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Measurement of the gas velocity distribution in galaxy clusters providesinsight into the physics of mergers, through which large scale structures formin the Universe. Velocity estimates within the intracluster medium (ICM) can beobtained via the Sunyaev-Zel'dovich (SZ) effect, but its observation ischallenging both in term of sensitivity requirement and control of systematiceffects, including the removal of contaminants. In this paper we reportresolved observations, at 150 and 260 GHz, of the SZ effect toward the triplemerger MACS J0717.5+3745 (z=0.55), using data obtained with the NIKA camera atthe IRAM 30m telescope. Assuming that the SZ signal is the sum of a thermal(tSZ) and a kinetic (kSZ) component and by combining the two NIKA bands, weextract for the first time a resolved map of the kSZ signal in a cluster. ThekSZ signal is dominated by a dipolar structure that peaks at -5.1 and +3.4sigma, corresponding to two subclusters moving respectively away and toward usand coincident with the cold dense X-ray core and a hot region undergoing amajor merging event. We model the gas electron density and line-of-sightvelocity of MACS J0717.5+3745 as four subclusters. Combining NIKA data withX-ray observations from XMM-Newton and Chandra, we fit this model to constrainthe gas line-of-sight velocity of each component, and we also derive, for thefirst time, a velocity map from kSZ data (i.e. that is model-dependent). Ourresults are consistent with previous constraints on the merger velocities, andthanks to the high angular resolution of our data, we are able to resolve thestructure of the gas velocity. Finally, we investigate possible contaminationand systematic effects with a special care given to radio and submillimetergalaxies. Among the sources that we detect with NIKA, we find one which islikely to be a high redshift lensed submillimeter galaxy.

Mapping the kinetic Sunyaev-Zel'dovich effect toward MACS J0717.5+3745 with NIKA