1 Introduction

Hickson Compact Groups (hereafter HCGs; Hickson 1982; Hickson 1993) are small systems of several galaxies (four or more) in an apparent close proximity in the sky. The debate on their physical reality as bounded systems is still open. A possibility exists that only a part of the sample of HCGs are bound systems and/or that HCG dynamical evolution depends on their environments. Important informations about their real nature could be obtained by studying the rate of merger and interaction between their galaxies. The studies carried out so far agree with the view of a low merging rate inside HCGs with respect to undisrupted systems of galaxies (Zepf et al. 1991; Zepf 1993). On the other hand it is not so clear which is the fraction of interacting galaxies in HCGs: photometric and spectroscopic studies (Rubin et al. 1991; Mendes de Oliveira et al. 1997; Moles et al. 1994; Mendes de Oliveira et al. 1994; Vilchez & Iglesias Paramo 1998a; Vilchez & Iglesias Paramo 1998b; Iglesias Paramo & Vilchez 1999) have often given contradictory results. It is expected that interaction and merger phenomena strongly affect the star formation rate (SFR) of galaxies. In particular, interacting galaxies should show an higher star formation rate than field galaxies. Thus the study of star formation of galaxies in HCGs gives important clues about the interaction and merger phenomena inside them. Powerful tools to investigate on the star formation activity are the ionization lines emitted by the heated gas surrounding the regions of star formation. The H$_\alpha$ emission line at 6563 Å can be used as a quantitative and spatial tracer of the rate of massive ($\geq$ 10 $M_\odot$) and therefore recent ($\leq 10^7$ years) star formation (Kennicutt 1983; Ryder & Dopita 1994), unlike the color indexes in the U,B,V filters, that give indications about the past star formation (> 10⁸ years). Therefore, by knowing the H$_\alpha$ emission of the HCG galaxies it is possible in principle to carry out important investigations about the present merger and interaction events in these systems. Up to now, only Rubin et al. (1991) and more recently Vilchez & Iglesias Paramo (1998a) have collected significant samples of H$_\alpha$ data on HCG galaxies. They published H$_\alpha$ emission-line images respectively for 14 and 16 HCGs. While Vilchez & Iglesias Paramo (1998a) estimate the H$_\alpha$ flux for each of the 63 galaxies of their sample, Rubin et al. do not use flux calibrated and they take into account a sample constituted by disk galaxies only. H$_\alpha$ data for the galaxies of single groups have been also obtained by Valluri & Anupama (1996); Mendes de Oliveira et al. (1998) and Plana et al. (1998). Valluri & Anupama presented H$_\alpha$ calibrated data for the galaxies of HCG62 and Mendes de Oliveira et al. and Plana et al. reported kinematic observations of H$_\alpha$ emission respectively for four late-type galaxies of HCG16 and for two early-type galaxies and one disk system of HCG90.

With the aim to obtain quantitative information about the H$_\alpha$ emission of HCG galaxies we have observed 31 HCGs in narrow-band interferometric filters deriving H$_\alpha$ calibrated fluxes for 95 galaxies, 22 out of which are upper limits. In this paper we present the catalogue containing these H$_\alpha$ data.

We first describe the sample and the observations in Sect. 2 and in Sect. 3. In Sect. 4 and Sect. 5 we present the data reduction and calibration procedures used. The Zero Point correction, Galactic and Internal extinction corrections applied to the fluxes are described in Sect. 6, while Sect. 7 contains the photometric error derivation. In Sect. 8 we present the H$_\alpha$ Catalogue of Galaxies, while in Sect. 9 we derive the star formation rate for the whole sample. Finally we briefly discuss some of the observed groups in Sect. 10.