Galaxies without dark matter halos?

greywolf42 writes:
[ ... ]
http://www.arxiv.org/abs/astro-ph/0201031

"Stellar Orbits Near Sagittarius A*," Eckart, Genzel, Schodel, Jan 2002

Mixes data types:
Combines the high precision but shorter time scale NIRC/Keck data with the
lower precision but longer time scale SHARP/NTT data set;

"Adjusts" data:
Statistically corrects the observed accelerations for theoretical projection
effects;

Drops "contradictory" data:
Excludes star S8 from the analysis of the amount and position of the central
mass. ("... this star either was or is subject to a close interaction with a
different object or that its position measurements are influenced by the
emission of a different cluster star.")

The three factors you bring up are *differences* between the Eckart
work and previous work by Ghez et al (2000). Despite this, the
results of Eckart and Ghez are consistent with one another, and with
the previous work by Genzel et al (2000). Furthermore the techniques
applied by the Eckart paper appear to be appropriate, and would tend
to *decrease* rather than increase observational biases. Therefore
your criticisms appear to be essentially irrelevant.



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.

23 authors (I'm afraid I'll always start out highly skeptical of such an
obviously political effort). One star as primary data. Mixing of VLBI and
infrared data. "Correcting" data to match theory (Sgr A* position was
'moved' to fit the theory). No mention of the other stars that don't seem
to be moving in this fashion.

Your "political" comment is irrelevant. Your other comments imply
your disapproval. However, as the authors say, "

The remarkable consequence of the orbital technique is that the mass
can be determined from a single stellar orbit, in comparison to the
statistical techniques that use several tens to hundreds of stellar
velocities at 10 to 300 light days from SgrA* (Fig. 3). In addition,
the orbital technique requires fewer assumptions than the other
estimates (for example, equilibrium and isotropy of orbits), and
thus is less vulnerable to systematic effects.

The combination of VLBI and infrared data allows an extremely precise
alignment of the stars (IR) and Sgr A* (radio). The position of Sgr
A* can be considered a nuisance parameter since it is not relevant to
the mass determination (and in any case is within 1.6 sigma of the
previously determined position).



http://www.arxiv.org/abs/astro-ph/0306214

"Stellar Dynamics in the Central arcsecond of our galaxy," June 2003,
Schodel et al Followup from 0210426. 6 Stars plotted. Retains the
"adjusted" data (postion of Sgr A*).


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. ..."


Genzel et al (2000) show, using an ensemble of stars, that the central
velocity dispersion is of order 280-350 km/s, and decreases with
radius between r^-1 and r^-0.5, so Rieke's findings appear to be
addressed. However, these statistical measurements really take second
place to a direct measurment of a Keplerian orbit around the central
mass, including both pericenter and apocenter, which both clinches the
mass of the central body, and highly constrains its density (since
pericenter is within 17 lt-hr = 120 AU).

CM

References
Genzel, R. et al 2000, MNRAS, 317, 348
Ghez, A. M. et al 2000, Nature, 407, 349

Craig Markwardt wrote in message
...
greywolf42 writes:
[ ... ]
http://www.arxiv.org/abs/astro-ph/0201031

"Stellar Orbits Near Sagittarius A*," Eckart, Genzel, Schodel, Jan 2002

Mixes data types:
Combines the high precision but shorter time scale NIRC/Keck data with
the
lower precision but longer time scale SHARP/NTT data set;

"Adjusts" data:
Statistically corrects the observed accelerations for theoretical
projection
effects;

Drops "contradictory" data:
Excludes star S8 from the analysis of the amount and position of the
central
mass. ("... this star either was or is subject to a close interaction
with a
different object or that its position measurements are influenced by the
emission of a different cluster star.")

The three factors you bring up are *differences* between the Eckart
work and previous work by Ghez et al (2000).

No. These are specific quotes from the specific references. I made no
attempt to compare these to 'previous work.'

