A Comparison of the Intrinsic Shapes of Two Different Types of Dwarf Galaxies: Blues Compact Dwarfs and Dwarf Ellipticals

Abstract

We measure the apparent shapes for a sample of 62 blue compact dwarf galaxies (BCDs), and compare them with the apparent shapes for a sample of 80 dwarf elliptical galaxies (dEs). The BCDs are flatter, on average, than the dEs, but the difference is only marginally significant. We then use both non-parametric and parametric techniques to determine possible distributions of intrinsic shapes for the BCDs. The hypothesis that BCDs are oblate spheroids can be ruled out with a high confidence level ($> 99%$), but the hypothesis that they are prolate spheroids cannot be excluded. The apparent shapes of BCDs are totally consistent with the hypothesis that they are triaxial ellipsoids. If the intrinsic axis ratios, $β$ and $γ$, are distributed according to a Gaussian with means $β_0$ and $γ_0$ and standard deviation $σ$, we find the best-fitting distribution for BCDs has $(β_0,γ_0,σ)= (0.66,0.55,0.16)$, while that for dEs has $(β_0,γ_0,σ)= (0.85,0.64,0.24)$. Our results are consistent with the hypothesis that BCDs have a close evolutionary relation with dEs.