The neutral interstellar medium in nearby dwarf galaxies .1. Leo A

We present VLA observations of the 21 cm emission at 15'' = 60 pc resolution in the nearby dwarf irregular galaxy Leo A. These new data reveal a previously unknown H I envelope or halo that extends to 9 kpc x 5 kpc (3 times the optical size of the galaxy) and surrounds the region of high column density where young stars and H II regions are found. H I line profiles in Leo A can be separated into a broad spectral component (sigma = 9 km s(-1)), observed everywhere in the galaxy and containing 80% of the H I mass, and a narrower component (3.5 km s(-1)), observed only in the central part of the galaxy near the H II regions. These spectral components may represent cold and warm phases of the neutral gas, analogous to those found in the Galaxy. The velocity field of H I in Leo A does show a gradient that may be due to rotation, but the observations are inconsistent with H I distributed in an inclined, circular disk supported by rotation. There is no compelling evidence that Leo A contains a significant amount of dark matter.