Despite this, the
results of Eckart and Ghez are consistent with one another, and with
the previous work by Genzel et al (2000). Furthermore the techniques
applied by the Eckart paper appear to be appropriate, and would tend
to *decrease* rather than increase observational biases. Therefore
your criticisms appear to be essentially irrelevant.

My observations of questionable manipulations of data are not irrelevant at
all.

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.

23 authors (I'm afraid I'll always start out highly skeptical of such an
obviously political effort). One star as primary data. Mixing of VLBI
and
infrared data. "Correcting" data to match theory (Sgr A* position was
'moved' to fit the theory). No mention of the other stars that don't
seem
to be moving in this fashion.

Your "political" comment is irrelevant.

It was a statement of a bias on my part.

Your other comments imply your disapproval.

Yes.

However, as the authors say, "
The remarkable consequence of the orbital technique is that the mass
can be determined from a single stellar orbit, in comparison to the
statistical techniques that use several tens to hundreds of stellar
velocities at 10 to 300 light days from SgrA* (Fig. 3). In addition,
the orbital technique requires fewer assumptions than the other
estimates (for example, equilibrium and isotropy of orbits), and
thus is less vulnerable to systematic effects.

The combination of VLBI and infrared data allows an extremely precise
alignment of the stars (IR) and Sgr A* (radio). The position of Sgr
A* can be considered a nuisance parameter since it is not relevant to
the mass determination (and in any case is within 1.6 sigma of the
previously determined position).

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.)


http://www.arxiv.org/abs/astro-ph/0306214

"Stellar Dynamics in the Central arcsecond of our galaxy," June 2003,
Schodel et al Followup from 0210426. 6 Stars plotted. Retains the
"adjusted" data (postion of Sgr A*).

No comment provided on this study.

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. ..."


Genzel et al (2000) show,

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?

using an ensemble of stars, that the central
velocity dispersion is of order 280-350 km/s, and decreases with
radius between r^-1 and r^-0.5, so Rieke's findings appear to be
addressed.

Nonsense. Rieke's findings are not addressed at all. That you have found a
study that appears to contradict Rieke, does not mean that Rieke is the
paper in error. One must actually 'address' Rieke's study to 'address' the
study. (i.e. identify a likely source of error in one or the other)

However, these statistical measurements really take second
place to a direct measurment of a Keplerian orbit around the central
mass, including both pericenter and apocenter, which both clinches the
mass of the central body, and highly constrains its density (since
pericenter is within 17 lt-hr = 120 AU).

In this you are flatly incorrect -- because you are 'picking' out the data
you prefer. One star does not a galaxy make.

References
Genzel, R. et al 2000, MNRAS, 317, 348
Ghez, A. M. et al 2000, Nature, 407, 349

Thanks, but I'll pass on bothering with these two -- until you address the
issue above, based on the first set of studies you claimed 'addressed'
Rieke.

greywolf42
ubi dubium ibi libertas

Craig Markwardt wrote in message
...
greywolf42 writes:
[ ... ]
http://www.arxiv.org/abs/astro-ph/0201031

"Stellar Orbits Near Sagittarius A*," Eckart, Genzel, Schodel, Jan 2002

Mixes data types:
Combines the high precision but shorter time scale NIRC/Keck data with
the
lower precision but longer time scale SHARP/NTT data set;

"Adjusts" data:
Statistically corrects the observed accelerations for theoretical
projection
effects;

Drops "contradictory" data:
Excludes star S8 from the analysis of the amount and position of the
central
mass. ("... this star either was or is subject to a close interaction
with a
different object or that its position measurements are influenced by the
emission of a different cluster star.")

The three factors you bring up are *differences* between the Eckart
work and previous work by Ghez et al (2000).

No. These are specific quotes from the specific references. I made no
attempt to compare these to 'previous work.'

