MASSIV: Mass Assembly Survey with SINFONI in VVDS III. Evidence for positive metallicity gradients in z similar to 1.2 star-forming galaxies

Astronomy and Astrophysics (Impact Factor: 4.38). 11/2011; 539(0004-6361). DOI: 10.1051/0004-6361/201117718
Source: arXiv


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.

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