|
|
Thread Tools | Display Modes |
#21
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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 |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
"Dark matter" forms dense clumps in ghost universe (Forwarded) | Andrew Yee | Astronomy Misc | 0 | November 21st 03 05:41 PM |
Galaxies without dark matter halos? | greywolf42 | Astronomy Misc | 34 | November 5th 03 01:34 PM |
A Detailed Map of Dark Matter in a Galactic Cluster Reveals How Giant Cosmic Structures Formed | Ron Baalke | Astronomy Misc | 3 | August 5th 03 02:16 PM |
Galaxies without dark matter halos? | Ed Keane III | Research | 4 | August 4th 03 12:39 PM |
Hubble tracks down a galaxy cluster's dark matter (Forwarded) | Andrew Yee | Astronomy Misc | 0 | July 17th 03 01:42 PM |