Annu. Rev. Astron. Astrophys. 2001. 39: 137-174
Copyright © 2001 by . All rights reserved

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Most of what we know about rotation curves we have learned in the last fifty years, due principally to instrumental and computational advances. It is likely that these advances will accelerate in the future. We can look forward to an exciting future. Specifically,

1. Extinction-free rotation kinematics in the central regions will come from high-J CO line spectroscopy and imaging using ALMA, the Andes large mm- and sub- mm wave interferometer at 5000 m altitude. This array will produce high spatial (0.01 arcsec) and high velocity (deltaV < 1 km s-1) resolution.

2. Extinction-free measurements will also come from eight- and ten-meter class telescopes using Br gamma, H2 molecular, and other infrared lines in K-band and longer-wavelength spectroscopy.

3. VLBI spectroscopic imaging of maser sources will teach us more about super massive black holes and rotation and mass distribution in nuclear disks.

4. Rotation of the Galaxy, separately for the disk and bulge, will be directly measured from proper motions, parallaxes, and hence, distances, and radial velocities of maser sources using micro-arcsecond radio interferometry. VERA (VLBI Experiment for Radio Astrometry) will become a prototype. This facility will also derive an accurate measurement of the Galactic Center distance, R0.

5. Optical interferometry may permit us to "watch" a nearby spiral (M31, M33, LMC, etc.) rotate on the plane of the sky, at least through a few microarcsecond. Radio interferometry of maser stars will be used to directly measure rotation on the sky of the nearest galaxies, from measures of proper motion (hence, distances) and radial velocities.

6. Rotation curves will be determined for galaxies at extremely high redshift; with luck we will observe protogalactic rotation and dynamical evolution of primeval galaxies. This may be a task for successive generations of space telescopes.

7. We may be lucky and ultimately understand details of barred spiral velocity fields from spectroscopic imaging. We may be able to separate the disk, bulge and bar potentials by fitting the number of parameters necessary for describing bar's mass and dynamical properties.

8. Polar ring kinematics will be understood, especially halo kinematics perpendicular to the disk, and therefore, 3-D halo structure. We are certain to learn details of galaxies which are unexpected and hence surprising. We may even be luckier and learn something new about the Universe.

9. Sophisticated methods of analysis, perhaps involving line shapes and velocity dispersions, will produce more accurate rotation curves for large samples of spirals. These will lead to more tightly constrained mass deconvolutions. Distribution of dark and luminous matters within the halo, disk, bulge, and core will be mapped in detail from more sophisticated mass-to-luminosity ratio analyses.

10. Dark halos will finally be understood. We will know their extent, and their relation to the intracluster dark mass. We may even know the rotation velocity of the halo. Will the concept of a "rotation curve" apply at such large distances from the disk? Will we learn if our halo brushes the halo of M31?

11. We will ultimately know what dark matter is, the major constituent of the Universe when measured in mass. Elementary particle physics will teach us its origin and physical properties.

12. Perhaps we will be able to put to rest the last doubt about the applicability of Newtonian gravitational theory on a cosmic scale, or enthusiastically embrace its successor.

Acknowledgements: The authors thank Dr. Yoichi Takeda for assisting in gathering and selecting the references from a huge number of related papers in the decades. They thank Drs. Linda Dressel, Jeffrey Kenney, Stacy McGaugh, and Rob Swaters for references and helpful comments. They also thank Mareki Honma for references, Jin Koda and Kotaro Kohno for NMA data analyses.

This is an unedited draft of a chapter submitted for publication in the Annual Review of Astronomy and Astrophysics, Volume 39, 2001

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