1 Introduction

This paper is the third of a series devoted to H$\alpha$+[NII] imaging observations of Virgo cluster galaxies: in Gavazzi et al. (2002; Paper I), we present H$\alpha$ imaging data obtained with the 2.1 m of San Pedro Martir, mostly of low luminosity spirals and irregulars. In Boselli & Gavazzi (2002; Paper II) we present data obtained with the 1.2 m telescopes at the Observatoire de Haute Provence and at Calar Alto for the brightest galaxies in the cluster. This paper presents H$\alpha$ imaging data obtained at the 2.5 m INT and NOT telescopes for blue compact dwarfs (BCDs) galaxies in Virgo. These data are aimed at completing the H$\alpha$ survey of BCD galaxies of Almoznino et al. (1998) and Heller et al. (1999) aimed at studying the star formation activity in low-mass, dwarf galaxies. These objects are particulary useful to test whether the scaling relations of giant spirals, as derived by Boselli et al. (2001), hold true at the low end of the luminosity function. Given the lack of spiral patterns in BCDs, star formation cannot be triggered by compression from gravitational density waves and rotational shear, as in giant spirals, but is probably governed by random collision of interstellar clouds (Hunter et al. 1998). These characteristics make BCDs and dwarf irregulars a unique class of objects for studying the physical conditions behind the star formation.

Data for other bright galaxies, serendipitously observed in the Wide Field Camera (WFC) at the INT are also presented.

The analysis based on these data will be discussed in future communications.