[AstroPy] astropy.cosmology

Benjamin Weiner bjw at as.arizona.edu
Wed Dec 11 15:07:25 EST 2013

> [Erik wrote]
> Suppose you have a galaxy where you think you know what its absolute
> magnitude should be (say its a standard candle) and have an apparent
> magnitude, and you want to use it to know what redshift it's at.  Then
> you need this, because the distance modulus is tied to the luminosity
> distance (this example came to mind because I've actually needed this
> for actual science in the past).  My point here is mainly that just
> because we don't want all of them, doesn't mean some other users don't
> have a need.


Not to belabor this point, but it is very common to
want an inverse function for redshift or comoving distance
when you are trying to deal with selection limits.  For example
the 1/Vmax luminosity function calculation asks, given an
object of known luminosity, and a selection limit, to what
Dmax could I see it?  So you know a distance modulus
and want to know comoving_volume(z(distmod)).

Wanting the inverse of angular distance is less common
but I could imagine using it, for example, answering to what
redshift can HST resolve a 1 kpc diameter?

As an aside, I didn't think astropy.cosmology included a
function for the comoving volume element dV_c or the
integrated comoving volume from 0 to z, which are both
useful.  However, looking in the source, comoving_volume
is there, but it and several other functions were not in the


Benjamin Weiner
Associate Astronomer, Steward Observatory
bjw at as.arizona.edu
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