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Dwarf galaxies play a crucial role in contemporary theories for the formation and evolution of galaxies. They are proposed to be the building blocks from which larger systems have been created by merging (Kauffmann, White, & Guiderdoni 1993).

Blue Compact Dwarf galaxies (BCDs) are spectroscopically characterized by a faint, blue optical continuum accompanied, in most cases, by intense emission lines. Their current star formation rates (Fanelli et al. 1988) and neutral gas content (Thuan & Martin 1981) imply gas consumption time-scales of ~ 109yr, much shorter than the age of the Universe. This fact, combined with the low metal abundances (1/3 > Z > 1/50 Zodot; Hunter and Hoffman 1999), led Searle, Sargent, & Bagnuolo (1973) to suggest earlier that either these objects are young galaxies or that they have experienced an episodic star-formation history.

The analysis of BCD surface brightness and color profiles (James 1994; Papaderos et al. 1996a, b; Doublier et al. 1997, 1999; Cairós et al. 2001a, b) combined with HST color-magnitude diagrams (Aloisi, Tosi, & Greggio 1999; Schulte-Ladbeck et al. 1999, 2000, 2001; Östlin 2000; Crone et al. 2000, 2002; Drozdovsky et al. 2001) has shown the existence of an underlying stellar population at least a few 109 yr old in most of these galaxies, i.e. these are not purely "young" galaxies. In this sense, Legrand et al. (2000) and Crone et al. (2000) have recently suggested that the formation of this evolved stellar population could have taken place at a low but continuous rate rather than in a purely bursting mode (see also Heller et al. 2000 concerning the lack of other evidence for random star formation in BCDs). Only in the case of the most metal poor BCDs (Z < 1/20 Zodot; Izotov & Thuan 1999) are the results regarding their evolutionary status still contradictory, mainly due to uncertain distances (Östlin 2000) and contamination of the galaxy outer halo colors by nebular emission (Izotov et al. 2001a, b; Papaderos et al. 1998, 2002).

Understanding of the role played by the collective supernovae-driven winds in the mass (i.e. luminosity) and chemical evolution, and in the propagation of the star formation in these galaxies is also mandatory. In this sense, the evolution of these winds in low-mass galaxies like the BCDs is thought to lead to the loss of a (still highly uncertain) fraction of metals (De Young & Heckman 1994; Mac-Low & Ferrara 1999; Silich & Tenorio-Tagle 2001), making these galaxies arguably one of the major polluters of the intergalactic medium (Ferrara & Tolstoy 2000).

In order to shed light on some of these questions we have obtained BRHalpha imaging data for a statistically significant sample of BCD galaxies. This sample includes 114 galaxies, all except 9 of them classified as BCDs. Similar recent studies include Papaderos et al. (1996a, b), Telles & Terlevich (1997), Doublier et al. (1997, 1999), and Cairós et al. (2001a, b). However, these studies analyze samples that contain at least a factor of 4 fewer galaxies than our sample and, excepting the recent work of Cairós et al. (2001b), none of these studies include emission-line imaging data.

In Section 2 we describe the properties of our sample. A new set of observational, quantitative criteria to define the Blue Compact Dwarf class of galaxies is proposed in Section 3. The observations and reduction procedures are described in Sections 4 and 5. Sections 6 and 7 present the images of the galaxies along with their morphological classification. The results from the analysis of the integrated photometry of the sample are given in Section 8. We summarize our conclusions in Section 9. In Paper II we will analyze the structural properties of the sample, both in the continuum and in the emission-lines, by carrying out a decomposition of the surface brightness profiles, and presenting the concentration and asymmetry parameters in the different bands.

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