Exploring the Gas-Phase Metallicity Gradients of Star-forming Galaxies at Cosmic Noon

Abstract: We explore the relationships between the [O/H] gas-phase metallicity radial gradients and multiple galaxy properties for 238 star-forming galaxies at 0.6<z<2.6 selected from the CANDELS Ly$\alpha$ Emission at Reionization (CLEAR) survey with stellar mass 8.5 < log $M_{*}/M_{\odot}$ < 10.5. The gradients cover the range from -0.11 to 0.22 dex kpc$^{-1}$, with the median value close to zero. We reconstruct the nonparametric star-formation histories (SFHs) of the galaxies with spectral energy distribution modeling using Prospector with more than 40 photometric bands from HST, Spitzer and ground-based facilities. In general, we find weak or no correlations between the metallicity gradients and most galaxy properties, including the mass-weighted age, recent star formation rate, dust attenuation, and morphology as quantified by both parametric and non-parametric diagnostics. We find a significant but moderate correlation between the gradients and the 'evolutionary time', a temporal metric that characterizes the evolutionary status of a galaxy, with flatter gradients observed in more evolved galaxies. Also, there is evidence that galaxies with multiple star-formation episodes in their SFHs tend to develop more negative gas-phase metallicity gradients (higher [O/H] at the center). We conclude that gas kinematics, e.g. radial inflows and outflows, is likely an important process in setting the gas-phase metallicity gradients, in addition to the evolution of the SFH radial profile. Since the gradients are largely independent on the galaxies' physical properties, and only weakly dependent on their SFH, it would appear that the timescale of the gas kinematics is significantly shorter than the evolution of star formation.