The SAURON project - XII. Kinematic substructures in early-type galaxies: evidence for discs in fast rotators

Abstract

We analysed two-dimensional maps of 48 early-type galaxies obtained with the SAURON and OASIS integral-field spectrographs using kinemetry, a gene ralisation of surface photometry to the higher order moments of the line-of-sight velocity distribution (LOSVD). The maps analysed include: reconstructed image, mean velocity, velocity dispersion, h3 and h4 GaussHermite moments. Kinemetry is a good method to recognise structures otherwise missed by using surface photometry, such as embedded disks and kinematic sub-components. In the SAURON sample, we find that 31% of early-type galaxies are sin gle component systems. 91% of the multi-components systems have two kinematic subcomponents, the rest having three. In addition, 29% of galaxies have kinematically decoupled components, nuclear components with significant kinematic twists. We differentiate betwee n slow and fast rotators using velocity maps only and find that fast rotating galaxies contain disks with a large range in mass fractions to the main body. Specifically, we find that the velo city maps of fast rotators closely resemble those of inclined disks, except in the transition r egions between kinematic subcomponents. This deviation is measured with the kinemetric k5/k1 ratio, which is large and noisy in slow rotators and about 2% in fast rotators. In terms of E/S0 classification, this means that 74% of Es and 92% of S0s have components with disk-like kinematics. We suggest that differences in k5/k1 values for the fast and slow rotators arise from their differ ent intrinsic structure which is reflected on the velocity maps. For the majority of fa st rotators, the kinematic axial ratios are equal to or less than their photometric axial rati os, contrary to what is predicted with isotropic Jeans models viewed at different inclinations. T he position angles of fast rotators are constant, while they vary abruptly in slow rotators. Velocity dispersion maps of face-on galaxies have shapes similar to the distribution of light. Veloci ty dispersion maps of the edge-on fast rotators and all slow rotators show differences which can only be partially explained with isotropic models and, in the case of fast rotators, often req uire additional cold components. We constructed local (bin-by-bin) h3 V/σ and h4 V/σ diagrams from SAURON observations. We confirm the classical anti-correlation of h3 and V/σ, but we also find that h3 is almost zero in some objects or even weakly correlated with V/σ. The distribution of h4 for fast and slow rotators is mildly positive on average. In general, fast rot ators contain flattened components characterised by a disk-like rotation. The difference betw een slow and fast rotators is traceable throughout all moments of the LOSVD, with evidence for different intrinsic shapes and orbital contents and, hence, likely different evolutionary paths.