I found in the literature that galaxies with an inclination angle relative to the line of sight greater than 60 degrees are considered highly inclined. Does anyone know why this particular angle is considered? Why not 50 deg or 70 deg?
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1I assume that, as it is a somewhat arbitrary measure, astronomers chose 60 degrees because for a spiral galaxy tilted at that angle, the semi-major axis will appear to be twice the length of the semi-minor axis. So it makes for a very simple measurement. – Penguino Oct 04 '20 at 21:15
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1@Penguino Yes, that's the basic reason. If you assume a (disk) galaxy is perfectly thin and circular, then an inclination of 60 deg will produce an observed axis ratio of exactly 0.5. If you assume a modest thickness, you still get something very close: e.g., for a typical $c/a$ of 0.2, an inclination of 60 deg projects to an axis ratio of 0.47, so 0.5 = 60 deg is a reasonable approximation. – Peter Erwin Oct 15 '20 at 11:13
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As a practical point, it becomes hard to see and accurately measure in-plane structures (bars, spiral arms, etc.) once the inclination gets about $\sim 70$ deg; a limit of 60 deg is a reasonably conservative one. – Peter Erwin Oct 15 '20 at 11:14