The dwarf stellar mass function in different environments and the lack of a generic missing dwarfs problem in ΛCDM

ABSTRACT We combine deep photometric data in the COSMOS and XMM-LSS fields with high-resolution cosmological hydrodynamical simulations to explore two key questions: (1) how does the galaxy stellar mass function, particularly in the dwarf ($M_\star$ $<$ 10$^{9.5}$ M$_\odot$) regime, vary with environment, defined as the distance from large-scale structure (LSS) traced by nodes and filaments in the cosmic web? (2) is there a generic ‘missing dwarfs’ problem in Lambda cold dark matter ($\Lambda$CDM) predictions when all environments – and not just satellites around Milky Way like galaxies – are considered? The depth of the observational data used here enables us to construct complete, unbiased samples of galaxies, down to $M_{\star }$ $\sim$ 10$^7$ M$_{\odot }$ and out to $z\sim 0.4$. Strong environmental differences are found for the galaxy stellar mass function when considering distance from LSS. As we move closer to LSS, the dwarf mass function becomes progressively flatter and the knee of the mass function shifts to larger stellar masses, both of which result in a higher ratio of massive to dwarf galaxies. While the stellar mass functions from the three simulations (NewHorizon, TNG50, and FIREbox) considered here do not completely agree across the dwarf regime, there is no evidence of a generic missing dwarfs problem in the context of $\Lambda$CDM, akin to the results of recent work that demonstrates that there is no missing satellites problem around Galactic analogues.