[Paper Review] MASSIV: Mass Assembly Survey with SINFONI in VVDS. III. Evidence for positive metallicity gradients in z~1.2 star-forming galaxies

J. Queyrel, T. Contini+12 more|arXiv (Cornell University)|Nov 16, 2011

Galaxies: Formation, Evolution, Phenomena68 references83 citations

TL;DR

This study uses spatially resolved SINFONI spectroscopy of 50 z~1.2 star-forming galaxies to measure metallicity gradients via the N2 ratio. It finds that 14% of the sample (7 galaxies) exhibit significant positive metallicity gradients—increasing metallicity from center to outskirts—suggesting infall of metal-poor gas, likely driven by interactions or cold accretion, challenging the typical negative gradients seen in local galaxies.

ABSTRACT

A key open issue for galaxy evolution and formation models is the understanding of the different mechanisms of galaxy assembly at various cosmic epochs. The aim of this study is to derive the global and spatially-resolved metal content in high-redshift galaxies. Using VLT/SINFONI IFU spectroscopy of a first sample of 50 galaxies at z~1.2 in the MASSIV survey, we are able to measure the Ha and [NII]6584 emission lines. Using the N2 ratio as a proxy for oxygen abundance in the interstellar medium, we measure the metallicity of the sample galaxies. We develop a tool to extract spectra in annular regions of these galaxies, leading to a spatially-resolved estimate of the oxygen abundance in each galaxy. We derive a metallicity gradient for 26 galaxies in our sample and discover a significant fraction of galaxies with a "positive" gradient. Using a simple chemical evolution model, we derive infall rates of pristine gas onto the disks. Seven galaxies display a positive gradient at a high confidence level. Four out of these are interacting and one is a chain galaxy. We suggest that interactions might be responsible for shallowing and even inverting the abundance gradient. We also identify two interesting correlations in our sample: a) galaxies with higher gas velocity dispersion have shallower/positive gradients; and b) metal-poor galaxies tend to show a positive gradient whereas metal-rich ones tend to show a negative one. This last observation can be explained by the infall of metal-poor gas into the center of the disks. We address the question of the origin of this infall under the influence of gas flows triggered by interactions and/or cold gas accretion.

Motivation & Objective

• To measure spatially resolved metallicity gradients in high-redshift star-forming galaxies at z~1.2 using integral-field spectroscopy.
• To investigate the origin of metallicity gradients in the context of galaxy assembly and evolution mechanisms.
• To explore the role of interactions and gas inflows in shaping metallicity profiles in high-redshift disk galaxies.
• To test the impact of gas infall on metallicity gradients using a simple chemical evolution model.
• To examine correlations between metallicity gradients, global metallicity, and kinematic properties such as gas velocity dispersion.

Proposed method

• Acquired VLT/SINFONI integral-field spectroscopy of 50 galaxies at z~1.2 in the MASSIV survey to extract Hα and [N ii]6584 emission lines.
• Used the N2 ratio (log([N ii]6584/Hα)) as a proxy for oxygen abundance in the interstellar medium.
• Developed a spectral extraction tool to obtain radial metallicity profiles by dividing galaxies into annular regions centered on the Hα peak.
• Applied a simple chemical evolution model with radial gas flows to estimate infall rates of pristine gas onto galactic disks.
• Correlated metallicity gradients with global metallicity, gas velocity dispersion, and morphological classification (e.g., isolated vs. interacting).
• Used statistical analysis to identify galaxies with high-confidence positive gradients and assess their physical significance.

Experimental results

Research questions

• RQ1 What is the distribution of metallicity gradients in z~1.2 star-forming galaxies, and how do they compare to local galaxies?
• RQ2 What physical mechanisms—such as interactions or cold gas accretion—can explain the presence of positive metallicity gradients in high-redshift galaxies?
• RQ3 How does the global metallicity of a galaxy correlate with the sign and steepness of its metallicity gradient?
• RQ4 To what extent do kinematic properties like gas velocity dispersion correlate with metallicity gradient morphology?
• RQ5 Can the observed metallicity gradients be explained by infall of metal-poor gas, and what are the inferred infall rates?

Key findings

• Seven galaxies in the sample exhibit a positive metallicity gradient at high confidence, representing 14% of the 50-galaxy sample.
• Four of the seven galaxies with positive gradients are classified as interacting systems, and one is a chain galaxy, suggesting interactions may drive or enhance positive gradients.
• Galaxies with higher gas velocity dispersion tend to have shallower or positive metallicity gradients, indicating more turbulent ISM conditions.
• A weak but notable correlation exists: metal-poor galaxies preferentially show flat or positive gradients, while metal-rich galaxies tend to have negative gradients.
• The observed trend can be explained by the infall of metal-poor gas into galactic centers, diluting central metallicity and flattening or reversing the gradient.
• Inferred infall rates of pristine gas reach several hundred solar masses per year, with a positive correlation between maximum infall rate and dark matter halo mass.