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Using hydrodynamical $N$-body simulations, we show that the observedstructure and kinematics of the extremely metal-poor, dwarf irregular galaxyDDO 68 is compatible with a merger event with at least two smaller satellitegalaxies. We were able to obtain a self-consistent model that simultaneouslyreproduces several of its observed features, including: the very asymmetric anddisturbed shape of the stellar component, the overall HI distribution and itsvelocity field, the arc-like stellar structure to the west, the low-surfacebrightness stellar stream to the north. The model implies the interaction ofthe main progenitor of DDO 68 with two systems with dynamical masses$7\times10^8\,M_{\odot}$ and almost $10^8\,M_{\odot}$ -- 1/20 and 1/150 timesthe dynamical mass of DDO 68, respectively. We show that the merger between DDO68 and the most massive of its satellites offers a route to explain the largeoffset of DDO 68 from the mass-metallicity relation. Assuming that theinteracting galaxies have metallicities prior to the merger compatible withthose of galaxies with similar stellar masses, we provide quantitative evidencethat gas mixing alone does not suffice at diluting the gas of the twocomponents; according to our simulations, the HII regions observed along theCometary Tail trace the low metallicity of the accreted satellite rather thanthat of DDO 68's main body. In this case, the mass corresponding to the lowmetallicity is that of the secondary body and DDO 68 becomes consistent withthe mass-metallicity relation.

Dancing in the void: hydrodynamical N-body simulations of the extremely metal poor galaxy DDO 68