GalaxyCount - Usage Instructions

GalaxyCount

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Usage Instructions

GalaxyCount calculates the number and standard deviation of galaxies in a magnitude limited observation of a given area. The methods to calculate both the number and standard deviation may be selected from different options. According to the options chosen various inputs will be activated/ deactivated. A detailed description of the calculator is given the paper.

Always remember to press the calculate button after changing the input.

Step-by step instructions

  • Start GalaxyCount by typing, java GalaxyCount, in a terminal, from the directory containing the code.
  • The intoduction panel will be displayed, containing a brief description of the program. Change to "Data Input and Output" by clicking on the tab at the top of the window.
  • Set the magnitude limits, filter and field size in the top three input rows.
  • Decide how to model the amplitude of the angular correlation function. If "Model" is chosen, the "Model Parameters" box will be activated, allowing a choice of epsilon.
  • Choose how to model the number counts of galaxies. Choices are:
  • "Define incompleteness function". This is the recommended technique. The incompleteness should be measured from your own data if at all possible, as this yields much more accurate results.
  • "Calculate average", which assumes your observations are 100% complete at all magnitudes;
  • "Input own value", if you have performed your own star/galaxy classification and already know the number of galaxies;
  • "Use observing conditions", which will estimate the completeness based on exposure, seeing, SNR, telescope aperture and total system throughput;
  • The system throughput may be explicitly defined or you may use Subaru or Gemini default values. Click on the help ("?") button for details on the default values.
  • Click "Calculate" to display the results in the box at the bottom of the screen.
  • Graphs of the number counts, correlation function and completeness function are displayed under the other tabs at the top of the screen. Note that these are not always displayed under certain options.

    • The sections below illustrate where to find the various input parameters. Note the images are already slightly outdated.

    General

    The buttons and text fields highlighted in the image below are general puropse and would be used in most applications of GalaxyCount.


    Amplitude of the angular correlation function

    You may choose the function form of the amplitude of the correlation function, as either log(A) = -K mag, or using an epsilon model to describe the evolution of clustering.

    If an epsilon model is chosen then select the value of epsilon.

    You may check the fit of your model choice with data compiled from the lierature.


    Modelling the number counts

    The number counts may be modelled in three ways.

  • Integrating the number density relation and multiplying by the appropriate area. This assumes your observations are 100% complete.
  • Specifying a completeness function, of a sigmoid form, which is convolved with the number density relation.
  • Specifying the observing conditions. From this a completeness function is estimated and treated as above.

    • Or if known, the actual number may be specified.

    The number density relation used and the integral under the curve (including completeness corrections if appropriate) can be checked in the "Number Counts" window.

    The completeness function and the 99, 90, 50 and 30% completeness limits are displayed in the "Completeness" window.


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    Simon Ellis 27-02-2006
    sce@aaoepp.aao.gov.au