The Net Radial Flow Velocity of the Neutral Hydrogen in the Oval Distortion of NGC 4736

Abstract

The net radial flow velocity of gas is an important parameter for understanding galaxy evolution. It is difficult to measure in the presence of the elliptical orbits of an oval distortion because the mathematical model describing the observed velocity is degenerate in the unknown velocity components. A method is developed in this paper that breaks the degeneracy using additional information about the angular frequency of the oval distortion. The method is applied to the neutral hydrogen in the oval distortion of NGC 4736. The neutral hydrogen is flowing inward at a mean rate of -6.1 $\pm$ 1.9 km s${}^{-1}$. At this rate, it takes 400 Myr, or 1.7 rotations of the oval distortion, for the neutral hydrogen to travel the 2.5 kpc from the end to the beginning of the oval distortion. The mean mass flow rate of the neutral hydrogen in this region is -0.25 $\pm$ 0.11 $M_{\odot }$ yr${}^{-1}$, which is similar to estimates for the star formation rate reported in the literature.

Accepted for publication in The Astrophysical Journal

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