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Astron. Astrophys. 344, 421-432 (1999)

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1. Introduction

It is generally admitted that gravitational interaction can modify the properties of galaxies in rich environments. Its effects are often invoked to explain different observed properties, from the distribution of morphological types in clusters of galaxies to the peculiarities sometimes seen in particular galaxies. Those effects, on the other hand, can be very diverse in nature and have very different time scales to manifest, so it is not straightforward to ascertain whether such or such peculiarity is actually the fact of the interaction. In other words, the absence of peculiarities in a given system cannot be given as a sign of isolation, whereas the presence of unusual features cannot be unambiguously given as a proof of gravitational interaction (see Moles et al. 1994, for the pair NGC 450/UGC 807).

It is clear that the characterization of the specific effects of the gravitational interaction needs to be preceded by the exhaustive analysis of the properties of galaxies that could be considered as isolated. The average values and the ranges they present in size, luminosity, bulge to disk ratio, etc, do constitute the starting point to which refer similar properties of spirals in richer environments, from isolated pairs and small groups to clusters. Only such a comparative analysis could eventually lead to the identification of the specific effects of the gravitational interaction. This interaction is expected to produce changes in some morphological aspects, the kinematics, and the stellar content of the involved galaxies. Thus, it is necessary to start with the study of those same properties for a well defined sample of isolated galaxies.

Important studies of samples of spiral galaxies do exist, but these, even if sometimes defined as containing normal or non-peculiar galaxies, include galaxies belonging to interacting systems. The first important contribution to the study of galactic kinematics was made by Rubin and collaborators (Rubin et al. 1991, and references therein). A total of about 60 galaxies, selected to cover a wide range in size, mass, and luminosity were observed. There was no aim to build up a complete sample, and most of the objects are non isolated. Moreover, the ulterior photometric analysis (Kent 1988, and references therein) has been done only for some of them, so complete data are only available for a relatively small number of field spirals. Further studies have considerably increased the number of objects, but none of them took into account the information on the environmental status of the galaxies, and were focused either on the photometric properties (de Jong & van der Kruit 1994; de Jong 1996a, b, c; see Table 1 from Héreaudeau & Simien 1996; Peletier & Balcells 1997; more recently, Baggett et al. 1998 for one photometric band data) or to spectroscopic and imaging surveys of field galaxies for which the existing information is long slit spectra together with just one broad band (Mathewson et al. 1992; Courteau 1996).

It is not a simple question to define what an isolated galaxy is. We only try here to establish operational criteria to identify isolated systems. The perturbations that a galaxy can suffer depend, apart the properties of the galaxy itself, on the mass, size, distance and relative velocity of the perturbing agent. Thus, the influence of very far away big galaxies will be negligible, but small galaxies can produce secular alterations on the dynamics of the primary system provided they are close enough (Athanassoula 1984; Sundelius et al. 1987; Byrd & Howard 1992). And they can manifest themselves on very different time scales. Here we define an isolated galaxy as that for which the possible past perturbations by neighboring galaxies, if any, have been completely erased by now. Accepting that typical time scales for the decay of the perturbation effects are not longer than a few times [FORMULA] years, a criterion for isolation can be given. As discussed in Márquez & Moles (1996; hereafter paper I), we consider a galaxy isolated when it has no neighbours in a volume defined by a radius of 0.5 Mpc in projected distance and a redshift difference of 500 km s-1. To be conservative, we also discarded all those galaxies which appear on the POSS prints with close neighbours for which there is no redshift information (see below).

The present work, the second of three, is devoted to the study of the properties of a sample of isolated spiral galaxies. The case of spirals in isolated pairs will be presented in Paper III, whereas the description of both samples, the details of the observations, data reduction and methods of analysis were given in Paper I. We have both, CCD multi-color (Johnson B, V and I bands) photometry and major axis long slit spectra information, for 15 isolated spiral galaxies. For some of those galaxies we also present minor axis long slit spectra and/or H[FORMULA] CCD photometry. We also present long slit spectra for 4 more galaxies. The properties we have measured are compared with those of other analyses to find whether they are different. This contribution is organized as follows: The sample is briefly described in Sect. 2. In Sect. 3 we comment the morphological aspects. In Sect. 4 we analyse the set of parameters obtained from the whole data and in Sect. 5, the relationships among them. The conclusions are presented in Sect. 6.

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© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999