Red Supergiants as cosmic abundance probes: the first direct metallicity determination of NGC 4038 in the Antennae

TL;DR

This study measures the metallicity of NGC 4038 using red supergiant stars, providing a direct and unbiased method that reveals a flat metallicity gradient likely caused by galaxy interaction.

Contribution

It introduces a novel direct stellar metallicity measurement technique using RSGs, avoiding biases of traditional HII region methods.

Findings

Measured metallicity of [Z]= +0.07 ± 0.03 in NGC 4038

Detected a flat metallicity gradient across the galaxy

Suggests metal redistribution due to galaxy interaction

Abstract

We present a direct determination of the stellar metallicity in the close pair galaxy NGC~4038 (D = 20 Mpc) based on the quantitative analysis of moderate resolution KMOS/VLT spectra of three super star clusters (SSCs). The method adopted in our analysis has been developed and optimised to measure accurate metallicities from atomic lines in the $J$-band of single red supergiant (RSG) or RSG-dominated star clusters. Hence, our metallicity measurements are not affected by the biases and poorly understood systematics inherent to strong line HII methods which are routinely applied to massive data sets of galaxies. We find [Z]= +0.07 $\pm$ 0.03 and compare our measurements to HII strong line calibrations. Our abundances and literature data suggest the presence of a flat metallicity gradient, which can be explained as redistribution of metal-rich gas following the strong interaction.