Despite this, the
results of Eckart and Ghez are consistent with one another, and with
the previous work by Genzel et al (2000). Furthermore the techniques
applied by the Eckart paper appear to be appropriate, and would tend
to *decrease* rather than increase observational biases. Therefore
your criticisms appear to be essentially irrelevant.

My observations of questionable manipulations of data are not irrelevant at
all.

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.

23 authors (I'm afraid I'll always start out highly skeptical of such an
obviously political effort). One star as primary data. Mixing of VLBI
and
infrared data. "Correcting" data to match theory (Sgr A* position was
'moved' to fit the theory). No mention of the other stars that don't
seem
to be moving in this fashion.

Your "political" comment is irrelevant.

It was a statement of a bias on my part.

Your other comments imply your disapproval.

Yes.

However, as the authors say, "
The remarkable consequence of the orbital technique is that the mass
can be determined from a single stellar orbit, in comparison to the
statistical techniques that use several tens to hundreds of stellar
velocities at 10 to 300 light days from SgrA* (Fig. 3). In addition,
the orbital technique requires fewer assumptions than the other
estimates (for example, equilibrium and isotropy of orbits), and
thus is less vulnerable to systematic effects.

The combination of VLBI and infrared data allows an extremely precise
alignment of the stars (IR) and Sgr A* (radio). The position of Sgr
A* can be considered a nuisance parameter since it is not relevant to
the mass determination (and in any case is within 1.6 sigma of the
previously determined position).

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.)


http://www.arxiv.org/abs/astro-ph/0306214

"Stellar Dynamics in the Central arcsecond of our galaxy," June 2003,
Schodel et al Followup from 0210426. 6 Stars plotted. Retains the
"adjusted" data (postion of Sgr A*).

No comment provided on this study.

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. ..."


Genzel et al (2000) show,

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?

using an ensemble of stars, that the central
velocity dispersion is of order 280-350 km/s, and decreases with
radius between r^-1 and r^-0.5, so Rieke's findings appear to be
addressed.

Nonsense. Rieke's findings are not addressed at all. That you have found a
study that appears to contradict Rieke, does not mean that Rieke is the
paper in error. One must actually 'address' Rieke's study to 'address' the
study. (i.e. identify a likely source of error in one or the other)

However, these statistical measurements really take second
place to a direct measurment of a Keplerian orbit around the central
mass, including both pericenter and apocenter, which both clinches the
mass of the central body, and highly constrains its density (since
pericenter is within 17 lt-hr = 120 AU).

In this you are flatly incorrect -- because you are 'picking' out the data
you prefer. One star does not a galaxy make.

References
Genzel, R. et al 2000, MNRAS, 317, 348
Ghez, A. M. et al 2000, Nature, 407, 349

Thanks, but I'll pass on bothering with these two -- until you address the
issue above, based on the first set of studies you claimed 'addressed'
Rieke.

greywolf42
ubi dubium ibi libertas

greywolf42 writes:

Craig Markwardt wrote in message
...
greywolf42 writes:
[ ... ]
http://www.arxiv.org/abs/astro-ph/0201031

"Stellar Orbits Near Sagittarius A*," Eckart, Genzel, Schodel, Jan 2002

[ CM: some reformatting and snipping below ]
Mixes data types:
Combines the high precision but shorter time scale NIRC/Keck data with the
lower precision but longer time scale SHARP/NTT data set;
"Adjusts" data:
Statistically corrects the observed accelerations for theoretical
projection effects;
Drops "contradictory" data:
Excludes star S8 from the analysis of the amount and position of the
central mass. [ ... ]

The three factors you bring up are *differences* between the Eckart
work and previous work by Ghez et al (2000).

No. These are specific quotes from the specific references. I made no
attempt to compare these to 'previous work.'


However, the abstract you were quoting from (Eckart et al 2002) says:
"Our analysis differs in three main points from Ghez et al: (1)
.... (2) ... (3) ...," which are the three points you listed above.
The mere fact that the Eckart paper contains different analysis from a
previous paper does not make it questionable. Indeed, it is usually a
sign of original research.

