The Central Velocity Field in NGC 253: Possible Indication of a Bar

Abstract

We have investigated whether motion of gas in a barlike potential can account for the peculiar but systematic velocity field observed in the nuclear region of the starburst galaxy NGC 253. This unusual velocity field with gradients along both major and minor axes was revealed in a high-resolution (1.8"×1.0") H92α recombination line observation by Anantharamaiah & Goss. A simple logarithmic potential is used to model the bar. Assuming that the bulk of the gas flows along closed and nonintersecting x1 bar and x2 antibar orbits of the bar potential, we have computed the expected velocity field and position-velocity diagrams and compared them with the observations. A comparison of the integrated CO intensity maps with the spatial distribution of the x1 and x2 orbits in the model indicates that the nuclear molecular gas in NGC 253 lies mainly on the x2 orbits. We also find that the velocity field observed in the central 100 pc region in the H92α recombination line is well accounted for by the bar model if most of the ionized gas resides in the inner x2 orbits. However, the model is unable to explain the velocity field on a larger scale of ~500 pc observed using the Owens Valley Radio Observatory interferometer with a resolution of 5''×3''. The direction of the observed CO velocity field appears twisted compared to the model. We suggest that this perturbation in the velocity field may be due to an accretion event that could have occurred 107 years ago.