3 Major axis position-velocity diagrams

The projected rotation curve, $v(r)\sin~i$, can be visualized as a position-velocity slice. We include such maps in Fig. 4, which show intensity as a function of position along the kinematic major axis and heliocentric velocity. Position angles and central positions were taken from Tables 1 or 3, in order of preference. We have also included in Fig. 4 the rotation curve points listed in Tables 2 and 4.

Figure 1: Histogram of the difference in position angle between the major axis of the $N_{\rm HI}=3 \times 10^{20}~{\rm cm^{-2}}$ isophote and the velocity gradient for 27 objects (Mkn 178 and DDO 101 were excluded because of missing data). The angle $\Delta \rm PA$ is the smallest angle between the velocity gradient and the major axis. The median difference is $\Delta \rm PA=15^\circ$. The object with $\Delta \rm PA=90^\circ$ is NGC 2537.

Virtually all of the observed galaxies show a velocity gradient. However, in poorly-ordered velocity fields, it is not clear whether or not this rotation represents. In Fig. 1 we show, however, that observed velocity gradients tend to align with the major axis of the HI isophotes, although a few significant misalignments exist suggesting a rotating disk (e.g. DDO 47, DDO 63, DDO 165 and also Sextans A as shown by Skillman et al. 1988). We suspect that much of the width of the distribution in Fig. 1 is caused by errors in the determination of major axis position angle from the HI distribution, although the magnitude of $\Delta$PA is not correlated with the total brightness of the galaxy in HI (or in blue light).

The position-velocity maps of three very-low luminosity dwarf galaxies ( $M_{\rm B} > -14$) betray high ratio of rotational to random velocities: those of DDO 47, DDO 52 and DDO 87. The existence of rotationally supported dwarfs of such low luminosity is remarkable if simple arguments based on the luminosity-linewidth relation for large spirals are applied to dwarf galaxies (Lo et al. 1993; Stil & Israel, in preparation). In addition, the rotation curves of DDO 52 and DDO 87 show clear signs of flattening. They illustrate the increase in the number of extremely low luminosity galaxies known to be largely supported by rotation (cf. Carignan & Beaulieu 1989; Côté 1997). Indeed, with the exception of DDO 47 Fig. 4 as well as Tables 2 and 4 include few convincing examples of pure solid-body rotation.