Observing EAGLE galaxies with JWST: predictions for Milky Way progenitors and their building blocks

We present predictions, derived from the EAGLE $\Lambda$CDM cosmological hydrodynamical simulations, for the abundance and of galaxies expected to be detected at high redshift by the {\it James Webb Space Telescope} (\JWST). We consider the galaxy as a whole and focus on the sub-population of progenitors of Milky Way (MW) analogues, defined to be galaxies with accretion histories similar to the MW's, that is, galaxies that underwent a merger the Gaia-Enceladus-Sausage (GES) event and that contain an analogue of the Large Magellanic Cloud (LMC) satellite today. We the luminosity function of all EAGLE galaxies in \JWST/NIRCam passbands, in the redshift range $z=2-8$, taking into account dust obscuration and different exposure times. For an exposure time of $T=10^5$s, average MW progenitors are observable as far back as $z\sim6$ in most bands, and this changes to $z\sim5$ and $z\sim4$ for the GES and LMC progenitors, respectively. The progenitors of GES and LMC analogues are, on average, $\sim 2$ and $\sim 1$ mag fainter than the MW progenitors at most redshifts. They lie, on average, within $\sim 60$ and $30$ arcsec, respectively, of their future MW host at all times, and thus will appear within the field-of-view of \JWST/NIRCam. We conclude that galaxies resembling the main progenitor of the MW and its major accreted components should be observable with \JWST\ beyond redshift $2$, providing a new and unique window in studying the formation history of our own galaxy.Comment: 17 page, 16 figures. Submitted to MNRAS. Contact Tilly Evans with comments or question