  Galaxy spectra are essential to probe the spatial distribution of galaxies in
our Universe. To better interpret current and future spectroscopic galaxy
redshift surveys, it is important to be able to simulate these data sets. We
describe Uspec, a forward modeling tool to generate galaxy spectra taking into
account some intrinsic galaxy properties as well as instrumental responses of a
given telescope. The model for the intrinsic properties of the galaxy
population, i.e., the luminosity functions, and size and spectral coefficients
distribu- tions, was developed in an earlier work for broad-band imaging
surveys [1], and we now aim to test the model further using spectroscopic data.
We apply Uspec to the SDSS/CMASS sample of Luminous Red Galaxies (LRGs). We
construct selection cuts that match those used to build this LRG sample, which
we then apply to data and simulations in the same way. The resulting real and
simulated average spectra show a good statistical agreement overall, with
residual differences likely coming from a bluer galaxy population of the
simulated sam- ple. We also do not explore the impact of non-solar element
ratios in our simulations. For a quantitative comparison, we perform Principal
Component Analysis (PCA) of the sets of spectra. By comparing the PCs
constructed from simulations and data, we find good agree- ment for all
components. The distributions of the eigencoefficients also show an appreciable
overlap. We are therefore able to properly simulate the LRG sample taking into
account the SDSS/BOSS instrumental responses. The differences between the two
samples can be ascribed to the intrinsic properties of the simulated galaxy
population, which can be reduced by further improvements of our modelling
method in the future. We discuss how these results can be useful for the
forward modeling of upcoming large spectroscopic surveys.
