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Quiescent Isolation: The Extremely Extended HI Halo of the Optically Compact Dwarf Galaxy ADBS 113845+2008

We present new optical imaging and spectroscopy and HI spectral line imaging of the dwarf galaxy ADBS 113845+2008 (hereafter ADBS 1138). This metal-poor (Z~30% Z_Sun), "post-starburst" system has one of the most compact stellar distributions known in any galaxy to date (B-band exponential scale length =0.57 kpc). In stark contrast to the compact stellar component, the neutral gas is extremely extended; HI is detected to a radial distance of ~25 kpc at the 10^19 cm^-2 level (>44 B-band scale lengths). Comparing to measurements of similar "giant disk" dwarf galaxies in the literature, ADBS 1138 has the largest known HI-to-optical size ratio. The stellar component is located near the center of a broken ring of HI that is ~15 kpc in diameter; column densities peak in this structure at the ~3.5x10^20 cm^-2 level. At the center of this ring, in a region of comparatively low HI column density, we find ongoing star formation traced by H alpha emission. We sample the rotation curve to the point of turn over; this constrains the size of the dark matter halo of the galaxy, which outweighs the luminous component (stars + gas) by at least a factor of 15. To explain these enigmatic properties, we examine "inside-out" and "outside-in" evolutionary scenarios. Calculations of star formation energetics indicate that "feedback" from concentrated star formation is not capable of producing the ring structure; we posit that this is a system where the large HI disk is evolving in quiescent isolation. In a global sense, this system is exceedingly inefficient at converting neutral gas into stars.