Alfalfa Discovery of the Nearby Gas-Rich Dwarf Galaxy Leo P. V. Neutral Gas Dynamics and Kinematics

Abstract

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.

Shield: Neutral Gas Kinematics and Dynamics

“…In a detailed H I study by Bernstein-Cooper et al (2014), the authors examine deep VLA H I 21 cm data that are very similar to the data presented here for the SHIELD galaxies. The conclusion is the same as that in the present work: extracting a meaningful and non-degenerate model of the gas kinematics is extremely challenging at rotation velocities lower than 20 km s −1 and without well-constrained gas inclination.…”

“…The gold diamonds represent the less massive gas-rich galaxies used to calibrate the model, and the red squares represent spheroidal dwarf galaxies with no detectable H I. The larger cyan diamond represents Leo P, the slowest rotating and lowest-mass galaxy known to still be relatively rich with interstellar gas (Bernstein-Cooper et al 2014). The gray bars represent 1σ and 3σ deviations from a fit of the BTFR to the gas-rich galaxy sample.…”

“…To reduce the number of free parameters in our model and explicitly determine the rotation-supported component of the circular velocities at each ring, we assume negligible asymmetric drift (see Bernstein-Cooper et al 2014); that is, we assume zero radial component to the motions of the rings. ROTCURʼs Levenberg-Marquardt solver fits kinematic parameters within concentric rings of finite thickness (typically half of the width of the resolving beam major axis).…”

“…In Figure 26, the SHIELD galaxies are each plotted on the BTFR alongside the galaxy populations from the comprehensive review of McGaugh (2012): gas-dominated spirals, gas-rich dwarfs, and gas-poor dwarf spheroidal galaxies, along with the least massive known H I-bearing galaxy in the local univsere, Leo P (Bernstein- Cooper et al 2014). In agreement with recent results (e.g., Lelli et al 2016;Papastergis et al 2016, although note that the method used to determine rotational velocity there uses W 50 ), the SHIELD galaxies fall on the BTFR within measurement error.…”

“…Empirically, this transition has been found to occur near a circular velocity of ∼20 km s −1 ; for example, the sample presented in McGaugh (2012) contains no such galaxies with rotation velocities significantly below this value. Bernstein-Cooper et al (2014) estimate that the extremely low-mass and metal-poor galaxy Leo P is rotating at 15±5 km s −1 . For the slowest rotating galaxies, the signatures of rotation become indistinguishable from the random statistical motion of the gaseous component.…”