The NIRVANDELS Survey: a robust detection of α-enhancement in star-forming galaxies at z ≃ 3.4

Abstract

We present results from the NIRVANDELS survey on the gas-phase metallicity (Zg, tracing O/H) and stellar metallicity (Z⋆, tracing Fe/H) of 33 star-forming galaxies at redshifts 2.95 < z < 3.80. Based on a combined analysis of deep optical and near-IR spectra, tracing the rest-frame far-ultraviolet (FUV; 1200–2000 Å) and rest-frame optical (3400–5500 Å) respectively, we present the first simultaneous determination of the stellar and gas-phase mass-metallicity relationships (MZRs) at z ≃ 3.4. In both cases, we find that metallicity increases with increasing stellar mass (M⋆), and that the power-law slope at M⋆ ≲ 1010M⊙ of both MZRs scales as $Z \propto M_{\star }^{0.3}$. Comparing the stellar and gas-phase MZRs, we present direct evidence for super-solar O/Fe ratios (i.e. α-enhancement) at z > 3, finding (O/Fe) = 2.54 ± 0.38 × (O/Fe)⊙, with no clear dependence on M⋆.

Chemical abundance scaling relations for multiple elements in z~2-3 star-forming galaxies

“…Likewise, there is no statistically significant anti-correlation between [O/Fe] and M * for the subset of galaxies in Figure 12 (Spearman ρ = −0.29, p = 0.02). Perhaps the most direct comparison of this finding to other samples is with the recent results for z ∼ 3 galaxies from Cullen et al (2021), who compared their rest-UV-measured Fe/H with the strong-line-based O-MZR from Sanders et al (2021). As we do, the authors found virtually identicalalbeit much steeper-slopes for their gas-phase metal-licity relation (O-MZR) and stellar metallicity relation (Fe-MZR).…”

“…As with the O-MZR, the Fe-MZR in Figure 10 is shallower than other recent studies at z 2 (Cullen et al 2019(Cullen et al , 2021, which found β Fe/H ∼ 0.3 using galaxies' rest-UV spectra to estimate stellar metallicity. Theios et al (2019) also showed a somewhat steeper increase in stellar metallicity as a function of M * (β Fe/H ≈ 0.29) Figure 10.…”

“…Using rest-UV photospheric spectra is a more direct method of inferring stellar metallicity than GalDNA, and we showed in Steidel et al (2016) that stellar metallicity inferred this way is relatively insensitive to the choice of stellar model. However, Cullen et al (2021) found that stellar metallicities inferred using BPASS were systematically lower than metallicities inferred using Star-burst99 models (Leitherer et al 2010). There is is also substantial diversity in the predicted ionizing spectra of the same massive stars, which is what GalDNA relies on to indirectly estimate Fe/H.…”

“…More recently, very deep spectra of massive, quiescent galaxies at high redshift have enabled the measurement of multiple elemental abundances (Kriek et al 2016(Kriek et al , 2019Jafariyazani et al 2020). Using deep rest-UV spectra, it is also becoming possible to measure iron abundances in high-z star-forming galaxies (e.g., Steidel et al 2016;Cullen et al 2019;Topping et al 2020;Cullen et al 2021). These studies represent a promising direction for future high-z galaxy research, because understanding the abundance patterns of galaxies provides additional insight regarding their assembly histories-particularly the comparison between iron and α elements (including Mg and O), as the α/Fe ratio is sensitive to star formation timescales (e.g., Tinsley 1979).…”

The impact of the FMR and starburst galaxies on the (low metallicity) cosmic star formation history

“…Recent studies indeed find evidence for supersolar O/Fe 2 (O/Fe)| in the ISM of z ∼ 2 star-forming galaxies (e.g. Steidel et al 2016;Topping et al 2020;Cullen et al 2021). Consequently, oxygen (or, more generally, α-element)-enhanced abundance ratios are also found in local stars that formed at earlier cosmic epochs (e.g.…”