Why exactly do we believe that stars and more importantly, gas in the outer regions of spiral galaxies move in circular orbits? This assumption lies at the heart of the infamous galaxy rotation curve problem, as the circular orbital velocity for a spiral galaxy (whose visible mass is concentrated in the central bulge) should be proportional to the inverse square root of the distance from the center; instead, observed rotation curves are “flat”, meaning that the velocity remains approximately the same at various distances from the center.
So why do we assume that stars and gas move in circular orbits? Well, it turns out that one key bit of evidence is in a 32-year old paper that was published by two Indian physicists: Radhakrishnan and Sarma (A&A 85, 1980) made observations of hydrogen gas in the direction of the center of the Milky Way, and found that the bulk of gas between the solar system and the central bulge has no appreciable radial velocity.
However, more recent observations may be contradicting this result. Just two years ago, the Radial Velocity Experiment (RAVE) survey (Siebert et al, MNRAS 412, 2010) found, using a sample of several hundred thousand relatively nearby stars, that a significant radial velocity exists, putting into question the simple model that assumes that circular orbits dominate.