MUSE sneaks a peek at extreme ram-pressure events

Abstract

We present the MUSE observations of UGC 6697, a giant (Mstar 10^{10} Msol) spiral galaxy infalling in the nearby cluster Abell 1367. During its high velocity transit through the intracluster medium (ICM), the hydrodynamical interactions with the ICM produce a ~ 100 kpc tail of ionized gas that we map with a mosaic of five MUSE pointings up to 60 kpc from the galaxy. CGCG 97087N, a small companion that lies at few arcminutes in projection from UGC 6697, is also suffering from the hydrodynamic action of the ICM of the cluster. Along the whole extent of the tail we detect diffuse H$\alpha$ emission and, to a lesser extent, H$\beta$, [OIII]$\lambda5007$, and [OI]$\lambda6300$. By comparing the kinematics and distribution of gas and stars (as traced by the CaII triplet) for both galaxies, we separate the ionized gas, as traced by the H$\alpha$ line, in a component still bound to the galaxy and a component that is stripped. We find that the "onboard" component shows low velocity dispersion and line ratios consistent with photoionization by hot stars. The stripped gas is more turbulent, with velocity dispersions up to > 100 km/s}, and is excited by shocks as traced by high values of [OI]/H$\alpha$ and [NII]/H$\alpha$ ratio. In the tail of UGC 6697 we identify numerous bright compact knots with line ratios typical of HII regions. These are distributed along the only streams of stripped gas retaining low velocity dispersions (< 35 km/s). Despite being in the stripped gas, their physical properties do not differentiate from normal HII regions in galactic disks. We find evidence of a past fast encounter between the two galaxies in the form of a double tail emerging from CGCG 97087N that connects with UGC 6697. This encounter might have increased the efficiency of the stripping process, leaving the stellar distribution and kinematics unaltered.