SHIELD: Neutral Gas Kinematics and Dynamics

TL;DR

This study analyzes the gas kinematics of 12 low-mass dwarf galaxies using VLA HI observations, revealing their rotation velocities and placing them on the baryonic Tully-Fisher relation, highlighting challenges in modeling such faint systems.

Contribution

The paper provides detailed kinematic analysis of extremely low-mass dwarf galaxies, demonstrating the limitations of current rotation curve methods at velocities below 15 km/s.

Findings

Most galaxies have circular velocities <30 km/s.

An empirical threshold of <15 km/s exists below which rotation cannot be distinguished from pressure support.

SHIELD galaxies are representative of a population poorly described by existing models.

Abstract

We present kinematic analyses of the 12 galaxies in the "Survey of HI in Extremely Low-mass Dwarfs" (SHIELD). We use multi-configuration interferometric observations of the HI 21cm emission line from the Karl G. Jansky Very Large Array (VLA) 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 circular velocities <30 km/s for the entire survey population. Baryonic masses are calculated using single-dish HI 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 <15 km/s, 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.