Hickson 62 I. Kinematics of NGC 4778

Detailed studies of the photometric and kinematical properties of compact groups of galaxies are crucial to understand the physics of galaxy interactions and to shed light on some aspects of galaxy formation and evolution. In this paper we present a kinematical and photometrical study of a member, NGC 4778, of the nearest (z = 0.0137) compact group: Hickson 62. The aim of this work was to investigate the existence of kinematical anomalies in the brightest group member, NGC 4778 in order to constrain the dynamical status and the formation history of the group. We used long-slit spectra obtained with FORS1 at VLT, to measure line-of-sight velocity distributions by means of the Fourier Correlation Quotient method, and to derive the galaxy rotation curve and velocity dispersion profile. Our analysis reveals that Hickson 62a, also known as NGC 4778, is an S0 galaxy with kinematical and morphological peculiarities, both in its central regions (r < 5 '') and in the outer halo. In the central regions, the rotation curve shows the existence of a kinematically decoupled stellar component, offset with respect to the photometric center. In the outer halo we find an asymmetric rotation curve and a velocity dispersion profile showing a rise on the SW side, in direction of the galaxy NGC 4776. The nuclear counterrotation, the distorted kinematics in the outer halo and the X-ray properties of the group suggest that NGC 4778 may be the product of a recent minor merger, more reliable with a small late-type galaxy.