Shield: Neutral Gas Kinematics and Dynamics

Abstract

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.

First Characterization of the Neutral ISM in Two Local Volume Dwarf Galaxies

“…Interestingly, these two galaxies probe the beginning of the transition regime from rotationally-dominated to pressure-dominated systems that is discussed in detail in McNichols et al (2016). In UGC 11411 the rotational velocity is much larger than the velocity dispersion at all radii.…”

“…UGC 11411, on the other hand, is a more dark matter dominated system that is more massive than all of the SHIELD galaxies discussed in McNichols et al (2016). Using the larger value of inclination (i = 52…”

“…As shown in Figure 1(c), the projected rotation of the galaxy shows the characteristic solid-body signature that is typical of low-mass galaxies (e.g., McNichols et al 2016). We use the 4 σ contours in Figure 1 to define the total projected velocity (∼87 km s −1 ) and angular extent (±60 ′′ from the dynamical center) of the gas.…”

“…We therefore apply the techniques presented in McNichols et al (2016) to create the position-velocity slices shown in Figures 1(c) and 2(c). The position angle of each slice is determined iteratively by maximizing the projected velocity along the kinematic major axis; this position angle is checked by examining the position angle of the isovelocity contours at the systemic velocity of each galaxy and then rotating each by +90…”

“…The data sets are part of the "Observing for University Classes" program, a service provided by NRAO as an opportunity for courses teaching radio astronomy theory to acquire new observations to be analyzed by students (Cannon & Van Moorsel 2017). Data handling and imaging followed the prescriptions described in McNichols et al (2016) and Teich et al (2016). Briefly, a 4.0 MHz bandwidth is separated into 1024 channels, delivering a native spectral resolution of 0.86 km s −1 ch −1 .…”