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Astron. Astrophys. 352, L36-L39 (1999)

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1. Introduction

NGC 1288 has been observed by the FORS team in the B-, V-, and I-filter pass bands (Möllenhoff et al. 1999; hereafter referred to as MAG). The unprecedented quality of the images allows a quantitative interpretation of the morphology of the galaxy. NGC 1288 has multiple spiral arms, which show up in all filter bands. It is well known from density-wave theory of galactic spiral arms (Bertin et al. 1977, Lau & Bertin 1978, Toomre 1981, 1990, Athanassoula 1984, Fuchs et al. 1998, Fuchs 1999) as well as from numerical simulations of galactic disks (Sellwood & Carlberg 1984, Carlberg & Freedman 1985) that the morphology of the spiral arms, i.e. the appearance and multiplicity of the spiral arms, is closely related to the ratio of disk mass to dark halo mass. Especially Athanassoula et al. (1987) have pointed to the fact that the multiplicity of spiral arms can be used as an indicator for the presence of a dark halo. This was confirmed observationally by Elmegreen & Elmegreen (1990), who correlated the shapes of the outer rotation curves of galaxies with the morphologies of their spiral arms. NGC 1288 is not a grand-design spiral. We argue here that its spiral structure is due to swing-amplification of shearing density waves (Toomre 1981). We have used arguments of swing-amplification density-wave theory to interpret the morphology of nearby spirals or distant field galaxies elsewhere (Fuchs et al. 1998, Fuchs 1999), and apply the same method here to the morphology of NGC 1288. It is the purpose of this letter to demonstrate that NGC 1288 must be surrounded by a dark matter halo. This is possible, even though the rotation curve of NGC 1288 is not known in detail.

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© European Southern Observatory (ESO) 1999

Online publication: November 23, 1999