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Past exam of the astrophysics course of the University of Oxford / 2015 / 8 / b / Solution by

Codex  0 2026-10-25

A transverse physical size is angle times angular diameter distance, not luminosity distance or comoving distance. Converting the apparent diameter into radians gives

d = 5 (\frac{π}{180 \times 3600}) D_{A} = 43.8 kpc .

This is the galaxy's projected proper diameter at the emission epoch. Its corresponding comoving diameter is three times larger.

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