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Stellar Formation History
The input Star Formation History gives the relative weight of the star formation episodes at various ages (Star Formation Rate (SFR)) in order of increasing age, and the mean metallicity ([Fe/H]) of the stars formed at that age (Age-Metallicity Relationship (AMR)) plus a 1 sigma Gaussian spread around this mean [Fe/H]. The SFH is tabulated for N key ages t_i (t=0 correspond to stars formed at present).

After the SFH is read by the code, the following cycle starts, running along the SFH index, from i=0 to N. The age difference between two generic tabulated t_i and t_(i+1) values is computed. The number of stars formed in this age interval is given by the product of the scale factor specified by the user, times the value of the SFR at t_i, and has to be less than 2000000. For each of the stars formed in this age interval, a random age t between t_i<= t < t_(i+1) is drawn from a flat probability distribution, together with a mass selected according to the specified IMF, and the value of [Fe/H] at time t_i from the AMR. With these three specified values of t, M and [Fe/H] the program interpolates quadratically in age, metallicity and mass among the isochrones in the grid, to provide the star's photometric properties. Once all stars formed between ages t_i and t_(i+1) are generated, the next interval is considered and the cycle continues, ending when all stars in the final age bin between t_(N-1) and t_N are generated. The value of the SFR and AMR at t_N are not considered and can be set to any arbitrary value; the last point in the SFH file provides only the upper age-limit for the synthetic stellar population.

Three examples of SFH are the following:

Age (yr) SFR [Fe/H] sigma([Fe/H])
12.0e+09 1.0 -1.27 0.0
12.0e+09 0.0 -1.27 0.0

This corresponds to a 12 Gyr, [Fe/H]=-1.27 single age - single metallicity population.

Age (yr) SFR [Fe/H] sigma([Fe/H])
4.0e+08 1.0 -1.27 0.0
4.0e+08 0.0 -1.27 0.0
12.0e+09 1.0 -1.27 0.0
12.0e+09 0.0 -1.27 0.0

This corresponds to two single age-single metallicity populations with ages equal to 400 Myr and 12 Gyr, [Fe/H]=-1.27. The strength of the SFR is the same for both star formation episodes

Age (yr) SFR [Fe/H] sigma([Fe/H])
6.0e+07 1.0 -0.20 0.1
4.0e+08 0.5 -0.55 0.1
1.0e+09 0.25 -1.00 0.1
12.0e+09 0.1 -1.27 0.1
13.0e+09 0.0 -1.27 0.1

This is a more complex SFH. There is a large age and [Fe/H] spread in the resulting composite stellar population. Stars with ages between 60 Myr and 400 Myr (upper age limit excluded) have formed with [Fe/H]=-0.20+-0.1; objects with ages between 400 Myr and 1 Gyr (upper age limit excluded) have formed with [Fe/H]=-0.55+-0.1, and so on. The number of objects formed in the first age interval is twice the number of objects formed in the second age interval, four times the number of objects formed in the third age interval, and 10 times the number of objects formed in the fourth age interval.

Predetermined SFHs for the following stellar systems are provided:

Object Reference
NGC6822 (global SFH) Gallart et al. 1996, AJ, 112, 2596
SMC (global SFH) Harris & Zaritsky 2004, AJ, 127, 1531
LMC (bar field) Holtzman et al. 1999, AJ, 118, 2262
Local disk Rocha Pinto et al. 2000, A&A 358, 869
Milky Way bulge Molla et al. 2000, MNRAS 316, 345
Sextans A Dolphin et al. 2003, MNRAS, 126, 187
LGS3 Miller et al. 2001, ApJ, 562, 713