The Intrinsic Shapes of Low Surface Brightness Dwarf Irregular Galaxies and Comparison to Other Types of Dwarf Galaxies

In this paper, we measure the ellipticities of 30 low surface brightness (LSB) dwarf irregular (dI) galaxies and compare the ellipticity distribution with that of 80 dwarf elliptical (dEs) and 62 blue-compact dwarfs (BCDs). We find that the ellipticity distribution of LSB dIs is very similar to that of BCDs, and marginally different from that of dEs. We then determine the distribution of intrinsic shapes of dI galaxies and compare this to the distributions of other types of dwarf galaxies under various assumptions. First, we assume that LSB dIs are either all oblate or all prolate, and use a nonparametric analysis to find the best-fitting distribution of intrinsic shapes. With this assumption, we find that the scarcity of nearly circular LSB dIs implies, at the 99% confidence level, that they cannot be a population of randomly oriented oblate or prolate objects, implying that LSB dIs are highly unlikely to be disk-shaped systems. Next, we assume that dIs are triaxial, and use a parametric analysis to find permissible distributions of intrinsic shapes. We find that if the intrinsic axis ratios β and γ are distributed according to a Gaussian with means β0 and γ0 and a common standard deviation of σ, the best-fitting set of parameters for LSB dIs is (β0, γ0, σ) = (0.66, 0.50, 0.15), and the best fit for BCDs is (β0, γ0, σ) = (0.66, 0.55, 0.16), while the best fit for dEs is (β0, γ0, σ) = (0.78, 0.69, 0.24). The dIs and BCDs thus have very similar shape distributions, given this triaxial hypothesis, while the dEs peak at a somewhat more spherical shape. Therefore, our results provide strong observational evidence to support the evolutionary scenario in which the three types of dwarf galaxy have a close relation with each other.