THEGRISM LENS-AMPLIFIED SURVEY FROM SPACE(GLASS). II. GAS-PHASE METALLICITY AND RADIAL GRADIENTS IN AN INTERACTING SYSTEM ATZ≃ 2

Abstract

We present spatially resolved gas-phase metallicity for a system of three galaxies at z = 1.85 detected in the Grism Lensed-Amplified Survey from Space (GLASS). The combination of HST's diffraction limit and strong gravitational lensing by the cluster MACS J0717+3745 results in a spatial resolution of 200 − 300 pc, enabling good spatial sampling despite the intrinsically small galaxy sizes. The galaxies in this system are separated by 50 − 200 kpc in projection and are likely in an early stage of interaction, evidenced by relatively high specific star formation rates. Their gas-phase metallicities are consistent with larger samples at similar redshift, star formation rate, and stellar mass. We obtain a precise measurement of the metallicity gradient for one galaxy and find a shallow slope compared to isolated galaxies at high redshift, consistent with a flattening of the gradient due to gravitational interaction. An alternative explanation for the shallow metallicity gradient and elevated star formation rate is rapid recycling of metal-enriched gas, but we find no evidence for enhanced gas-phase metallicities which should result from this effect. Notably, the measured stellar masses log M * /M = 7.2 − 9.1 probe to an order of magnitude below previous massmetallicity studies at this redshift. The lowest mass galaxy has properties similar to those expected for Fornax at this redshift, indicating that GLASS is able to directly study the progenitors of local group dwarf galaxies on spatially resolved scales. Larger samples from the full GLASS survey will be ideal for studying the effects of feedback, and the time evolution of metallicity gradients. These initial results demonstrate the utility of HST spectroscopy combined with gravitational lensing for characterizing resolved physical properties of galaxies at high redshift.

Mild evolution of the stellar metallicity gradients of disc galaxies

“…The available observations of HII regions for high-z galaxies do not allow the formation of a robust conclusion on the evolution of the metallicity gradients (e.g. Yuan et al 2011;Queyrel et al 2012;Stott et al 2014;Jones et al 2015). In fact, high-redshift observations show a complex situation with gas-phase components showing a variety of metallicity gradients that could respond to the action of different physical processes (e.g.…”

A magnified view of galaxy formation

“…As gas is converted into stars, metallicity increases at a rate modulated by the degree of mass loss. Right: Metallicity as a function of stellar mass for galaxies at z 2, from Jones et al [75]. Blue triangles are from composites of non-lensed galaxies demonstrating a steep mass-metallicity relation especially at masses below 10 9 M .…”

“…To date several measurements have been made for gravitationally lensed galaxies which provide the most reliable results due to their high spatial resolution [75,79,80]. A growing number of non-lensed galaxies also have limited spatially resolved metallicity measurements which show interesting trends with global demographic properties [81].…”

“…A negative gradient is apparent with a slope of −0.28 dex/kpc. Right: Metallicity gradients in star forming galaxies as a function of redshift, from Jones et al [75] (see also Leethochawalit et al [79]). Red points are gravitationally lensed galaxies while blue points are nonlensed galaxies observed with adaptive optics such that the effective disk radii are resolved.…”

A framework for empirical galaxy phenomenology: the scatter in galaxy ages and stellar metallicities

“…In this section, we test the effect of an additional starburst contribution to galaxy star formation histories that complete observations of the star-forming main sequence may not reflect but may, nevertheless, be necessary to explain the density profiles of dwarf systems (e.g., Weisz et al 2012;Amorisco et al 2014;Madau et al 2014) and observed in a lensed dwarf systems at high redshift (Jones et al 2014). Such bursts would represent an further random process that produces spikes in the star formation rate, perhaps as some tidal event funnels gas into the central regions of the galaxy.…”