|
|
Thread Tools | Display Modes |
#11
|
|||
|
|||
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 |
#12
|
|||
|
|||
Galaxies without dark matter halos?
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 |
#13
|
|||
|
|||
Galaxies without dark matter halos?
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 |
#14
|
|||
|
|||
Galaxies without dark matter halos?
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 |
#15
|
|||
|
|||
Galaxies without dark matter halos?
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 |
#16
|
|||
|
|||
Galaxies without dark matter halos?
"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). -- Lt. Lazio, HTML police | e-mail: No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html |
#17
|
|||
|
|||
Galaxies without dark matter halos?
"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). -- Lt. Lazio, HTML police | e-mail: No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html |
#18
|
|||
|
|||
Galaxies without dark matter halos?
"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. -- Lt. Lazio, HTML police | e-mail: No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html |
#19
|
|||
|
|||
Galaxies without dark matter halos?
"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. -- Lt. Lazio, HTML police | e-mail: No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html |
#20
|
|||
|
|||
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 |
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 04:41 PM |
Galaxies without dark matter halos? | greywolf42 | Astronomy Misc | 34 | November 5th 03 12:34 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 |