Despite this, the
results of Eckart and Ghez are consistent with one another, and with
the previous work by Genzel et al (2000). Furthermore the techniques
applied by the Eckart paper appear to be appropriate, and would tend
to *decrease* rather than increase observational biases. Therefore
your criticisms appear to be essentially irrelevant.

My observations of questionable manipulations of data are not irrelevant at
all.

You do not substantiate your claims of "questionable manipulations."
Merely claiming the above to be questionable does not make them so.
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.
(2) 2D projection effects are a fact of our natural world. They
must be corrected for, in order to infer physical accelerations.
(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*.


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.

23 authors (I'm afraid I'll always start out highly skeptical of such an
obviously political effort). One star as primary data. Mixing of VLBI and
infrared data. "Correcting" data to match theory (Sgr A* position was
'moved' to fit the theory). No mention of the other stars that don't seem
to be moving in this fashion.
[ ... ]

However, as the authors say, "
The remarkable consequence of the orbital technique is that the mass
can be determined from a single stellar orbit, in comparison to the
statistical techniques that use several tens to hundreds of stellar
velocities at 10 to 300 light days from SgrA* (Fig. 3). In addition,
the orbital technique requires fewer assumptions than the other
estimates (for example, equilibrium and isotropy of orbits), and
thus is less vulnerable to systematic effects.

The combination of VLBI and infrared data allows an extremely precise
alignment of the stars (IR) and Sgr A* (radio). The position of Sgr
A* can be considered a nuisance parameter since it is not relevant to
the mass determination (and in any case is within 1.6 sigma of the
previously determined position).

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

[ ... ]
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. ..."


Genzel et al (2000) show,

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?

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



using an ensemble of stars, that the central
velocity dispersion is of order 280-350 km/s, and decreases with
radius between r^-1 and r^-0.5, so Rieke's findings appear to be
addressed.

Nonsense. Rieke's findings are not addressed at all. That you have found a
study that appears to contradict Rieke, does not mean that Rieke is the
paper in error. One must actually 'address' Rieke's study to 'address' the
study. (i.e. identify a likely source of error in one or the other)

Borrowed from a different thread:
Rieke & Rieke (1988) is largely outdated, and so your reliance on it
is unfounded. Their study used an imager with ~1 arcsec seeing, and
did not resolve any stars within 1-2 arcsec of Sgr A. In contrast,
the works of Genzel et al ([2003]) and Ghez et al (2000) are able to
resolve stars within 0.1 arcsec, and many more fainter stars. There
are of order ~50 such stars [within 2 arcsec]. Furthermore, the
work of Genzel et al ([2000]) demonstrates that there is indeed a
velocity cusp, with a central velocity dispersion of 280-350 km/s.
Which is consistent with the measurements of gas motion of order 300
km/s.
[some changes entered in brackets]

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

[ .. reordered .. ]
Thanks, but I'll pass on bothering with these two [references
below] -- until you address the issue above, based on the first set
of studies you claimed 'addressed' Rieke.

You must have me confused with someone else. This is my first reply
to this thread in s.a.r, and I did not "proffer" up the first set of
studies.

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
Ghez, A. M. et al 2000, Nature, 407, 349
Rieke, G. H. & Rieke, M. J. 1988, ApJL, 330, L33

greywolf42 writes:

Craig Markwardt wrote in message
...
greywolf42 writes:
[ ... ]
http://www.arxiv.org/abs/astro-ph/0201031

"Stellar Orbits Near Sagittarius A*," Eckart, Genzel, Schodel, Jan 2002

[ CM: some reformatting and snipping below ]
Mixes data types:
Combines the high precision but shorter time scale NIRC/Keck data with the
lower precision but longer time scale SHARP/NTT data set;
"Adjusts" data:
Statistically corrects the observed accelerations for theoretical
projection effects;
Drops "contradictory" data:
Excludes star S8 from the analysis of the amount and position of the
central mass. [ ... ]

The three factors you bring up are *differences* between the Eckart
work and previous work by Ghez et al (2000).

