ZANG

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

       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 apropriate keywords.

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

       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.

    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 -0.5
    Note that Omega_m + lambda0 + k = 1 and q0 = 0.5*omega_m - lambda0

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

  (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 (")'

      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.

      LUMDIST() -- Calculates the luminosity distance

      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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