ProGeny I: a new simple stellar population spectra generator and impact of isochrones / stellar spectra / initial mass functions

In this work we introduce progeny, a new stellar population library (SPL) software package written in r. This release encapsulates the core open source software to generate simple/single stellar populations and their associated spectra (SSPs); the various data inputs required (in particular isochrones and stellar spectra); example scripts to generate the SSPs; and a number of pre-generated static SSPs available for immediate use. The most novel feature of progeny is its ability to produce SSPs with evolving initial mass functions (IMFs), allowing functional dependencies on stellar age or metallicity. We perform both internal comparisons (within the progeny SPL) and external comparisons (to SSPs associated with other public SPLs) and tests. The main conclusion is that the choice of isochrone has significantly more impact on the predicted SSP spectra than the choice of stellar spectra and/or IMF (comparing Chabrier and Kroupa variants). A number of limiting uncertainties and corrections for star formation rates and stellar masses are also presented.