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 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 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, MB = -13.5, and I Zw 49, MB = -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 (MB = -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 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 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.
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