errors in kinematic distances and our image of the milky way galaxy

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No. 6, 2006KINEMATIC DISTANCE ERRORS 2381Fig. 7.—Same as Fig. 6, but using the full velocity field to recover the densitydistribution. Most of the characteristics of Fig. 1 are recovered, althoughsome spurious structure appears due to the new distance ambiguities introducedby the noncircular motions.line broadening. When these are considered, some of the newambiguities are spread over a range of distances, effectively disappearing.Therefore, some of the spurious structures blend withreal structures. So, the observer might get an image of the modelgalaxy closer to reality than Figure 7 suggests.5. SUMMARY AND DISCUSSIONThe effect of the circular orbits assumption on our perceptionof the large-scale structure of the Galaxy was explored. Sincethese errors might be quite large at the positions of the spiral arms,the study of the spiral structure of the Galaxy and objects associatedwith it is particularly affected. By simulating the way animaginary observer inside a model galaxy might try to infer thestructure of the gaseous disk, it was found that the circular orbitsassumption destroys the spiral structure and creates spuriousfeatures in the measured distribution.The method of kinematic distances is a powerful one, since itallows measurement of distances to diffuse sources and is easilyapplicable to a large fraction of the Galactic disk. Even if the measuredrotation curve includes deviations that do not reflect the truelarge-scale mass distribution, Figure 5a shows that the errors inthe distance are, in fact, not very large for most of the Galacticdisk; in fact, the distance errors that arise from using the truerotation curve are larger. In both cases, however, the errors arequite large at the positions of the spiral arms. If we want to usethis distance method for objects associated with the spiral structure,we need to consider noncircular motions (as has been successfullyshown by Foster & MacWilliams [2006] for a set ofH ii regions and supernova remnants).One possibility for achieving this is to try to determine the fullvelocity field of the Galactic disk. But direct measures of distancesto the diffuse gas components is quite difficult (thereforethe strength of the kinematic distance method). So, we need touse discrete objects and assume that they share their velocitywith the diffuse component (e.g., Brand & Blitz 1993; see alsothe discussion in Minn & Greenberg 1973). Yet another difficultyarises when tangential velocities and distances are required beyondthe solar neighborhood.Another approach to determining the full velocity field is tomodel it. Recently, Foster & MacWilliams (2006) used an analyticalmodel of the density and velocity fields of the diffuse gas,with parameters for the model fitted to H i observations of theouter Galaxy. Despite the fact that their density and velocitymodels are not consistent in the hydrodynamics sense, and thatthe model does not include the dynamical effects of magneticfields, they were able to add features of the Galaxy that are currentlydifficult to incorporate into numerical models, such asthe disk’s warp or the rolling motions associated with the spiralarms. Further numerical studies should allow the developmentof a more realistic analytical model.Instead of an analytic model, a numerical model was used inthe present work to obtain density and velocity fields. Since thefocus is on large-scale velocity structures, an Eulerian code providesa good approach. Also, since the Galactic magnetic fieldhas been proved to be an important component of the total interstellarmedium pressure (Boulares & Cox 1990), its effect in thegas dynamics is likely to be important; therefore, a full MHD simulationwas required. The large-scale forcing is also transcendent;since the azimuthal shape of the spiral perturbation appears tohave an influence on the gaseous response (Franco et al. 2002),the usual sinusoidal perturbation was deemed too simplistic, anda self-consistent model for the perturbing arms was chosen. Atthe present time, the Galactic warp and the vertical motions associatedwith the spiral arms (Gómez&Cox2004a,2004b)couldnot be considered at the necessary resolution.In this work it has been shown that it is possible to recovermost of the gaseous structure of a galactic disk using kinematicdistances, as long as the full velocity field is considered. Nevertheless,applying these results to the Milky Way is a whole newissue, since obtaining the full velocity field is not trivial. For theprocedure used here, how close the numerical simulation is to thereal Galaxy remains the weak point of this approach. The computationcost of a realistic enough simulation is still too high toallow a parameter fitting analysis. So, the remaining question iswhether the velocity field that results from the simulation yields adetermination of the distance to a given object, or only an estimationof the distance error. The answer to that question is left tothe reader’s criterion.This author wishes to thank J. Ballesteros-Paredes, E. Vázquez-Semadeni, C. Watson, J. Franco, L. Loinard, S. Kurtz, and an anonymousreferee for their encouragement and useful commentsduring the preparation of this manuscript.Binney, J., & Merrifield, M. 1998, Galactic Astronomy ( Princeton: PrincetonUniv. Press)Blitz, L., & Spergel, D. N. 1991, ApJ, 370, 205Boulares, A., & Cox, D. P. 1990, ApJ, 365, 544Brand, J., & Blitz, L. 1993, A&A, 275, 67Dehnen, W., & Binney, J. 1998, MNRAS, 294, 429Drimmel, R. 2000, A&A, 358, L13REFERENCESDrimmel, R., & Spergel, D. N. 2001, ApJ, 556, 181Fish, V. L., Reid, M. J., Wilner, D. J., & Churchwell, E. 2003, ApJ, 587, 701Foster, T., & MacWilliams, J. 2006, ApJ, 644, 214Franco, J., Martos, M., Pichardo, B., & Jongsoo, K. 2002, in ASP Conf.Ser. 275, Disks of Galaxies: Kinematics, Dynamics, and Perturbations, ed.E. Athanassoula, A. Bosma, & R. Mujica (San Francisco: ASP), 343Georgelin, Y. M., & Georgelin, Y. P. 1976, A&A, 49, 57
2382GÓMEZGómez, G. C., & Cox, D. P. 2004a, ApJ, 615, 744———. 2004b, ApJ, 615, 758Kerr, F. J. 1962, MNRAS, 123, 327———. 1969, ARA&A, 7, 39Linblad, P. O. 1967, Bull. Astron. Inst. Netherlands, 19, 34Maciel, W. J., & Lago, L. G. 2005, Rev. Mex. AA, 41, 383Martos, M., Hernández, X., Yáñez, M., Moreno, E., & Pichardo, B. 2004a,MNRAS, 350, L47Martos, M., Yáñez, M., Hernández, X., Moreno, E., & Pichardo, B. 2004b,J. Korean Astron. Soc., 37, 199Minn, Y. K., & Greenberg, J. M. 1973, A&A, 24, 393Oort, J. H., Kerr, F. J., & Westerhout, G. 1958, MNRAS, 118, 379Pichardo, B., Martos, M., Moreno, E., & Espresate, J. 2003, ApJ, 582, 230Sewilo, M., Watson, C., Araya, E., Churchwell, E., Hofner, P., & Kurtz, S.2004, ApJS, 154, 553Sinha, R. P. 1978, A&A, 69, 227Stone, J. M., & Norman, M. L. 1992a, ApJS, 80, 753———. 1992b, ApJS, 80, 791Taylor, J. H., & Cordes, J. M. 1993, ApJ, 411, 674Watson, C., Araya, E., Sewilo, M., Churchwell, E., Hofner, P., & Kurtz, S.2003, ApJ, 587, 714Yáñez, M. A. 2005, Ph.D. thesis, UNAM
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