We investigate the [3.6]−[4.5] Spitzer‐IRAC colour behaviour of the early‐type galaxies of the SAURON survey, a representative sample of 48 nearby ellipticals and lenticulars. We investigate how this colour, which is unaffected by dust extinction, can be used to constrain the stellar populations in these galaxies. We find a tight relation between the [3.6]−[4.5] colour and effective velocity dispersion, a good mass indicator in early‐type galaxies: ([3.6]−[4.5])e = (−0.109  0.007)+ (0.154  0.016). Contrary to other colours in the optical and near‐infrared, we find that the colours become bluer for larger galaxies. The relations are tighter when using the colour inside re (scatter 0.013 mag), rather than the much smaller re/8 aperture (scatter 0.023 mag), due to the presence of young populations in the central regions. We also obtain strong correlations between the [3.6]−[4.5] colour and three strong absorption lines (H, Mgb and Fe 5015). Comparing our data with the models of Marigo et al., which show that more metal rich galaxies are bluer, we can explain our results in a way consistent with results from the optical, by stating that larger galaxies are more metal rich. The blueing is caused by a strong CO absorption band, whose line strength increases strongly with decreasing temperature and which covers a considerable fraction of the 4.5‐m filter. In galaxies that contain a compact radio source, the [3.6]−[4.5] colour is generally slightly redder (by 0.015 0.007 mag using the re/8 aperture) than in the other galaxies, indicating small amounts of either hot dust, non‐thermal emission, or young stars near the centre. We find that the large majority of the galaxies show redder colours with increasing radius. Removing the regions with evidence for young stellar populations (from the H absorption line) and interpreting the colour gradients as metallicity gradients, we find that our galaxies are more metal poor going outwards. The radial [3.6]−[4.5] gradients correlate very well with the metallicity gradients derived from optical line indices. We do not find any correlation between the gradients and galaxy mass; at every mass, galaxies display a real range in metallicity gradients. Consistent with our previous work on line indices, we find a tight relation between local [3.6]−[4.5] colour and local escape velocity. The small scatter from galaxy to galaxy, although not negligible, shows that the amount and distribution of the dark matter relative to the visible light cannot be too different from galaxy to galaxy. Due to the lower sensitivity of the [3.6]−[4.5] colour to young stellar populations, this relation is more useful to infer the galaxy potential than the Mgb–vesc relation.

We present an investigation of galaxy-galaxy interactions and their effects on the velocity fields of disc galaxies in combined N-body/hydrodynamic simulations, which include cooling, star formation with feedback, and galactic winds. Rotation curves (RCs) of the gas are extracted from these simulations in a way that follows the procedure applied to observations of distant, small, and faint galaxies as closely as possible. We show that galaxy-galaxy mergers and fly-bys disturb the velocity fields significantly and hence the RCs of the interacting galaxies, leading to asymmetries and distortions in the RCs. Typical features of disturbed kinematics are significantly rising or falling profiles in the direction of the companion galaxy and pronounced bumps in the RCs. In addition, tidal tails can leave strong imprints on the rotation curve. All these features are observable for intermediate redshift galaxies, on which we focus our investigations. We use a quantitative measure for the asymmetry of rotation curves to show that the appearance of these distortions strongly depends on the viewing angle. We also find in this way that the velocity fields settle back into relatively undisturbed equilibrium states after unequal mass mergers and fly-bys. About 1 Gyr after the first encounter, the RCs show no severe distortions anymore. These results are consistent with previous theoretical and observational studies. As an illustration of our results, we compare our simulated velocity fields and direct images with rotation curves from VLT/FORS spectroscopy and ACS images of a cluster at z = 0.53 and find remarkable similarities.

Aims. We trace the interaction processes of galaxies at intermediate redshift by measuring the irregularity of their ionized gas kinematics, and investigate these irregularities as a function of the environment (cluster versus field) and of morphological type (spiral versus irregular). Methods. We obtain the gas velocity fields by placing three parallel and adjacent VLT/FORS2 slits on each galaxy. To quantify irregularities in the gas kinematics, we use three indicators: the standard deviation of the kinematic position angle (σ PA ), the mean deviation of the line of sight velocity profile from the cosine form which is measured using high order Fourier terms (k 3,5 /k 1 ) and the average misalignment between the kinematical and photometric major axes (Δφ). These indicators are then examined together with some photometric and structural parameters (measured from HST and FORS2 images in the optical) such as the disk scale length, rest-frame colors, asymmetry, concentration, Gini coefficient and M 20 . Our sample consists of 92 distant galaxies. 16 cluster (z ∼ 0.3 and z ∼ 0.5) and 29 field galaxies (0.10 ≤ z ≤ 0.91, mean z = 0.44) of these have velocity fields with sufficient signal to be analyzed. To compare our sample with the local universe, we also analyze a sample from the SINGS survey. Results. We find that the fraction of galaxies that have irregular gas kinematics is remarkably similar in galaxy clusters and in the field at intermediate redshifts (according to σ PA ≈ 10%, k 3,5 /k 1 ≈ 30%, Δφ ≈ 70%). The distribution of the field and cluster galaxies in (ir)regularity parameters space is also similar. On the other hand galaxies with small central concentration of light, that we see in the field sample, are absent in the cluster sample. We find that field galaxies at intermediate redshifts have more irregular velocity fields as well as more clumpy and less centrally concentrated light distributions than their local counterparts. Comparison with a SINS sample of 11 z ∼ 2 galaxies shows that these distant galaxies have more irregular gas kinematics than our intermediate redshift cluster and field sample. We do not find a dependence of the irregularities in gas kinematics on morphological type. We find that two different indicators of star formation correlate with irregularity in the gas kinematics. Conclusions. More irregular gas kinematics, also more clumpy and less centrally concentrated light distributions of spiral field galaxies at intermediate redshifts in comparison to their local counterparts indicate that these galaxies are probably still in the process of building their disks via mechanisms such as accretion and mergers. On the other hand, they have less irregular gas kinematics compared to galaxies at z ∼ 2.

Context. We continue our investigation on how the cluster environment affects the evolution of galaxies. Aims. By examining both galaxy structure and internal kinematics of cluster galaxies at lookback times of ∼5 Gyr we study the nature and impact of possible interactions at the peak epoch of cluster assembly. Methods. 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), ∆φ (the average misalignment between kinematic and photometric position angles) and k 3,5 (squared sum of the higher order Fourier terms).Results. 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 , ∆φ 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. Conclusions. 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.

Abstract. We investigate the morphologies and velocity fields of spiral galaxies in distant clusters (z ∼ 0.5) focussing on signatures from interactions. Structural parameters and peculiarities are determined with HST/ACS images. To derive the internal kinematics and rotation curves we have performed 3D-spectroscopy allowing the construction of the full velocity field for each galaxy. Combining both approaches, transformation mechanisms are revealed that affect not only the stellar populations but also the mass distribution. The observations are supported by N-body/SPH simulations of different interaction processes. Figure 1. An example for a member galaxy of cluster MS 0451-03 at z = 0.53. Next to the HST/ACS thumbnail, the 3 slit positions are shown with which we simulate a matched IFU with FORS2. To the left is the observed and reconstructed velocity field. To the right is the extracted gas and stellar rotation curve, below which the change of the position angle of the kinematic axis is plotted. Keywords. galaxies: evolution