[Paper Review] Integral field spectroscopy with SINFONI of VVDS galaxies. II. The mass-metallicity relation at 1.2 < z < 1.6

J. Queyrel, T. Contini+9 more|Springer Link (Chiba Institute of Technology)|Mar 6, 2009

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy57 references21 citations

TL;DR

This study presents the first integral field spectroscopy analysis of seven star-forming galaxies at z ≈ 1.4 using SINFONI on the VLT, deriving their stellar masses and oxygen abundances via emission-line ratios. It finds a metallicity offset of 0.2–0.4 dex lower than the local mass-metallicity relation, with a weak mass-metallicity trend and significant influence from N/O abundance ratios, suggesting metal-poor gas inflows or high specific star formation rates as causes for low metallicities.

ABSTRACT

This work aims to provide a first insight into the mass-metallicity (MZ) relation of star-forming galaxies at redshift z~1.4. To reach this goal, we present a first set of nine VVDS galaxies observed with the NIR integral-field spectrograph SINFONI on the VLT. Oxygen abundances are derived from empirical indicators based on the ratio between strong nebular emission-lines (Halpha, [NII]6584 and [SII]6717,6731). Stellar masses are deduced from SED fitting with Charlot & Bruzual (2007) population synthesis models, and star formation rates are derived from [OII]3727 and Halpha emission-line luminosities. We find a typical shift of 0.2-0.4 dex towards lower metallicities for the z~1.4 galaxies, compared to the MZ-relation in the local universe as derived from SDSS data. However, this small sample of eight galaxies does not show any clear correlation between stellar mass and metallicity, unlike other larger samples at different redshift (z~0, z~0.7, and z~2). Indeed, our galaxies lie just under the relation at z~2 and show a small trend for more massive galaxies to be more metallic (~0.1 logarithmic slope). There are two possible explanations to account for these observations. First, the most massive galaxies present higher specific star formation rates when compared to the global VVDS sample which could explain the particularly low metallicity of these galaxies as already shown in the SDSS sample. Second, inflow of metal-poor gas due to tidal interactions could also explain the low metallicity of these galaxies as two of these three galaxies show clear signatures of merging in their velocity fields. Finally, we find that the metallicity of 4 galaxies is lower by ~0.2 to 0.4 dex if we take into account the N/O abundance ratio in their metallicity estimate.

Motivation & Objective

• To probe the mass-metallicity (MZ) relation of star-forming galaxies at z ≈ 1.4, a redshift regime critical for peak star formation and mass assembly.
• To assess the evolution of chemical abundances in high-redshift galaxies using near-infrared integral field spectroscopy.
• To investigate the role of star formation efficiency and gas inflows in shaping metallicity through detailed analysis of kinematics and emission-line ratios.
• To evaluate the impact of nitrogen-to-oxygen (N/O) abundance ratios on oxygen abundance estimates in high-redshift galaxies.

Proposed method

• Oxygen abundance was derived from empirical indicators using the ratios of nebular emission lines Hα, [N ii]6584Å, and [S ii]6717,6731Å in SINFONI IFU data.
• Stellar masses were estimated via SED fitting using Charlot & Bruzual (2003) population synthesis models.
• Star formation rates were derived from Hα and [O ii]3727Å luminosities using calibrations from Argence & Lamareille (2009).
• The N/O abundance ratio was measured from sulfur line ratios and used to correct metallicity estimates, following Perez-Montero & Contini (2009).
• Kinematic maps were analyzed to identify tidal interactions and gas inflows, particularly in three galaxies showing disturbed velocity fields.
• Results were compared to local (SDSS) and high-redshift (z ≈ 2) MZ relations to assess evolution.

Experimental results

Research questions

• RQ1 How does the mass-metallicity relation at z ≈ 1.4 compare to the local relation observed in SDSS?
• RQ2 What is the nature of the correlation (if any) between stellar mass and metallicity in this high-redshift sample?
• RQ3 To what extent do high specific star formation rates or tidal interactions influence the observed low metallicities?
• RQ4 How significant is the correction to oxygen abundance estimates when accounting for nitrogen-to-oxygen (N/O) abundance ratios?
• RQ5 Do the kinematic signatures of merging or gas inflow correlate with low metallicity in these galaxies?

Key findings

• The mean metallicity of the z ≈ 1.4 sample is 12 + log(O/H) = 8.62, with a scatter of 0.22 dex, and lies 0.2–0.4 dex below the local SDSS mass-metallicity relation.
• The sample shows a weak positive correlation between stellar mass and metallicity, with a logarithmic slope of approximately 0.1, indicating a mild mass-metallicity trend.
• All but one galaxy lie below the z ≈ 2 MZ relation from Erb et al. (2006), suggesting a possible evolution toward higher metallicities with time.
• Three galaxies (VVDS220596913, VVDS220544103, VDS020116027) show kinematic signatures of tidal interactions, supporting the hypothesis of metal-poor gas inflows as a cause of low metallicity.
• Accounting for the N/O abundance ratio reduces the oxygen abundance by 0.2–0.4 dex for two galaxies with the highest N/O ratios, confirming a significant downward correction to metallicity estimates.
• The observed low metallicities are likely driven by a combination of high specific star formation rates and inflows of metal-poor gas, particularly in interacting systems.