However, little is known at present about the role of the environment on the evolution of dwarf galaxies though they represent ideal laboratories to study it, as they are numerous and their low mass should make them very sensitive to any external perturbation. Some preliminary studies have put forward a number of intriguing properties of dwarf galaxies in clusters. Hoffman et al. (1988) found that dwarf galaxies are not more HI deficient than spirals, while Valluri & Jog (1990) claimed that the deficiency actually depends on the galaxy size, with dwarfs being the least deficient. These are apparently surprising results, since the HI gas in dwarfs, especially in dwarf irregulars, tends to be relatively more diffuse and extended than in spirals. However how easily the HI will be stripped from the galaxy depends on how much it is gravitationally bound to the stellar component and to the dark matter halo which might be the dominant mass.
The complete, three-dimensional coverage made in HI with the VLA of two clusters, Hydra (McMahon 1993; Valluri et al. 1999) and Hercules (Dickey 1997), has shown the presence of HI sources not associated with previously identified massive galaxies. Similar HI sources were also found in VLA fields centered on bright galaxies in two other clusters, Virgo (Cayatte et al. 1990) and Coma (Bravo et al. 1999). The survival of such low-mass HI-clouds in the close environment of stripped cluster spirals, as observed in Hercules, Virgo and Coma, is puzzling.
Numerous surveys of optically selected dwarf galaxies in nearby clusters
have already been performed and were not very conclusive as regards the
role of the environment: e.g.
Bothun et al. (1986);
Gallagher & Hunter (1989),
Impey et al. (1988)
and
Almoznino & Brosch (1998)
in Virgo;
Secker et al. (1997)
and
Trentham (1998)
in Coma. However, most of the abovementioned works
were mainly based on the study of stellar properties which are a priori less
affected by the environment
than the gas. When available, the HI data of these clusters suffer from severe
incompleteness. In a cluster as close as Virgo, HI maps
have been obtained for only a limited number of galaxies. This lead us to
adopt a different approach in the study of
environmental effects on dwarf galaxies, i.e., through
selection of HI sources likely to be associated with dwarf galaxies
in complete surveys of the atomic gas in clusters, and studies of
the properties of their stellar counterparts.
Among the nearby clusters of galaxies, the Hydra I cluster (Abell 1060), at an adopted distance of 45 Mpc (H0 = 75 km s-1 Mpc-1), has the advantage of not being as hugely extended over the sky as Virgo. Hence a quasi-complete radio synthesis HI survey towards this cluster would be feasible with a reasonable amount of pointings, and has in fact been made by McMahon (1993) at the VLA. Hydra I is classified as an irregular cluster of richness class 1 and morphological type III in the nomenclature of Bautz & Morgan (1970). This cluster, which appears to have a regular structure in X-ray (Fitchett & Merritt 1988), is at first sight dynamically relaxed and its large spirals show apparently only moderate HI deficiencies (Richter & Huchtmeier 1983). However, various analyses of the dynamics of the cluster indicate evidence of substructures that could explain the lack of on-going gas stripping from the massive galaxies (Fitchett & Merritt 1988; McMahon et al. 1992 and recently Valluri et al. 1999). A major result of the HI survey of Hydra I by McMahon (1993) is the discovery of numerous HI sources likely to be associated with low-mass galaxies.
Several optical catalogs of galaxies in Hydra have already been compiled by Richter et al. (1982) [hereafter R82], Richter (1987) [R87] - updated in 1989 (Richter 1989), and more recently by Hamabe (1993). However, they include galaxies brighter than V=17 only and have therefore missed a lot of dwarf galaxies, in particular, as we will see, most of the optical counterparts of the faint VLA HI sources.
In this paper, we present HI line spectra and optical/near-infrared photometric data for a sample of HI-selected dwarf galaxies in the Hydra I cluster. The selection criteria are detailed in Sect. 2, while the observations made at Nançay and at ESO are described in Sect. 3. Section 4 deals with the principal results: the identification of the HI sources, a careful surface brightness and color profiles analysis of their stellar counterparts and a preliminary comparison of the structural properties of the Hydra dwarfs with other field and cluster dwarf galaxies. Finally, in Sect. 5 notes on individual objects in the sample are presented. Long-slit spectroscopic data obtained in March 1998 with the ESO 3.6 m telescope will be presented in Paper II (Duc et al., in preparation) whereas the general discussion on the environmental effects on the HI-detected Hydra dwarfs will be the subject of another paper of these series.
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