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Astron. Astrophys. 364, L97-L100 (2000)

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1. Introduction

Investigating the distribution of spin vectors (SVs) of disk galaxies (with respect to a proper reference plane, e.g. the parent cluster plane) can be regarded as a clue to study and understand the process of formation of large scale structures: these SVs can be an indicator of the initial conditions when galaxies and clusters formed, provided the angular momenta of the galaxies have not been altered too much since their formation.

Contemporary theories advocate three different predictions concerning the spatial orientation of SVs of galaxies. First, the `Pancake model' (see, e.g. Doroshkevich 1973, Doroshkevich & Shandarin 1978) predicts that the SVs of galaxies tend to lie within the cluster plane. Second, according to the `Hierarchy model' (see, e.g. Peebles 1969) the directions of the SVs should be distributed randomly. Third, the `primordial vorticity theory' (see, e.g. Ozernoy 1978) predicts that the SVs of galaxies are distributed primarily perpendicular to the cluster plane.

In most of the recently published papers on the origin of angular momenta of galaxies a `position angle (PA) - inclination method' was used to study the spatial orientation of SVs of galaxies (e.g. Flin & Godlowski 1986, Kashikawa & Okamura 1992, Godlowski 1993, 1994, Hu et al. 1995, 1997, 1998, Yuan et al. 1997, Godlowski & Ostrowski 1999. This method was initially proposed by Jaaniste & Saar (1978) and later refined by Flin & Godlowski (1986). In this method, the measured PAs of galaxies (e.g. measured on photographic plates) are converted into 3-dimensional SVs using inclination angles which are obtained from the measured axial ratios. The distribution of these SVs can now be compared with a hypothesis, e.g. an isotropic spatial distribution.

In all previous work an underlying isotropic distribution of the SVs of galaxies in 3-dimensional space was assumed for analysis. This assumption leads to specific distributions of other parameters, e.g. the polar and azimuthal angles (see later), which we call the expected isotropic distribution curve for the parameter in question. Any deviation from this curve was in general considered as an indication of anisotropic distribution. However, the expected isotropic distribution curves could be seriously influenced by selection effects. So its nature should be examined for the various types of these effects.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001