Metal deficiency in cluster star-forming galaxies at z=2

TL;DR

This study reveals that star-forming galaxies in a z=2 galaxy cluster have significantly lower metallicities and higher star formation indicators compared to similar field galaxies, suggesting environmental effects on galaxy evolution.

Contribution

It provides the first evidence of metal deficiency in cluster star-forming galaxies at z=2, highlighting environmental influence on galaxy chemical enrichment.

Findings

Cluster SFGs are up to 0.25 dex poorer in metals than field SFGs.

Cluster SFGs show higher Hα luminosity and equivalent width.

Lower metallicity confirmed with multiple indicators.

Abstract

We investigate the environmental effect on the metal enrichment of star-forming galaxies (SFGs) in the farthest spectroscopically confirmed and X-ray detected cluster, CL~J1449+0856 at $z=1.99$. We combined HST/WFC3~G141 slitless spectroscopic data, our 13-band photometry, and a recent Subaru/MOIRCS near infrared spectroscopic follow-up to constrain the physical properties of SFGs in CL~J1449+0856 and in a mass-matched field sample. After a conservative active galactic nuclei (AGN) removal, stacking individual MOIRCS spectra of $6$ ($31$) sources in the cluster (field) in the mass range $10 \leq \mathrm{log(M/M_{\odot}}) \leq 11$, we find a $\sim4\sigma$ significant lower [N II]/H$\alpha$ ratio in the cluster than in the field. Stacking a subsample of $16$ field galaxies with H$\beta$ and [O III] in the observed range, we measure a [O III]/H$\beta$ ratio fully compatible with the cluster value. Converting these ratios into metallicities, we find that the cluster SFGs are up to $0.25\,\mathrm{dex}$ poorer in metals than their field counterparts, depending on adopted calibrations. The low metallicity in cluster sources is confirmed using alternative indicators. Furthermore, we observe a significantly higher H$\alpha$ luminosity and equivalent width in the average cluster spectrum than in the field. This is likely due to enhanced specific star formation rate, even if lower dust reddening and/or an uncertain environmental dependence of the continuum-to-nebular emission differential reddening may play a role. Our findings might be explained by the accretion of pristine gas around galaxies at $z=2$ and from cluster-scale reservoirs, possibly connected with a phase of rapid halo mass assembly at $z>2$ and of high galaxy merging rate.