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Galaxy Evolutionary Synthesis Models
help you understand your data on star clusters and galaxies from the early universe until today in terms of their relevant physical and chemical properties and their evolutionary state.

Overview - Help topics

Input parameters
Burst parameters
Cosmological parameters
Output configuration

Input Physics: stellar evolution, stellar spectral library and initial mass function

The choice of "Initial Mass Function" determines the full set of input physics, not only the IMF alone.

This input physics is:
1) Stellar evolution currently includes:
- stellar isochrones of the Padova group as it was in 1999, hence including TP-AGB, but not yet the later Girardi et al. 2000 or Salasnich et al. 2000 or Marigo 2001 models
- isochrones for 0.15, 0.3 and 0.45 Msun have been added from the work of Baraffe and Chabrier

2) Stellar spectral atmosphere library
described in Lejeune (1997, A&AS 125, 229) and Lejeune (1998, A&AS 130, 65) (called BaSeL 2.0)

3) Initial Mass Function
Here you can choose between a range of common IMFs
The lower mass limit is defaulted to be 0.1 Mo, the upper mass-limit can be chosen to be either 100Mo or 120Mo.

Comparison of Kroupa and
Salpeter IMFs

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Gaseous emission: Continuum and lines

Here you have three options to choose from:
  1. No gaseous emission
  2. Continuum emission only
    This includes bound-free emission from hydrogen and helium, free-free emission and emission from the hydrogen two-photon process.
  3. Contiuum and line emission
    In addition to the continuum emission, this also contains emission lines for a wide range of elements. Line emission for hydrogen lines is computed on basis of atomic physics and the total number of ionizing photons. Lines of heavier elements (e.g. C, N, O) are computed from observed line-ratios relative to H-beta.
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Initial/Total mass

The total mass of the galaxy, given in solar masses Msun. At the beginning all the mass is "gaseous". This changes due to star formation and stellar evolution.
Note that our models currently do not reproduce observed mass-metallicity relations, i.e. the mass is a mere scaling factor. (back to top)

Galaxy types

The crucial factor describing and determining the galaxy type is the galaxy's star formation history (SFH).
In principal there are three different possibilities:
  1. An exponentially declining star formation rate (SFR), typical for early-type galaxies, i.e. ellipticals
  2. A SFR that is proportional to the available gasmass as predicted from the Kennicutt-Schmidt law. This SFH is typically observed in spiral galaxies
  3. A constant SFR that is commonly used to describe the SFH of late-type spirals (Sd's)
By choosing the respective spectral galaxy type E, Sa...Sd you automatically choose the predefined parameters for the SFH of above galaxies that best represent today's observations.
If you, however, want to define your own model parameters (e.g. a different characteristic for an elliptical galaxy), you should instead choose one of the free types. The available parameters are described below (SFR, DECLFAK, SFRFAK)

The plot below shows the SFHs of the five predefined types, all normalized to a total mass of 106 solar masses.
Star formation histories
for 5 different galaxy types

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SFR0 (only for free Sd)

This parameter that is only available for the galaxy type "free Sd" specifies the constant SFR in units of solar masses per year.
SFR = SFR0 x Mtot / 1010
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SFRFAK (only for free E & Sabc types)

This parameter specifies the star formation rate of a user-defined spiral type model. The time-dependent star formation rate in this case is given by
SFR(t) = Mgas/109 x SFRFAK

This is hence equivalent to an inverse characteristic timescale for the star formation. A SFRFAK of 0.2 means a timescale of 5 Gyr, while a SFRFAK=1 means a timescale of just 1 Gyr.
Impact of the SFRFAK
parameter on the SFH

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DECLFAK (Decline factor, only for free E-type)

The DECLFAK (Decline factor) parameter specified the characteristic timescale for the SFR of an elliptical galaxy. The SFR is then given by:
SFR(t) = (Mtot / SFRFAK) x exp(-t/DECLFAK)
Besides the decline time the second important parameter is SFRFAK. The value of this parameter can be tuned in such a way to minimize the remaining gasmass.
Impact of DECLFAK
parameter on the SFH of Free-E models

Impact of SFRFAK on the
SFH of Free-E models

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All stars in the galaxy will have the same metallicity specified here, unless "chemically consistent" has been chosen. The choices are:
  1. chemically consistent modelling: Switch to chemically consistent modelling.
  2. [Fe/H] = -1.7, equivalent to Z = 0.0004 = 1/50 Zo
  3. [Fe/H] = -0.7, equivalent to Z = 0.004 = 1/5 Zo
  4. [Fe/H] = -0.4, equivalent to Z = 0.008 = 2/5 Zo
  5. [Fe/H] = 0.0, equivalent to Z = 0.02 = Zo (this means solar metallicity)
  6. [Fe/H] = +0.4, equivalent to Z = 0.05 = 2.5 Zo
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GALEV defines the strength of burst as the fraction of the available gas at the onset of burst that is transformed into stars during the burst:
Burststrength = Mass of formed stars / Mass of available gas

The burst strength therefore can only have values of between 0 and 1. Furthermore note that since the burststrength includes the mass of gas that is available at the beginning of the burst, the same value for the burststrength can result in different amounts of newly formed stars depending on galaxy type and time of burst. A weak burst (e.g. 20 %) early in the evolution of a Sd-type galaxy can form more stars than a strong burst (75 %) in a already old Sa galaxy.
SFHs of galaxies
undergoing bursts with different burst strengths

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Burst duration

The parameter burst duration specifies the e-folding time for the exponentially declining SFR during the burst. Before the onset of the burst (see parameter burst time the SFR is determined by the parameters for the undisturbed galaxy.
Influence of different
burst durations on the SFH

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This parameter simply determines at which age of the galaxy the burst should occur. The value needs to be given in years, so an age of 10 Gyr should be entered as 10e9.
SFHs for galaxies with
different burst times

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We currently offer two different extinction laws:
  1. Calzetti (2000, ApJ 533, 682)
  2. Cardelli (1989, ApJ 345, 245)
You can choose a maximum extinction value and the step width between the individual extinction values. All magnitudes and spectra (depending on which output was requested, see below) are then computed for each extinction value in this range. (back to top)

Cosmology: Hubble constant, Omega-factors and formation redshifts

Here you can choose the requested cosmology. Note that if no cosmology output is requested, the values are ignored during the execution of COCOS and hence should be left at their default values.

You can change the following parameters: (back to top)

Time evolution - Spectra

Sample output spectrum
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Time evolution - Magnitudes and Colours

Sample output of absolute

Sample output of colours

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Time evolution - Statistics & Diagnostics:
SFR, stellar and gas-mass, gaseous metallicity

Physical parameter
output: Star formation rate as function of time

Physical parameter
output: Stellar and gaseous masses as function of time

Physical parameter
output: Metallicity as function of time

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Cosmology - specs

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Cosmology - absolute magnitudes

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Cosmology - appmag

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Cosmology - attmag

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Cosmology - ecorr

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Cosmology - kcorr

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Cosmology - stat

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