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- TL;DR Summary
- projected linear separation between companion stars

A quasar with a bolometric flux of approximately 10−12 erg s−1 cm−2 is observed at

a redshift of 1.5, i.e. its comoving radial distance is about 4.4 Gpc.

Assume that the quasar in the previous question is observed to have a

companion galaxy which is 5 arcseconds apart. What is the projected linear separation of the

companion galaxy from the quasar?

The answer key given divides the comoving distance multiplied by angle by (1 + z), z = 1.5 such that

##d = \frac{4.4 Gpc * 5"}{1 + 1.5}##

This answer doesn't make sense because the 5" should stay as a constant as both the distance between the companion and the distance from companion to the Earth should grow at the same rate (Hubble's law). Therefore the correct answer I believe should just be ##5.5 Gpc * 5"## Can anyone more knowledgeable explain if the above reasoning is correct and which solution is correct (even if the question may be silly)?

Thanks,

a redshift of 1.5, i.e. its comoving radial distance is about 4.4 Gpc.

Assume that the quasar in the previous question is observed to have a

companion galaxy which is 5 arcseconds apart. What is the projected linear separation of the

companion galaxy from the quasar?

The answer key given divides the comoving distance multiplied by angle by (1 + z), z = 1.5 such that

##d = \frac{4.4 Gpc * 5"}{1 + 1.5}##

This answer doesn't make sense because the 5" should stay as a constant as both the distance between the companion and the distance from companion to the Earth should grow at the same rate (Hubble's law). Therefore the correct answer I believe should just be ##5.5 Gpc * 5"## Can anyone more knowledgeable explain if the above reasoning is correct and which solution is correct (even if the question may be silly)?

Thanks,