An Empirical Approach to Cosmological Galaxy Survey Simulation:
  Application to SPHEREx Low-Resolution Spectroscopy
  Highly accurate models of the galaxy population over cosmological volumes are
necessary in order to predict the performance of upcoming cosmological
missions. We present a data-driven model of the galaxy population constrained
by deep 0.1-8 $\rm \mu m$ imaging and spectroscopic data in the COSMOS survey,
with the immediate goal of simulating the spectroscopic redshift performance of
the proposed SPHEREx mission. SPHEREx will obtain over the full-sky $R\sim41$
spectrophotometry at moderate spatial resolution ($\sim6"$) over the wavelength
range 0.75-4.18 $\rm \mu m$ and $R\sim135$ over the wavelength range 4.18-5
$\rm \mu m$. We show that our simulation accurately reproduces a range of known
galaxy properties, encapsulating the full complexity of the galaxy population
and enables realistic, full end-to-end simulations to predict mission
performance. Finally, we discuss potential applications of the simulation
framework to future cosmology missions and give a description of released data
products.
