The Origin and Evolution of Metallicity Gradients: Probing the Mode of Mass Assembly at z=2

TL;DR

This study measures gas-phase metallicity gradients in gravitationally lensed galaxies at z=2, revealing diverse gradient patterns and showing that these gradients tend to flatten over cosmic time, supporting inside-out galaxy growth.

Contribution

First to provide a coherent picture of metallicity gradient evolution at z=2, combining observations with a simple chemical evolution model including gas migration and outflows.

Findings

Metallicity gradients are generally negative at z=2, with some steeper than local values.

One galaxy shows an inverted gradient, indicating complex kinematic processes.

Metallicity gradients flatten by a factor of approximately 2.6 from z=2.2 to today.

Abstract

We present and discuss measurements of the gas-phase metallicity gradient in gravitationally lensed galaxies at z=2.0-2.4 based on adaptive optics-assisted imaging spectroscopy with the Keck II telescope. Through deep exposures we have secured high signal to noise data for four galaxies with well-understood kinematic properties. Three galaxies with well-ordered rotation reveal metallicity gradients in the sense of having lower gas-phase metallicities at larger galactocentric radii. Two of these display gradients much steeper than found locally, while a third has one similar to that seen in local disk galaxies. The fourth galaxy exhibits complex kinematics indicative of an ongoing merger and reveals an "inverted" gradient with lower metallicity in the central regions. By comparing our sample to similar data in the literature for lower redshift galaxies, we determine that, on average, metallicity gradients must flatten by a factor of 2.6 +/- 0.9 between z=2.2 and the present epoch. This factor is in rough agreement with the size growth of massive galaxies suggesting that inside-out growth can account for the evolution of metallicity gradients. Since the addition of our new data provides the first indication of a coherent picture of this evolution, we develop a simple model of chemical evolution to explain the collective data. We find that metallicity gradients and their evolution can be explained by the inward radial migration of gas together with a radial variation in the mass loading factor governing the ratio of outflowing gas to the local star formation rate. Average mass loading factors of \lsim 2 are inferred from our model in good agreement with direct measurements of outflowing gas in z \simeq 2 galaxies.