The First Census of Precise Metallicity Gradients in Dwarf Galaxies at Cosmic Noon

Abstract: Dwarf galaxies (Mstar < 10^9 Msun) provide unique probes of cosmic structure formation on the smallest scales, and represent the most compelling testbeds for galactic feedback since they are more susceptible to feedback processes than their high-mass counterparts. However, their chemo-structural properties at cosmic noon (i.e. z~2) have remained unexplored, due to their intrinsically small size and insufficient spatial sampling limited by seeing. To address this, we develop a highly effective method to obtain sub-kiloparsec resolution (i.e. precise) gas-phase metallicity maps of strongly lensed galaxies using space-based slitless spectroscopy. Applying our method to the deep Hubble Space Telescope near-infrared grism data, we obtain precise metallicity maps of 81 star-forming galaxies at z~1.2-2.3, over half of which reside in the dwarf mass regime. Our work presents the first statistically representative sample of high-z dwarf galaxies with their metallicity spatial distribution measured with sufficient resolution. These metallicity maps reveal a variety of baryonic physics, such as efficient radial mixing from tidal torques, rapid accretion of low-metallicity gas, and various feedback processes which can significantly influence the chemo-structural properties of dwarf galaxies. In particular, we find two galaxies at z~2 displaying strongly inverted metallicity radial gradients, suggesting that powerful galactic winds triggered by central star bursts carry the bulk of stellar nucleosynthesis yields to the outskirts. Furthermore, 10% of the metallicity gradients measured in our sample are inverted, which are hard to explain by current cosmological zoom-in hydro-simulations and analytical chemical evolution models. Our method can also be readily applied to data from future space missions employing grism instruments, e.g. JWST, WFIRST, Euclid.

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