HI study of the warped spiral galaxy NGC5055: a disk/dark matter halo offset?

G. Battaglia, F. Fraternali, T. Oosterloo, R. Sancisi

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Abstract: We present a study of the HI distribution and the dynamics of the nearby spiral galaxy NGC5055 based on observations with the Westerbork Synthesis Radio Telescope. The gaseous disk of NGC5055 extends out to about 40 kpc, equal to 3.5 R_25 and shows a pronounced warp, starting at the end of the bright optical disk (R_25= 11.6 kpc). The warp is very extended and symmetric. Its large-scale symmetry and the long rotation period of its outer parts (~ 1.5 Gyr at 40 kpc) suggest a long-lived phenomenon. The rotation curve has a steep rise in the central parts up to the maximum velocity (v_{max}= 206 km/s). Beyond the bright stellar disk (R_25) it shows a decline of about 25 km/s and then it remains flat out to the last measured point. The standard analysis with luminous and dark matter components shows the dynamical importance of the disk. The best fit to the rotation curve is obtained with a ``maximum disk''. Less satisfactory fits with lighter disks help to set a firm lower limit of 1.4 to the mass-to-light ratio in F band of the disk. Such a ``minimum disk'' is contributing about 60% of the observed maximum rotational velocity. NGC5055 shows remarkable overall regularity and symmetry. A mild lopsidedness is noticeable, however, both in the distribution and in the kinematics of the gas. The tilted ring analysis of the velocity field has led us to the adoption of different values for the kinematical centre and for the systemic velocity for the inner and the outer parts of the system. This has produced a remarkable result: the kinematical and geometrical asymmetries have disappeared, both at the same time. These results point at two different dynamical regimes: an inner region, dominated by the stellar disk, and an outer one, dominated by a dark matter halo offset with respect to the disk.
Originele taal-2English
Aantal pagina's17
TijdschriftAstronomy & astrophysics
Volume3210
StatusPublished - okt-2005

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