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