edge-on or straight
: down onto the plane of the orbit.

Actually, Kepler gave us more than the elliptical shape of orbits.
He also discovered that the body treated as "at rest" occupies one
focus of the ellipse and that the angular velocity varies inversely
with the radial distance.


Historically, the latter is Newton, not Kepler. Kepler discovered that
planets sweep out equal areas in equal time. Similar, but not the same.

Using those two additional properties of
orbits allows us to tell what the inclination is.


The problem is that we don't know the 'angular velocity' of the orbit. We
can only directly measure the radial portion of the speed projected in our
direction.

greywolf42
ubi dubium ibi libertas
 




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