No. These are specific quotes from the specific references. I made no
attempt to compare these to 'previous work.'


However, the abstract you were quoting from (Eckart et al 2002) says:
"Our analysis differs in three main points from Ghez et al: (1)
.... (2) ... (3) ...," which are the three points you listed above.
The mere fact that the Eckart paper contains different analysis from a
previous paper does not make it questionable. Indeed, it is usually a
sign of original research.

Despite this, the
results of Eckart and Ghez are consistent with one another, and with
the previous work by Genzel et al (2000). Furthermore the techniques
applied by the Eckart paper appear to be appropriate, and would tend
to *decrease* rather than increase observational biases. Therefore
your criticisms appear to be essentially irrelevant.

My observations of questionable manipulations of data are not irrelevant at
all.

You do not substantiate your claims of "questionable manipulations."
Merely claiming the above to be questionable does not make them so.
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.
(2) 2D projection effects are a fact of our natural world. They
must be corrected for, in order to infer physical accelerations.
(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*.


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.

23 authors (I'm afraid I'll always start out highly skeptical of such an
obviously political effort). One star as primary data. Mixing of VLBI and
infrared data. "Correcting" data to match theory (Sgr A* position was
'moved' to fit the theory). No mention of the other stars that don't seem
to be moving in this fashion.
[ ... ]

However, as the authors say, "
The remarkable consequence of the orbital technique is that the mass
can be determined from a single stellar orbit, in comparison to the
statistical techniques that use several tens to hundreds of stellar
velocities at 10 to 300 light days from SgrA* (Fig. 3). In addition,
the orbital technique requires fewer assumptions than the other
estimates (for example, equilibrium and isotropy of orbits), and
thus is less vulnerable to systematic effects.

The combination of VLBI and infrared data allows an extremely precise
alignment of the stars (IR) and Sgr A* (radio). The position of Sgr
A* can be considered a nuisance parameter since it is not relevant to
the mass determination (and in any case is within 1.6 sigma of the
previously determined position).

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

[ ... ]
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. ..."


Genzel et al (2000) show,

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?

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



using an ensemble of stars, that the central
velocity dispersion is of order 280-350 km/s, and decreases with
radius between r^-1 and r^-0.5, so Rieke's findings appear to be
addressed.

Nonsense. Rieke's findings are not addressed at all. That you have found a
study that appears to contradict Rieke, does not mean that Rieke is the
paper in error. One must actually 'address' Rieke's study to 'address' the
study. (i.e. identify a likely source of error in one or the other)

Borrowed from a different thread:
Rieke & Rieke (1988) is largely outdated, and so your reliance on it
is unfounded. Their study used an imager with ~1 arcsec seeing, and
did not resolve any stars within 1-2 arcsec of Sgr A. In contrast,
the works of Genzel et al ([2003]) and Ghez et al (2000) are able to
resolve stars within 0.1 arcsec, and many more fainter stars. There
are of order ~50 such stars [within 2 arcsec]. Furthermore, the
work of Genzel et al ([2000]) demonstrates that there is indeed a
velocity cusp, with a central velocity dispersion of 280-350 km/s.
Which is consistent with the measurements of gas motion of order 300
km/s.
[some changes entered in brackets]

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

[ .. reordered .. ]
Thanks, but I'll pass on bothering with these two [references
below] -- until you address the issue above, based on the first set
of studies you claimed 'addressed' Rieke.

You must have me confused with someone else. This is my first reply
to this thread in s.a.r, and I did not "proffer" up the first set of
studies.

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
Ghez, A. M. et al 2000, Nature, 407, 349
Rieke, G. H. & Rieke, M. J. 1988, ApJL, 330, L33

"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. 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?

* 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. 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).

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"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. 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?

* 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. 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).

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"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?

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

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. 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.

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"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?

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

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. 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.

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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