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Issue A&A
Volume 447, Number 1, February III 2006
Page(s) 49 - 62
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20053210

A&A 447, 49-62 (2006)
DOI: 10.1051/0004-6361:20053210

$\ion{H}{i}$ study of the warped spiral galaxy NGC 5055: a disk/dark matter halo offset?

G. Battaglia1, F. Fraternali2, T. Oosterloo3 and R. Sancisi4

1  Kapteyn Astronomical Institute, Postbus 800, 9700 AV, Groningen, The Netherlands
    e-mail: gbattagl@astro.rug.nl
2  Theoretical Physics, University of Oxford, 1 Keble Road, OX1 3NP, Oxford, UK
3  ASTRON, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
4  INAF - Osservatorio Astronomico, via Ranzani 1, 40127, Bologna, Italy and Kapteyn Astronomical Institute, University of Groningen, PO Box 800, The Netherlands

(Received 7 April 2005 / Accepted 8 September 2005 )

Abstract
We present a study of the $\ion{H}{i}$ distribution and dynamics of the nearby spiral galaxy NGC 5055 based on observations with the Westerbork Synthesis Radio Telescope. The gaseous disk of NGC 5055 extends out to about 40 kpc, equal to 3.5 R25, and shows a pronounced warp that starts at the end of the bright optical disk ( R25= 11.6 kpc). This very extended warp has large-scale symmetry, which along with the rotation period of its outer parts ($\simeq$1.5 Gyr at 40 kpc), suggests a long-lived phenomenon.

The rotation curve rises steeply in the central parts up to the maximum velocity ( $v\rm _{max}\simeq 206$ km s-1). Beyond the bright stellar disk (R25), it shows a decline of about 25 km s-1 and then 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" contributes about 60% of the observed maximum rotational velocity.

NGC 5055 shows remarkable overall regularity and symmetry. A mild lopsidedness is noticeable, however, both in the distribution and kinematics of the gas. The tilted ring analysis of the velocity field led us to adopt 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 disappear, 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.


Key words: galaxies: individual: NGC 5055 -- galaxies: kinematics and dynamics -- galaxies: structure -- galaxies: ISM -- cosmology: dark matter

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© ESO 2006


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