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ZANG

ZANG

## Purpose

Determine the angular size of an object as a function of redshift

## Explanation

Requires an input size in kpc and returns an angular size in arc seconds
Default cosmology has a Hubble constant of 70 km/s/Mpc, Omega (matter)
=0.3 and a normalized cosmological constant Lambda = 0.7; however these
values can be changed with appropriate keywords.

## Calling Sequence

angsiz = zang( dl, [ z, H0 =, Omega_m =, Lambda0 = , q0 = , k =,
/SILENT] )

## Inputs

dl - linear size of the object *in kpc*, non-negative scalar or vector
z - redshift of object, postive scalar or vector
Either dl and z must have the same number of elements, or at least
one of them must be a vector.

## Optional Input Keywords

H0 - Hubble constant in km/s/Mpc, default is 70
No more than two of the following four parameters should be
specified. None of them need be specified, default values are given
k - curvature constant, normalized to the closure density. Default is
0, indicating a flat universe
Omega_m - Matter density, normalized to the closure density, default
is 0.3. Must be non-negative
Lambda0 - Cosmological constant, normalized to the closure density,
default is 0.7
q0 - Deceleration parameter, numeric scalar = -R*(R'')/(R')^2, default
is -0.55
Note that Omega_m + lambda0 + k = 1 and q0 = 0.5*omega_m - lambda0

## Output

angsiz - Angular size of the object at the given redshift in
arc seconds

## Example

(1) What would be the angular size of galaxy of diameter 50 kpc at a redshift
of 1.5 in an open universe with Lambda = 0 and Omega (matter) = 0.3.
Assume the default Hubble constant value of 70 km/s/Mpc.

IDL> print,zang(50,1.5, Lambda = 0,omega_m = 0.3)
====> 6.58 arc seconds
(2) Now plot the angular size of a 50 kpc diameter galaxy as a function of
redshift for the default cosmology (Lambda = 0.7, Omega_m=0.3) up to
z = 0.5
IDL> z = findgen(50)/10. + 0.1 ;Angular size undefined at z = 0
IDL> plot,z,zang(50,z),xtit='z',ytit='Angular Size (")'

## Notes

This procedure underwent a major revision in April 2000 to allow for a
cosmological constant, ***including a change of the calling sequence***
Be sure to supply the input linear size dl in units of kpc.

## Procedures Called

LUMDIST() -- Calculates the luminosity distance

## Revision History

Written J. Hill STX July, 1988
Converted to IDL V5.0 W. Landsman September 1997
Major rewrite to call LUMDIST function W. Landsman April 2000

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