The ATLAS project – II. Morphologies, kinemetric features and alignment between photometric and kinematic axes of early-type galaxies

We use the ATLAS sample of 260 early-type galaxies to study the apparent kinematic misalignment angle, Ψ, defined as the angle between the photometric and kinematic major axis. We find that 71% of nearby early-type galaxies are strictly aligned systems (Ψ ≤ 5◦), an additional 14% have 5◦ < Ψ ≤ 10◦ and 90% of galaxies have Ψ ≤ 15. Taking into account measurement uncertainties, 90% of galaxies can be considered aligned to better than 5, suggesting that only a small fraction of early-type galaxies (∼ 10%) are not consistent with axisymmetry within the projected half-light radius. We identify morphological features such as bars and rings (30%), dust structures (16%), blue nuclear colours (6%) and evidence of interactions (8%) visible on ATLAS galaxies. We use kinemetry to analyse the mean velocity maps and separate galaxies in two broad types of regular and non-regular rotators. We find 82% of regular rotators and 17% non-regular rotators, with 2 galaxies that we were not able to classify due to data quality. The non-regular rotators are typically found in dense regions and are massive. We characterise the specific features in the mean velocity and velocity dispersion maps. The majority of galaxies does not have any specific features, but we highlight here the frequency of the kinematically distinct cores (7% of galaxies) and the aligned double peaks in the velocity dispersion maps (4% of galaxies). We separate galaxies into 5 kinematic groups based on the kinemetric features, which are then used to interpret the (Ψ − ǫ) diagram. Most of the galaxies that are misaligned have complex kinematics and are non-regular rotators. In addition, some show evidence of interaction and might not be in equillibrium, while some are barred. While the trends are weak, there is a tendency that large values of Ψ are found in galaxies at intermediate environmental densities and among the most massive galaxies in the sample. Taking into account the kinematic alignment and the kinemetric analysis, the majority of early-type galaxies have velocity maps more similar to the spiral disks than to the remnants of equal mass mergers. We suggest that the most common formation mechanism for early-type galaxies preserves the axisymmetry of the disk progenitors and their general kinematic properties. Less commonly, the formation process results in a triaxial galaxy with much lower net angular momentum.