walter: A Tool for Predicting Resolved Stellar Population Observations with Applications to the Roman Space Telescope

TL;DR

This paper introduces 'walter', a tool that predicts resolved stellar populations in galaxies, aiding survey planning for the Roman Space Telescope by estimating star densities, exposure times, and crowding limits.

Contribution

The paper presents a new formalism and a publicly available tool for predicting stellar densities and observational requirements for galaxy surveys, specifically tailored for upcoming Roman Space Telescope observations.

Findings

Validated predictions for star densities in target fields.

Provided estimates for exposure times to detect key stellar features.

Assessed crowding limits for planned observations.

Abstract

Studies of resolved stellar populations in the Milky Way and nearby galaxies reveal an amazingly detailed and clear picture of galaxy evolution. Within the Local Group, the ability to probe the stellar populations of small and large galaxies opens up the possibility of exploring key questions such as the nature of dark matter, the detailed formation history of different galaxy components, and the role of accretion in galactic formation. Upcoming wide-field surveys promise to extend this ability to all galaxies within 10~Mpc, drastically increasing our capability to decipher galaxy evolution and enabling statistical studies of galaxies' stellar populations. To facilitate the optimum use of these upcoming capabilities we develop a simple formalism to predict the density of resolved stars for an observation of a stellar population at fixed surface brightness and population parameters. We provide an interface to calculate all quantities of interest to this formalism via a public release of the code: \texttt{walter}. This code enables calculation of (i) the expected number density of detected stars, (ii) the exposure time needed to reach certain population features, such as the horizontal branch, and (iii) an estimate of the crowding limit, among other features. These calculations will be very useful for planning surveys with NASA's upcoming Nancy Grace Roman Space Telescope (Roman, formerly WFIRST), which we use for example calculations throughout this work.