Detection of a slow H I bar in the dwarf irregular galaxy DDO 168

Abstract

We examine the HI total intensity maps of the VLA LITTLE-THINGS galaxies and identify an H I bar in the dwarf irregular galaxy DDO 168 which has a dense and compact dark matter halo that dominates at all radii. This is only the third galaxy found to host an H I bar. Using the H I kinematic data, we apply the Tremaine–Weinberg method to estimate the pattern speed of the bar. The H I bar is found to have an average pattern speed of 23.3 ± 5.9 kms−1 kpc−1. Interestingly, for the first time, we find that the observed pattern speeds of the bar in the two kinematic halves are different. We identify the origin of this difference to be the kinematic asymmetry. This observed offset in the pattern speed serves to put a stringent constraint on the lifetime of the bar set by the winding time-scale. The lifetime of the bar is found to be 5.3 × 108 yr, which is two times the dynamical time-scale of the disc.We also find the H I bar in DDO 168 to be a weak bar with a strength of 0.2. If H I bar being weak can be easily disturbed, this could possibly explain why it is extremely rare to observe H I bars in galaxies. We estimate the bar radius to be 1 kpc and the dimensionless ratio, RL/Rb to be ≥2.1 indicating a ‘slow’ bar in DDO 168. Our results confirm the proposition that the dynamical friction with the halo slows down a rotating bar in a galaxy dominated by dark matter halo from inner radii