The discovery of a previously unknown 21cm HI line source identified as an ultra-compact high velocity cloud in the ALFALFA survey is reported. The HI detection is barely resolved by the Arecibo 305m telescope ∼4 ′ beam and has a narrow HI linewidth (HPFW of 24 km s −1 ). Further HI observations at Arecibo and with the VLA corroborate the ALFALFA HI detection, provide an estimate of the HI radius, ∼ 1 ′ at the 5 × 10 19 cm −2 isophote, and show the cloud to exhibit rotation with an amplitude of ≃ 9.0±1.5 km s −1 . In other papers, Rhode et al. (2013) show the HI source to have a resolved stellar counterpart and ongoing star forming activity, while Skillman et al. (2013) reveal it as having extremely low metallicity: 12 + log(O/H) = 7.16 ± 0.04. The HI mass to stellar mass ratio of the object is found to be 2.6. We use the Tully-Fisher template relation in its baryonic form (McGaugh 2012) to obtain a distance estimate D Mpc = 1.3 +0.9 −0.5 . Additional constraints on the distance are also provided by the optical data of Rhode et al. (2013) and McQuinn et al. (private communication), both indicating a distance in the range of 1.5 to 2.0 Mpc. The three estimates are compatible within their errors. The object appears to be located beyond the dynamical boundaries of, but still in close proximity to the Local Group. Its pristine properties are consistent with the sedate environment of its location. At a nominal distance of 1.75 Mpc, it would have an HI mass of ≃ 1.0 × 10 6 M ⊙ , a stellar mass of ≃ 3.6 × 10 5 M ⊙ , and a dynamical mass within the HI radius of ≃ 1.5 × 10 7 M ⊙ . This discovery supports the idea that optically faint -or altogether dark -low mass halos may be detectable through their non-stellar baryons.

We present new H i spectral line imaging of the extremely metal-poor, star-forming dwarf irregular galaxy Leo P. Our H i images probe the global neutral gas properties and the local conditions of the interstellar medium (ISM). The H i morphology is slightly elongated along the optical major axis. We do not find obvious signatures of interaction or infalling gas at large spatial scales. The neutral gas disk shows obvious rotation, although the velocity dispersion is comparable to the rotation velocity. The rotation amplitude is estimated to be V c =15 ± 5 km s −1 . Within the H i radius probed by these observations, the mass ratio of gas to stars is roughly 2:1, while the ratio of the total mass to the baryonic mass is 15:1. We use this information to place Leo P on the baryonic Tully-Fisher relation, testing the baryonic content of cosmic structures in a sparsely populated portion of parameter space that has hitherto been occupied primarily by dwarf spheroidal galaxies. We detect the signature of two temperature components in the neutral ISM of Leo P; the cold and warm components have characteristic velocity widths of 4.2 ± 0.9 km s −1 and 10.1 ± 1.2 km s −1 , corresponding to kinetic temperature upper limits of ∼1100 K and ∼6200 K, respectively. The cold H i component is unresolved at a physical resolution of 200 pc. The highest H i surface densities are observed in close physical proximity to the single H ii region. A comparison of the neutral gas properties of Leo P with other extremely metal-deficient (XMD) galaxies reveals that Leo P has the lowest neutral gas mass of any known XMD, and that the dynamical mass of Leo P is more than two orders of magnitude smaller than any known XMD with comparable metallicity.

We present kinematic analyses of the 12 galaxies in the "Survey of H I in Extremely Low-mass Dwarfs" (SHIELD). We use multi-configuration interferometric observations of the H I 21 cm emission line from the Karl G. Jansky Very Large Array (VLA) 22 to produce image cubes at a variety of spatial and spectral resolutions. Both two-and three-dimensional fitting techniques are employed in an attempt to derive inclination-corrected rotation curves for each galaxy. In most cases, the comparable magnitudes of velocity dispersion and projected rotation result in degeneracies that prohibit unambiguous circular velocity solutions. We thus make spatially resolved position-velocity cuts, corrected for inclination using the stellar components, to estimate the circular rotation velocities. We find v circ  30 km s −1 for the entire survey population. Baryonic masses are calculated using singledish H I fluxes from Arecibo and stellar masses derived from HST and Spitzer imaging. Comparison is made with total dynamical masses estimated from the position-velocity analysis. The SHIELD galaxies are then placed on the baryonic Tully-Fisher relation. There exists an empirical threshold rotational velocity, Vrot < 15 km s −1 , below which current observations cannot differentiate coherent rotation from pressure support. The SHIELD galaxies are representative of an important population of galaxies whose properties cannot be described by current models of rotationally dominated galaxy dynamics.

We analyze the relationships between atomic, neutral hydrogen (H I) and star formation (SF) in the 12 low-mass SHIELD galaxies. We compare high spectral (∼0.82 km s −1 ch −1 ) and spatial resolution (physical resolutions of 170 pc -700 pc) H I imaging from the VLA with Hα and far-ultraviolet imaging. We quantify the degree of co-spatiality between star forming regions and regions of high H I column densities. We calculate the global star formation efficiencies (SFE, Σ SFR / Σ H I ), and examine the relationships among the SFE and H I mass, H I column density, and star formation rate (SFR). The systems are consuming their cold neutral gas on timescales of order a few Gyr. While we derive an index for the Kennicutt-Schmidt relation of N ≈ 0.68±0.04 for the SHIELD sample as a whole, the values of N vary considerably from system to system. By supplementing SHIELD results with those from other surveys, we find that HI mass and UV-based SFR are strongly correlated over five orders of magnitude. Identification of patterns within the SHIELD sample allows us to bin the galaxies into three general categories: 1) mainly co-spatial H I and SF regions, found in systems with highest peak H I column densities and highest total H I masses; 2) moderately correlated H I and SF regions, found in systems with moderate H I column densities; and 3) obvious offsets between H I and SF peaks, found in systems with the lowest total H I masses. SF in these galaxies is dominated by stochasticity and random fluctuations in their ISM.