GALAGE

   Determine the age of a galaxy given its redshift and a formation redshift.

   age = galage(z, [zform,  H0 =, k=, lambda0 =, Omega_m= , q0 =, /SILENT])'

    z - positive numeric vector or scalar of measured redshifts
    zform - redshift of galaxy formation (> z), numeric positive scalar
        To determine the age of the universe at a given redshift, set zform
        to a large number (e.g. ~1000).

    H0 - Hubble constant in km/s/Mpc, positive scalar, default is 70
    /SILENT - If set, then the adopted cosmological parameters are not
         displayed at the terminal.
        No more than two of the following four parameters should be
        specified.   None of them need be specified -- the adopted defaults
        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

    age -  age of galaxy in years, will have the same number of elements
           as the input Z vector

    (1) Determine the age of a galaxy observed at z = 1.5 in a cosmology with
    Omega_matter = 0.3 and Lambda = 0.0.    Assume the formation redshift was
    at z = 25, and use the default Hubble constant (=70 km/s/Mpc)
    IDL> print,galage(1.5,25,Omega_m=0.3, Lambda = 0)
             ===> 3.35 Gyr
    (2) Plot the age of a galaxy in Gyr out to a redshift of z = 5, assuming
        the default cosmology (omega_m=0.3, lambda=0.7), and zform = 100
    IDL> z = findgen(50)/10.
    IDL> plot,z,galage(z,100)/1e9,xtit='z',ytit = 'Age (Gyr)'

    For a given formation time zform and a measured z, integrate dt/dz from
    zform to z. Analytic formula of dt/dz in Gardner, PASP 110:291-305, 1998
    March  (eq. 7)

    (1) Integrates using the IDL Astronomy Library procedure QSIMP.    (The
    intrinsic IDL QSIMP() function is not called because of its ridiculous
    restriction that only scalar arguments can be passed to the integrating
    function.)    The function 'dtdz' is defined at the beginning of the
    routine (so it can compile first).
    (2) Should probably be fixed to use a different integrator from QSIMP when
    computing age from an "infinite" redshift of formation.    But using a
    large value of zform seems to work adequately.
     (3) An alternative set of IDL procedures for computing cosmological
    parameters is available at
            http://cerebus.as.arizona.edu/~ioannis/research/red/

    COSMO_PARAM, QSIMP

     STIS version by P. Plait (ACC)                  June 1999
     IDL Astro Version   W. Landsman (Raytheon ITSS)      April 2000
     Avoid integer overflow for more than 32767 redshifts  July 2001

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