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Originally Posted by suitti
The Milky Way is nearby. NGC 3031 appears to be M81, a galaxy so bright that there are claims of people seeing it naked eye. NGC 3198 is mag 10.9, NGC 7331 is mag 10.3. I'd guess that the galaxies were chosen as bright, more or less face on galaxies, perhaps with good analysis by others.
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NGC 3031, 3198, and 7331 have inclinations of 64 deg, 74 deg, and 71 deg respectively. This would actually be closer to edge on. Generally, studies of rotation curves are best done for galaxies with inclinations of 30 degrees or greater where 0 deg is face on and 90 deg is edge on.
This is because the observed rotational velocity Vrot is corrected for incl.
Vrot corrected = Vrot obs/sin i
At inclinations closer to 90 deg the effect of the inclination correction is smaller and the uncertainty in the inclination is generally smaller. With inclinations less than 30 degrees the inclinations become very uncertain and the effect of small errors in the estimated inclination become much larger.
They looked at these particular galaxies because they have very well studied rotation curves.