Prospects for Near-Far Constraints on Reionization from Simulated LSST Observations of Milky Way Dwarf Galaxies and Stellar Streams

Abstract

The NSF-DOE Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) will detect resolved stellar populations of intact dwarf galaxies and disrupted stellar streams in the Milky Way (MW) halo and beyond, opening a new avenue for near-field cosmology, particularly for constraining the faint galaxies during reionization. To interpret these data, we present and publicly release mock LSST catalogs of satellites, stellar streams, phase-mixed galaxies, and field galaxies around 13 MW-mass hosts in the FIRE-2 zoom-in simulations, generated with \textsf{py-ananke}. Using a surface brightness limit of $μ_{g} \leq 34$ $\rm{mag}$ $\rm{arcsec^{-2}}$, we find that $\approx29\%$ of streams with $M_{\star} \gtrsim 5 \times 10^5$ $M_{\odot}$ are detectable with tidal tails, while another $\approx38\%$ are detectable only through their bound progenitors and would appear as satellites. We estimate that $\approx96\%$ of satellites and $\approx64\%$ of field galaxies (out to $\sim$3.5 Mpc) with $M_{\star} \gtrsim 10^{5}$ $M_{\odot}$ are detectable, decreasing to $\approx$59\% and $\approx$42\% for $M_{\star} \gtrsim 10^{4.5}$ $M_{\odot}$ owing to resolution limits. Including streams detectable as satellites reproduces the radial distribution of observed MW satellites, suggesting that some observed satellites may be bound remnants of disrupted systems. Our mock galaxies match the sizes of observed satellites with $M_{\rm{V}} \lesssim -6$. Star-formation histories computed from only the detectable stars broadly recover the simulated star-formation histories. As an application, we test the near-far technique for reconstructing the proto-MW ultraviolet luminosity function (UVLF) during reionization ($z \gtrsim 6$). Adding the detectable streams improves the recovered faint-end slope at $z = 7$ and reduces the halo-to-halo scatter by $\approx33\%$.