Towards studying hierarchical assembly in real time: a Milky Way progenitor galaxy at z=2.36 under the microscope

We use Hubble Space Telescope (HST) imaging and near-infrared spectroscopy from Keck/Multi-Object Spectrometer For Infra-Red Exploration (MOSFIRE) to study the substructure around the progenitor of a Milky Way-mass galaxy in the Hubble Frontier Fields (HFF). Specifically, we study an r(e) = 40(-30)(+70) pc, M-star similar to 10(8.2) M-circle dot rest-frame ultraviolet luminous 'clump' at a projected distance of similar to 100 pc from a M-star similar to 10(9.8) M-circle dot galaxy at z = 2.36 with a magnification mu = 5.21. We measure the star formation history of the clump and galaxy by jointly modelling the broad-band spectral energy distribution from HST photometry and Ha from MOSFIRE spectroscopy. Given our inferred properties (e.g. mass, metallicity, dust) of the clump and galaxy, we explore scenarios in which the clump formed in situ (e.g. a star-forming complex) or ex situ (e.g. a dwarf galaxy being accreted). If it formed in situ, we conclude that the clump is likely a single entity as opposed to a aggregation of smaller star clusters, making it one of the most dense star clusters catalogued. If it formed ex situ, then we are witnessing an accretion event with a 1:40 stellar mass ratio. However, our data alone are not informative enough to distinguish between in situ and ex situ scenarios to a high level of significance. We posit that the addition of high-fidelity metallicity information, such as [O III] 4363 angstrom, which can be detected at modest signal-to-noise ratio with only a few hours of James Webb Space Telescope (JWST)/Near-Infrared Spectrograph (NIRSpec) time, may be a powerful discriminant. We suggest that studying larger samples of moderately lensed substructures across cosmic time can provide unique insight into the hierarchical formation of galaxies like the Milky Way.