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

TL;DR

This study uses hydrodynamical N-body simulations to demonstrate that the peculiar features of the extremely metal-poor galaxy DDO 68 can be explained by a merger with smaller satellite galaxies, shedding light on its unusual properties.

Contribution

The paper presents a self-consistent hydrodynamical N-body model showing how a merger explains DDO 68's structure, kinematics, and metallicity offset, which is a novel explanation for its properties.

Findings

The observed features of DDO 68 are consistent with a merger with two smaller satellites.

The merger explains the galaxy's offset from the mass-metallicity relation.

Gas mixing alone does not account for the low metallicity observed in the tail regions.

Abstract

Using hydrodynamical $N$-body simulations, we show that the observed structure and kinematics of the extremely metal-poor, dwarf irregular galaxy DDO 68 is compatible with a merger event with at least two smaller satellite galaxies. We were able to obtain a self-consistent model that simultaneously reproduces several of its observed features, including: the very asymmetric and disturbed shape of the stellar component, the overall HI distribution and its velocity field, the arc-like stellar structure to the west, the low-surface brightness stellar stream to the north. The model implies the interaction of the 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 times the dynamical mass of DDO 68, respectively. We show that the merger between DDO 68 and the most massive of its satellites offers a route to explain the large offset of DDO 68 from the mass-metallicity relation. Assuming that the interacting galaxies have metallicities prior to the merger compatible with those of galaxies with similar stellar masses, we provide quantitative evidence that gas mixing alone does not suffice at diluting the gas of the two components; according to our simulations, the HII regions observed along the Cometary Tail trace the low metallicity of the accreted satellite rather than that of DDO 68's main body. In this case, the mass corresponding to the low metallicity is that of the secondary body and DDO 68 becomes consistent with the mass-metallicity relation.