2008A&A...488..117K

We continue our investigation on how the cluster environment affects the evolution of galaxies. By examining both galaxy structure and internal kinematics of cluster galaxies at lookback times of ∼5Gyr we study the nature and impact of possible interactions at the peak epoch of cluster assembly. Going beyond our previous measurements of two-dimensional rotation curves, we here observe the whole velocity field of the galaxies of the sample. We achieve a complete coverage and optimal spatial sampling of galaxy sizes by placing three adjacent and parallel FORS2 MXU (Mask eXchange Unit) slits onto each object yielding simultaneously several emission and absorption lines. We reconstruct the gas velocity field and decompose it into circular rotation and irregular motions using a harmonic decomposition method called kinemetry. To measure the irregularity in the gas kinematics, we define 3 parameters: σ_PA (the standard deviation of the kinematic position angle within a galaxy), Δφ (the average misalignment between kinematic and photometric position angles) and k_3,5 (squared sum of the higher order Fourier terms). We present the analysis of the velocity fields and morphology of 22 distant galaxies in the MS 0451.6-0305 field with 11 members at z=0.54 and a local sample from SINGS. Using local, undistorted galaxies the three parameters σ_PA, Δφ and k_3,5 can be used to establish the regularity of the gas velocity fields. Among the galaxies for which we could measure these parameters, we find both field ones (4 of 8) and cluster members (3 of 4), which have a velocity field that we consider both irregular and asymmetric. We show that these fractions are underestimates of the total number of objects with irregular velocity fields. The values of the irregularity parameters for cluster galaxies are not very different from those of the field galaxies, implying that there are isolated field galaxies that are as distorted as the cluster members. None of the deviations in our small sample correlate with photometric/structural properties like luminosity or disk scale length in a significant way. We have demonstrated that our 3D-spectroscopic method successfully maps the velocity field of distant galaxies. Together with a structural analysis the importance and efficiency of cluster specific interactions can be assessed quantitatively.