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Astron. Astrophys. 324, 877-887 (1997)

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1. Introduction

Two H I surveys of spiral and irregular galaxies have been performed by means of short observations in the 21-cm line of neutral hydrogen with the Westerbork Synthesis Radio Telescope (WSRT). They provide one-dimensional information about the kinematics and distribution of the neutral hydrogen gas projected on the so-called resolution axis. The observations of each galaxy were made in such a way that the resolution axis coincides with the major axis of that galaxy, so that a position-velocity map (XV-map) of the H I distribution projected on the major axis was obtained. From these maps global H I profiles and H I strip integrals, [FORMULA], representing the projections of the entire neutral hydrogen distributions on the major axes, were derived. Subsequently these data were used to estimate for each galaxy a number of H I properties, like H I flux, H I mass, systemic velocity, global profile widths, and isophotal diameter of the hydrogen disc. The results have been published in Broeils & van Woerden (1994, hereafter Paper I) and Rhee & van Albada (1996, hereafter Paper II).

In the present paper these properties are analysed and combined with optical information on the sample galaxies. Because many studies of the integral properties (including neutral hydrogen) of larger and more complete samples have been published (e.g. Roberts 1969; Shostak 1978; Huchtmeier & Richter 1988; Roberts & Haynes 1994, hereafter RH), we will concentrate our attention on the specific characteristics of our data: spatial information on the H I distribution of an intermediate-sized sample of galaxies. The analysis of the kinematics (H I linewidths, rotation velocities and the Tully-Fisher relation) is discussed in an accompanying paper (Rhee & Broeils 1996).

In Sect.  2 we will investigate correlations between H I and optical properties, like morphological type, luminosity and optical diameter. We will show that the H I diameter correlates well with the optical size of a galaxy and also with the total neutral hydrogen mass. The latter correlation indicates that the average H I surface density per galaxy is constant to first order. Possible dependences of this quantity on morphological type and luminosity will also be investigated. Furthermore, the total mass of each galaxy inside the H I diameter is determined and correlated with optical properties. The edge of the H I distribution is generally the outermost point of the mass distribution of a galaxy that can be measured. The total mass defined within this point is therefore the best galaxy mass estimator that we have for an intermediate-sized sample of galaxies.

In the second part of this paper (Sect.  3) we will study the radial H I surface density distributions of the sample galaxies by means of Principal Component Analysis (PCA). It has been shown (Paper I; Cayatte et al. 1994) that although H I density profiles show quite some diversity, when they are normalized by a certain radius scale and averaged per morphological type, there seems to exist a characteristic profile for each morphological type. PCA will be used to investigate how many independent dimensions (principal components) are required to explain the observed variance in the H I surface density distributions and we will try to link these components to actual physical parameters.

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998