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Arxiv: Linking high-z and low-z: Are We Observing the Progenitors of the Milky Way with JWST? Published: 7/8/2024 9:00:00 PM Updated: 7/8/2024 9:00:00 PM

Paper abstract: The recent JWST observation of the Firefly Sparkle at z=8.3 offers a uniqueopportunity to link the high- and the low-z Universe. Indeed, the claim of itbeing a Milky Way (MW) type of assembly at the cosmic dawn opens thepossibility of interpreting the observation with locally calibratedgalaxy-formation models. Here, we use the MW-evolution model NEFERTITI toperform forward modeling of our Galaxy's progenitors at high-z. We build aset of mock spectra for the MW building blocks to make predictions for JWST andto interpret the Firefly Sparkle observation. First, we find that the mostmassive MW progenitor becomes detectable in a deep survey like JADES fromz~ 8.2, meaning that we could have already observed MW-analogs thatstill need interpretation. Second, we provide predictions for the number ofdetectable MW progenitors in lensed surveys like CANUCS, and interpret theFirefly Sparkle as a group of MW building blocks. Both the number of detectionsand the observed NIRCam photometry are consistent with our predictions. Byidentifying the MW progenitors whose mock photometry best fits the data, wefind bursty and extended star-formation histories, lasting > 150-300~Myr, andestimate their properties: M_h ~ 10^{8-9} \, M_{\odot}, M_*~ 10^{6.2-7.5}\, M_{\odot}, SFR ~ 0.04-0.20 \, M_{\odot}yr^{-1} and Z_{gas} ~ 0.04 - 0.24 \, Z_{\odot}. Uncovering theproperties of MW-analogs at cosmic dawn by combining JWST observations andlocally-constrained models, will allow us to understand our Galaxy's formation,linking the high- and low-z perspectives.