Deep neutral hydrogen observations of Leo T with the Westerbork Synthesis Radio Telescope

Leo T is the lowest mass gas-rich galaxy currently known and studies of its gas content help us understand how such marginal galaxies survive and form stars. We present deep neutral hydrogen (HI) observations from the Westerbork Synthesis Radio Telescope in order to understand its HI distribution and potential for star formation. We find a larger HI line flux than the previously accepted value, resulting in a 50% larger HI mass of 4.1 x 10^5 Msun. The additional HI flux is from low surface brightness emission that was previously missed; with careful masking this emission can be recovered even in shallower data. We perform a Gaussian spectral decomposition to find a cool neutral medium component (CNM) with a mass of 3.7 x 10^4 Msun, or almost 10% of the total HI mass. Leo T has no HI emission extending from the main HI body, but there is evidence of interaction with the Milky Way circumgalactic medium in both a potential truncation of the HI body and the offset of the peak HI distribution from the optical center. The CNM component of Leo T is large when compared to other dwarf galaxies, even though Leo T is not currently forming stars and has a lower star formation efficiency than other gas-rich dwarf galaxies. However, the HI column density associated with the CNM component in Leo T is low. One possible explanation is the large CNM component is not related to star formation potential but rather a recent, transient phenomenon related to the interaction of Leo T with the Milky Way circumgalactic medium.