The stellar and gaseous contents of the Orion dwarf galaxy

Abstract

We present new Kitt Peak National Observatory 0.9 m optical and Very Large Array H I spectral line observations of the Orion dwarf galaxy. This nearby (D ≃ 5.4 Mpc), intermediate-mass (M dyn≃ 1.1×10 10 M ⊙) dwarf displays a wealth of structure in its neutral interstellar medium, including three prominent "hole/ depression" features in the inner H I disk. We explore the rich gas kinematics, where solid-body rotation dominates and the rotation curve is flat out to the observed edge of the H I disk (∼6.8 kpc). The Orion dwarf contains a substantial fraction of dark matter throughout its disk: comparing the 4.7×108 M ⊙ of detected neutral gas with estimates of the stellar mass from optical and near-infrared imaging (3.7×108 M ⊙) implies a mass-to-light ratio ≃13. New Hα observations show only modest-strength current star formation (SF; 0.04 M ⊙yr-1); this SF rate is consistent with our 1.4 GHz radio continuum non-detection. © 2010. The American Astronomical Society. All rights reserved.