2002AJ....123.1476W

We present a radio interferometer mosaic of H I line emission within a radius of 45' of the small irregular galaxy Sextans A at a resolution of about 1'. Sextans A lies on the edge of the Local Group and was targeted because single-dish observations showed it to have an unusually extended disk of H I. However, we detected H I emission only to a radius of 9' at 8x10¹⁸ cm^–2, whereas single-dish observations detected H I to a radius 3 times that at approximately the same column density. We also recover only 62% of the flux detected in the single-dish maps. We conclude that the missing H I is real and is most likely to be found in a very smooth, low column density distribution whose scale is too large to be resolved with the interferometer. We put limits on the size of density fluctuations in this extended gas on the order of 10'-15'. We argue that this unusually smooth structure implies that Sextans A has not been perturbed by another galaxy for at least 6-10 Gyr. The fact that the current star formation rate is 4-5 times higher than the rate averaged over the galaxy's lifetime implies that internal processes are driving the modest fluctuation in the current star formation rate.