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Astron. Astrophys. 360, 439-446 (2000)

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1. Introduction

Most of the kinematically decoupled structures observed in galaxies have been considered to have an external origin, and to represent the end result of the acquisition of material coming from outside the boundaries of an already formed galaxy via infall, accretion or merging. The angular momenta of the acquired and pre-existing material are generally misaligned with respect to each other. In fact the angular momentum of the acquired material depends on the impact parameters of the capture process and on the geometry of the potential of the host galaxy. This is the case of the elliptical galaxies with a dust lane along their minor axis (see Bertola 1987 for a review) and of the polar-ring galaxies (see the photographic atlas of Whitmore et al. 1990), where the acquired gas (and its associated stars) is rotating around an axis orthogonal to the minor axis of the main stellar body. Moreover this is also the case of the Sa galaxy NGC 4698, where the presence of an orthogonal decoupling between bulge and disk has been recently discussed by Bertola et al. (1999).

The mass of the orthogonally rotating material is negligible ([FORMULA] [FORMULA]) with respect to the total dynamical mass of the host galaxy in the minor-axis dust-lane elliptical galaxies (Sage & Galletta 1993). The mass of the acquired gas ([FORMULA] [FORMULA]) is comparable or even greater than the mass of the pre-existing galaxy in the polar-ring galaxies, like the elliptical AM 2020-504 (Arnaboldi et al. 1993a), the S0 NGC 4650A (Sackett et al. 1994) and the spiral NGC 660 (van Driel et al. 1995). In NGC 4650A, the polar structure is massive enough to lead to the formation of a spiral pattern, as pointed out by Arnaboldi et al. (1997). This is a strong indication that in this object the acquired material settled onto a disk rather than forming a ring. Besides, there is a number of similarities between the global properties (e.g., total luminosity, integral colors, gas-to-dust ratio and star formation rate) of the polar-rings around S0's, and spirals and the properties of the disks of late-type spiral galaxies (see Reshetnikov & Sotnikova 1997 and references therein).

The case of the Sa galaxy NGC 4698 (Bertola et al. 1999) could represent the first example of an orthogonal acquisition involving an even larger amount of external material, which produced an intrinsic change of the galaxy morphology. In fact the geometric and kinematical orthogonal decoupling between the bulge and disk of this early-type spiral galaxy has been interpreted as the end product of the accretion of material which built up the disk component around a previously formed bare spheroid.

In the framework of massive acquisition processes, we present NGC 4672 as a new case of a disk galaxy characterized by a geometric and kinematical orthogonal decoupling between its bulge and disk. This paper is organized as it follows. In Sect. 2 we present our spectroscopic observations of NGC 4672. The morphology of this object and its classification are discussed in Sect. 3. The kinematical results for both the gaseous and the stellar components are described in Sect. 4 and we move towards our conclusions in Sect. 5.

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© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000