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GALAGE

GALAGE

## Purpose

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

## Calling Sequence

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

## Inputs

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).

## Optional Keyword Inputs

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 -0.55

## Outputs

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

## Example

(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)'

## Procedure

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/

## Procedures Called

COSMO_PARAM, QSIMP

## History

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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