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Date and Time of the Query: 2019-04-22 T22:00:39 PDT
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For refcode 1992A&A...257...69B:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1992A&A...257...69B The stellar velocity dispersion of the spiral galaxies NGC 1566 and NGC 2815 Roelof Bottema Kapteyn Astronomical Institute, University of Groningen, P. O. Box 800, NL-9700 AV Groningen, The Netherlands Received April 12, accepted September 27, 1991 Abstract. Long slit absorption line spectroscopy has been obtained of the close to face-on Sc galaxy NGC 1566 and inclined Sb galaxy NGC 2815. These observations provided stellar radial velocities and stellar velocity dispersions as a function of radius for both galaxies. For NGC 1566 a radially decreasing velocity dispersion is observed. When, for a face-on galaxy, the dispersion is proportional to the square root of the surface brightness, the M/L ratio of the disk is constant. It appeared that for the B-band light the observed dispersions do not allow a constant mass-to-light ratio; it has to change from (M/L)_B_ = 2 to 1 going outwards over the extent of the observed region. The near infrared H-band photometry, however, shows a much steeper decrease of the brightness and a constant (M/L)_H_ is in accordance with the observed dispersions. Then, most likely, a considerable population gradient is apparent over the inner half of the disk of NGC 1566. The stellar velocity dispersion of the Seyfert like core is constant at 115 km s^-1^; this must be related to an underlying mass distribution which existed before the onset of nuclear activity. With some uncertainty, because of the small inclination, a rotation curve is calculated from the determined mass distribution. It falls below the observed rotation and it is concluded that at least a small amount of dark matter is required. NGC 2815 has a considerable bulge. In order to disentangle the bulge and disk kinematics and to correct for line of sight integration effects a galaxy model has been constructed. The model consists of a bulge with constant velocity dispersion and a disk with a dispersion decreasing exponentially with a scale-length twice the photometric scalelength. It appears that, extrapolated to the centre, the radial disk dispersion, <V^2^_R_>^1/2^_R=0_, is 110 +/- 20 km s^-1^. The bulge dispersion amounts to 180 +/- 20 km s^-1^ and the bulge has a rotation considerably smaller than that of the disk. The disk rotation curve calculated from the disk dispersion is lower than the rotation implied by the so called maximum disk solution. Key words: galaxies: general - kinematics and dynamics structure of- NGC 1566-NGC 2815
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