Other
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...438..155r&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 438, no. 1, p. 155-169
Other
49
Disk Galaxies, Galactic Rotation, Gas Dynamics, Interstellar Gas, Ionized Gases, Kinematics, Dynamic Models, Galactic Structure, H I Regions, Velocity Distribution
Scientific paper
We present an investigation of the kinematics of the stars and the ionized gas along the principal axes of the galaxy NGC 4826. This galaxy is known to contain two nested, counterrotating, gas disks of a few 108 solar mass each, with the inner disk extending to approximately 1 kpc and the outer disk extending beyond. The stellar kinematics along the major axis, extending across the transition region between the two gas disks, show no hint of velocity reversal or increased velocity dispersion. The stars always rotate in the same sense as the inner gas disk, and thus it is the outer disk which 'counterrotates'. The projected circular velocities inferred from the stellar kinematics and from the H I disks agree to within approximately 10 km/s, supporting other evidence that the stellar and gaseous disks are coplanar to approximately 7 deg. Using a detailed analysis of the stellar velocity distributions, we can limit the fraction of counterrotating stars in the outer disk to approximately less than or = 0.05, or approximately less than or = 2 x 108 solar mass. This upper limit is comparable to the mass of detected counterrotating gas. This low mass of counterrotating material, combined with the low-velocity dispersion in the stellar disk, implies that NGC 4826 cannot be the product of a retrograde merger of galaxies, unless they differed by at least an order of magnitude in mass. The velocities of the ionized gas along the major axis are in agreement with that of the stars for R less than 0.75 kpc. The subsequent transition toward apparent counterrotation of the ionized gas is spatially well resolved, extending over approximately 0.6 kpc in radius. The kinematics of this region are not symmetric with respect to the galaxy center. On the southeast side there is a significant region in which vproj (H II) much less than vcirc approximately 150 km/s, but sigma (H II) approximately 65 km/s. The kinematic asymmetries cannot be explained with any stationary dynamical model, even is gas inflow or warps were invoked. The gas in this transition region shows a diffuse spatial structure, strong (N II) and (S II) emission, as well as the high-velocity dispersion. These data present us with the conundrum of explaining a galaxy in which a stellar disk, and two counterrotating H I disks, at smaller and much larger radii, appear in equilibrium and nearly coplanar, yet in which the transition region between the gas disks is not in steady state.
Braun Robert
Kennicutt Robert C. Jr.
Rix Hans-Walter R.
Walterbos Rene A. M.
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