# What is projected separation and how can I make sense of its unit (h^-1 kpc)?

I have been reading this paper on the separation of a binary quasar in which it states:

At a projected separation of 8.3/h kpc (Omega_matter = 1), CTQ 839 would be the smallest projected separation binary quasar currently known.

What is meant by projected separation and where does this unit come from?

## 2 Answers

It means the separation between the two objects if they were both at the same distance away. This separation is found by multiplying the angular separation (in radians) by the distance to the objects.

Often, the distance to extragalactic objects depends on the assumed value of the Hubble parameter. In this case, $h$ is a dimensionless number corresponding to $h=H_0/100$, where $H_0$ is the Hubble parameter in km/s per Mpc.

The current best value of $H_0$ is about 70 km/s per Mpc, so $h\simeq 0.7$.

Projected separation is the minimum physical separation of two astronomical objects, as determined from their angular separation and estimated distance.The Parallax shift of a star at a distance of one parsec as seen from the Earth. Not to scale For planets and double stars, this distance is usually given in Astronomical Units. The actual separation of the two objects depends on the angle of the line between the two objects to the line-of-sight of the observer.