1 Introduction

A Blue Compact Galaxy (BCG) is characterised by its blue optical colours, an H II region like emission line spectrum (therefore sometimes also referred to as an H II galaxy) and compact appearance on photographic sky survey plates. BCGs have small to intermediate sizes (as measured e.g. by R₂₅). Optical spectroscopy of BCGs in general reveal high star formation rates and low chemical abundances (Searle & Sargent 1972; Lequeux et al. 1979; French 1980; Kunth & Sargent 1983; Masegosa et al. 1994). Moreover most BCGs are rich in neutral hydrogen (Thuan & Martin 1981), a requisite for the intense star formation generally seen. However, the gas consumption time-scale is generally much shorter than the age of the universe, indicating that the high star formation rate must be transient. Such a galaxy is commonly referred to as a starburst galaxy. Thus BCGs are either genuinely young galaxies, or old galaxies that, for some reason, have resumed to form stars at a prodigious rate (Searle & Sargent 1972; Searle et al. 1973). It is believed that BCGs undergo short (on the order of a few times 10 Myr) starbursts intervened by longer (on the order of a Gyr) passive periods. A cyclic scenario has been proposed and may result from statistical effects (Searle et al. 1973; Gerola et al. 1980). Another possibility is that supernova driven winds halt star formation by expelling the gas. Later, the lost gas might accrete back on the galaxy and create a new starburst (Dekel & Silk 1986; Silk et al. 1987, and Babul & Rees 1992). Other ideas incorporate galaxy interactions as the triggering mechanism behind starburst activity (cf. e.g. Lacey et al. 1993; Sanders et al. 1988). Most BCGs are found outside galaxy clusters and in general seems to be fairly isolated, although there are indications that HI-companions, sometimes without obvious optical counterparts, may be common (Taylor 1997).

These different scenarios have been quite widely debated over the years, and while there is now ample evidence that most BCGs contain old stars indicating that the present burst is not the first one (for references see Paper II), there is no consensus on the process(es) that trigger the bursts of star formation now evident. Most arguments have been based on photometry alone. On the other hand the dynamics of these systems are not well explored, still the creation of an energetic event like a sudden burst of star formation is likely to have dynamical causes and impacts.

To improve our understanding of the dynamics and the triggering mechanisms behind the starburst activity we have obtained H$\alpha$ velocity fields, using scanning Fabry-Perot (FP) interferometry, of a sample of BCGs. With a FP it is possible to achieve a two dimensional velocity field with both high spatial and spectral (velocity) resolution. Thus we can get a much better view of the gas motions as compared to long slit spectroscopy. The velocity fields can also be used to estimate the dynamical masses of the galaxies, and from the H$\alpha$ intensities it is possible to estimate the mass of ionised gas. Previous integral field studies of BCGs at high spectral resolution are rare. Thuan et al. (1987) used Fabry-Perot interferometry to study two BCGs, both fainter than ours (VII Zw 403, M_B = -13.5, and I Zw 49, M_B = -17.3). While VII Zw403 showed no well ordered large scale motion, I Zw49 to some degree did. Petrosian et al. (1997) studied I Zw 18 (M_B = -13.9) and found indications of solid body rotation and asymmetric line profiles, which they interpreted to be caused by gas motions in the centre of the galaxy.

In this paper we will present the FP observations of six luminous BCGs. These were selected to be bright in H$\alpha$ emission. Two of the galaxies have un-catalogued but confirmed star forming companion galaxies and these were also observed. Most galaxies are from the sample by Bergvall & Olofsson (1986) and a newer extended version of it. In addition, one galaxy has been taken from the catalogue by Terlevich et al. (1991). In this paper (Paper I) we will present the observations (Sect. 2), reductions (Sect. 3) and the results: the derived H$\alpha$ images, velocity fields and continuum images (Sect. 4). In Sect. 5 we describe how the rotation curves (RCs) were constructed and provide rough mass estimates based on these. In Sect. 6 we give comments on the velocity fields and RCs of the individual target galaxies. In Sect. 7 we give a short summary of the results presented in this paper. Throughout this paper we will use a Hubble constant of H₀ = 75 km s^-1/Mpc. In Paper II (Östlin et al. 1999) we will discuss the interpretation of these results and their implications on the masses and dynamics of the galaxies and the triggering mechanism behind their starbursts.