The End of Galaxy Surveys

For nearly a century, imaging and spectroscopic surveys of galaxies have given us information about the contents of the universe and the galaxies themselves. This paper attempts to define the logical endpoint of such surveys by defining not the next galaxy survey, but the final galaxy survey at NIR wavelengths; this would be the galaxy survey that exhausts the information content useful for addressing extant questions. As we show in the paper, such a survey would require incredible advancements in a number of technologies (e.g., telescope apertures, detector properties, pixel count, orbit, and data transmission) and the final details depend on the as yet poorly constrained properties of the earliest galaxies (including their intrinsic brightness and redshift of formation). As such, the details and requirements of the last galaxy surveys are not well defined. However, using an standard exposure time calculator, we define nominal surveys for extracting the useful information for three science cases: dark energy cosmology, galaxy evolution, and supernovae. We show the scaling relations that trade off sky background, telescope aperture, and focal plane size to allow for a survey of a given depth over a given area. We find that for optimistic assumptions, a telescope pf 280m with a marginally resolved focal plane of 20 deg$^2$ operating at L2 and observing at in a single band at $\sim2\mu$m could potentially exhaust the cosmological information content of galaxies in a 10 year survey. For galaxy evolution (making use of gravitational lensing to magnify the earliest galaxies) and SN the same telescope would suffice. We discuss the technological advances that would be needed for such a telescope and a technological advance (placing telescopes $\sim$ 1 AU outside the plane of the Solar System) that could reduce the sky background by a factor of $\sim50$, significantly reducing the required telescope aperture. While the final galaxy survey remains well outside of our technical reach today, we present scaling relations that show how we can progress toward the goal of day exhausting the information content encoded in the shapes, positions, and colors of galaxies.