Velocity Profile

The figure above shows a galactic velocity versus radius profile, using data collected from my telescope.   Green points were determined using the tangent-point method.  Black points were determined by radius-specific trigonometric calculation.  The orange point is the Sun, using assumed values of 8 kpc for radius and 220 km/s for velocity.  

The profile is consistent with the findings of many others, showing that velocity remains relative constant across the radius of the galaxy, around 220 km/s (the radius of visible stars is about 15 kpc.)  

In order to generate this plot, I wrote a suite of Python scripts for the following:  1) fitting the data (i.e. determining the doppler-shifts), 2) modeling the doppler-shifts that would be expected from different regions of the galactic plane, and 3) calculating these velocities and generating the graph shown above.  The code is available on Github (see "Software.")  The following assumptions were made:

-Movement of the Sun relative to our local standard of rest (LSR) was accounted for in the calculated velocities shown above.  It was assumed that the Sun is 8 kpc from the galactic center, that the LSR travels at 220 km/s around the galactic center, and that the Sun travels about 13 km/s in the direction of the Solar Apex.

-The tangent point method was used to estimate data points for galactic longitudes between 35 and 70 degrees.  This method uses estimates of the most red-shifted part of spectra for inner portions of the galaxy.  It requires only an estimate of the Sun's distance from the center of the galaxy.

-Trigonometric calculations were used to determine velocities for galactic longitudes ranging from 35 to 240 degrees.  For these calculations, it was necessary to estimate the galactic radii of the emitting regions of the galaxy; these radii were estimated from a map of Milky Way, assuming that most of the emitting hydrogen is concentrated in the same regions where visible matter is present.  Several assumptions are in play here: 1) that we have an roughly accurate map of visible matter, and 2) that hydrogen is concentrated in these visible arms.