2 The sample selection

  The originality of the present study mainly resides in the selection criteria used and the completeness of our sample. Our HI-selected sample has been extracted from the catalog of HI sources in the Hydra I cluster compiled by McMahon (1993) using VLA mapping. The HI observations are also described in detail by Valluri et al. (1999). McMahon's survey covered the area of the Hydra cluster core with a series of 26 hexagonally positioned pointings. At each pointing the entire radial velocity range of the cluster was covered. Seventy percent of the cluster core volume has thus been imaged. The spatial resolution in the VLA C/D configuration was 45$^{\prime\prime}$ and the velocity resolution was 42 km s^-1. The 5$\sigma$ sensitivity of the observations varied between $2.5 \, 10^{7} \mbox{$M_{\odot}$}$ and $1.1\, 10^{8} \mbox{$M_{\odot}$}$ per 42 km s^-1 wide channel for unresolved objects, with column densities in the range $2.4\, 10^{19}-1.3\, 10^{20} \mbox{cm$^{-2}$}$ (Valluri et al. 1999).

We initially selected for our catalog all objects with a 21 cm line width below 130 km s^-1, as objects with such narrow HI lines are good dwarf candidates. The rather low velocity resolution of the VLA, 42 km s^-1, degrades the rigor of this selection criterion, though. Nevertheless, our new 21 cm line observations obtained at Nançay with a considerably higher velocity resolution, 15 km s^-1, have confirmed the narrowness of the HI lines in almost all cases (see Sect. 4.1). Among the 23 HI-preselected galaxies, 3 bright face-on spiral galaxies could already be excluded after inspection of their images in the Digital Sky Survey. The coordinates and, when available, the optical identification in the NED and LEDA databases of the 20 objects of our primary list are indicated in Table 1. The original VLA HI data from McMahon (1993), used for our sample selection, are given in Table 2, where it should be noted that her quoted VLA line width is not a strictly measured physical parameter, but only the velocity coverage of the line channels in which a particular object was detected. This is roughly equivalent to the width at 20% of the peak flux density value.

  

Table 1: Primary list of HI-selected dwarf galaxy candidates in the Hydra cluster

  

Table 2: HI data of the primary list dwarf galaxy candidates

We will see in Sect. 4 that, based on morphological and new HI data, 4 galaxies in this list cannot be classified as dwarf galaxies and/or do not meet the velocity width criteria. Moreover, one HI source has no optical counterpart. Therefore our final list includes 15 HI-selected objects that we confirmed to be dwarf galaxies. These are the objects listed in the first part of Tables 1-3. They have integrated 21 cm line fluxes ranging from 0.16 Jy km s^-1 to 1.36 Jy km s^-1.

  

Table 3: Total effective exposure times in minutes