Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...306..345b&link_type=abstract
Astronomy and Astrophysics, v.306, p.345
Astronomy and Astrophysics
Astrophysics
38
Galaxies: Individual: Ngc 4013, Galaxies: Ism, Galaxies: Kinematics And Dynamics, Galaxies: Structure, Radio Lines: Galaxies
Scientific paper
The detailed observations of the highly warped neutral hydrogen layer of the edge-on galaxy NGC 4013 have been analysed. To this aim a tilted ring model has been used, which is able to explain all the major features of the Hi structure and kinematics. A gas layer with constant thickness can give a reasonable, but not perfect reproduction of the gas structure. Therefore a description is given of the thickness, which is increasing as a function of radius, expected for a gaslayer being embedded in the potential of a spiral galaxy. Using such a layer in the model calculations produces a satisfactory fit to the observations. It appears that the line of nodes is directed nearly along the line of sight and must be straight to within 20deg over most of the warped region. Although there is some difference in surface density between the two sides of the warp, the amount of warping is nearly identical, proving the high degree of symmetry. The warp starts abruptly at the optical edge of the galaxy and the warp angle gradually increases with radius until around 25deg. A precise rotation curve has been determined by matching the model to the observed position-velocity diagram. In the stellar disk region the rotation slightly decreases towards larger radii, then there is the sudden drop at the optical edge, and the rotation is seen to increase slightly beyond this edge. A decomposition of the rotation curve into the individual contributions of disk, bulge, and dark halo reveals that a large core radius of the halo is needed. This immediately forces the bulge and disk to be kinematically dominant in the inner regions. The differential precession of a tilted ring structure subjected to the forcefield of NGC 4013 has been calculated. It appears that the observed structure can only exist for a period shorter than one rotation time. On the other hand, to achieve the high degree of symmetry, several rotation periods are needed. Consequently, by the time the warp has symmetrized, it has disappeared again. Thus a stabilization mechanism is required. Although attempts have been made to device such a mechanism, a final self consistent solution has not yet been found. The rigid tilted ring model seems unable to explain faint Hi emission high above the stellar disk. It is also unable to reproduce the observed Hi thickness in the disk region. Likely irregularities of the Hi gaslayer, like the observed corrugations of the dustlayer cause a much thicker line of sight integrated gaslayer than the model predicts.
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