Hi everyone. This isn't a specific homework problem, I'm just trying to understand a concept. I've been studying the HI profiles (from the 21cm emission line). Almost all of them look like the attached profile. The velocity on the x-axis corresponds to the amount that the line has been redshifted, and the flux on the y-axis tells how much gas has that particular velocity. With a simple model of an observer viewing a galaxy that is a solid, uniform disk edge-on, could someone help me understand why one would expect a double-peak HI profile. I've drawn tons of circles and tried to determine the flux at each velocity and the velocity at each angle, but I'm not getting it. Ideally, could someone show me how to derive a function for flux as a function of velocity for the simplest case? I understand that the velocity of the center of the peak gives the total recession velocity of the galaxy and the width of the peak gives rotation velocity, but I don't see why there's a double peak rather than a single one. Thanks so much!