Examples of Log(Star mass fraction) and ISM Mg/Fe ratio versus time from one of our models with a merger occurring at 8 Giga years (click on images to enlargen):

How do we Observe GCE?

The model is then compared , using statistical methods,  to reduced data from our group's own acquired observations of galaxy spectrums. For spectra, detailed spectra over as wide a range of wavelengths as possible are employed.

In future observations, work will be done to "tie-down" the age of galaxies to help break age-metallicity degeneracy (see below for more on this).

Example Observations Follow:

Optical image and WHT spectrum of the Sa galaxy NGC 3623:

Below, are notes on some of the functions and archived data sources the GCE code employs.

Salpeter Initial Mass Function

The IMF (E) describes the distribution of masses after an initial star burst. It specifies the distribution in mass of a newly formed stellar population and it is frequently assumed to be a simple power law (see below). A still much used function that describes this distribution is the modern equivalent of Salpeter’s formula (Salpeter, 1955). This function is often represented as a plot of E (which is usually the number density normalised to unity) against stellar mass. The IMF :

E(M)=cM^-(1+x)

with M being in solar masses, and x without units. The coefficient c is given often as a fraction of a solar mass (0.15 M¤ in the case of Salpeter).

The IMF may be normalised to unity. In general,  E is assumed to extend from a lower to an (non-observational) upper cut-off, chosen to be M₁=0.1 M¤ and M₂=125 M¤   (Bolzonella et al, 2000).

Worthey Spectral Indices

Models that predict the line strengths of spectra of galaxies, specifically spheroidal galaxies, are used in the GCE code (of Sansom & Proctor, CFA UCLan, 2002). Indices have been formulated by a number of researchers; for example, Worthey et al (1994)^* , who generate 21 indices from optical observations of stellar spectra. These indices are collectively referred to as the "Lick Indices". The more convincingly a GCE code predicts these line strengths (i.e. indices), the closer to the truth the theories as to GCE that produce that code are thought to be.

The Indices Worthey et al (1994) refer to are:

HdelA, HdelF, CN1, CN2, Ca4227, G4300, HgamA, HgamF, Fe4383, Ca4455, Fe4531, Fe4668, Hbeta, Fe5015, Mg1, Mg2, Mgb, Fe5270, Fe5335, Fe5406 and the Ca triplet line.  

*Worthey et al, 1994, ApJS, 95, 1, 107-149.

Vazdekis Spectral Indices
The GCE code reads in data tabulated from an SSP, from Vazdekis et al (1996)^*. This paper provided model predictions for SSPs; Broadband colours, mass-to-light ratios and absorption line-strengths at low spectral resolution for SSPs and full chemo-evolutionary models. SSPs spectral energy distributions (SEDs) at moderately high resolution (FWHM=1.8Å) in the optical (ranges 3820-4500 Å and 4780-5460 Å) are used from tables presented in the Vazdekis 1999 paper^*. The 1996 paper referred back to tables of model stellar data that predict line strengths for SSPs from

* Vazdekis et al, 1996, ApJS,107,306 and also Vazdekis 1999, ApJ,513,224.