The evolved splash bridge system UGC 6944: a population synthesis, X-ray and neutral hydrogen study

We present a study of the interacting galaxy group UGC 6944 (NGC 3991/4/5) and demonstrate that it is a potential evolved splash bridge system. Using multi-wavelength observations of UGC 6944 ranging from far-ultraviolet to far-infrared wavelengths, we have performed stellar population synthesis modelling using CIGALE. Our modelling reveals that NGC 3991 and NGC 3995 both experienced a burst of star formation ~600 Myr ago, thus establishing the time-scale of their latest interaction. In addition, we have analysed X-ray and 21-cm observations of UGC 6944 obtained by the Einstein Observatory and the Westerbork Synthesis Radio Telescope (WSRT) respectively. Our analysis of the 21-cm observations reveals a huge clump of neutral hydrogen gas in the H i bridge connecting NGC 3991 and NGC 3995. The formation of H i clumps after a disc-disc collision between gas rich galaxies had been predicted by previous simulations, and our study provides the first ever observational evidence of such structures. Further, we have identified asymmetric X-ray emission associated with NGC 3991. We have provided a possible origin scenario of the X-ray emitting gas and a simplified model to explain the observed asymmetric X-ray distribution.