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: 4.48). 11/2011; 539. DOI: 10.1051/0004-6361/201117718
Source: arXiv

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.


Available from: Bianca Garilli, May 18, 2015
1 Follower
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present results on the z~2.3 mass-metallicity relation (MZR) using early observations from the MOSFIRE Deep Evolution Field (MOSDEF) survey. We use an initial sample of 87 star-forming galaxies with spectroscopic coverage of H\beta, [OIII]\lambda 5007, H\alpha, and [NII]\lambda 6584 rest-frame optical emission lines, and estimate the gas-phase oxygen abundance based on the N2 and O3N2 strong-line indicators. We find a positive correlation between stellar mass and metallicity among individual z~2.3 galaxies using both the N2 and O3N2 indicators. We also measure the emission-line ratios and corresponding oxygen abundances for composite spectra in bins of stellar mass. Among composite spectra, we find a monotonic increase in metallicity with increasing stellar mass, offset ~0.15-0.3 dex below the local MZR. When the sample is divided at the median star-formation rate (SFR), we do not observe significant SFR dependence of the z~2.3 MZR among either individual galaxies or composite spectra. We furthermore find that z~2.3 galaxies have metallicities ~0.1 dex lower at a given stellar mass and SFR than is observed locally. This offset suggests that high-redshift galaxies do not fall on the local "fundamental metallicity relation" among stellar mass, metallicity, and SFR, and may provide evidence of a phase of galaxy growth in which the gas reservoir is built up due to inflow rates that are higher than star-formation and outflow rates. However, robust conclusions regarding the gas-phase oxygen abundances of high-redshift galaxies await a systematic reappraisal of the application of locally calibrated metallicity indicators at high redshift.
    The Astrophysical Journal 08/2014; 799(2). DOI:10.1088/0004-637X/799/2/138 · 6.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have observed a sample of typical z=1 star forming galaxies, selected from the HiZELS survey, with the new KMOS near-infrared, multi-IFU instrument on the VLT, in order to obtain their dynamics and metallicity gradients. The majority of our galaxies have a metallicity gradient consistent with being flat or negative (i.e. higher metallicity cores than outskirts). Intriguingly, we find a trend between metallicity gradient and specific star formation rate (sSFR), such that galaxies with a high sSFR tend to have relatively metal-poor centres, a result which is strengthened when combined with datasets from the literature. This result appears to explain the discrepancies reported between different high redshift studies and varying claims for evolution. From a galaxy evolution perspective, the trend we see would mean that a galaxy's sSFR is governed by the amount of metal poor gas that can be funnelled into its core, triggered either by merging or through efficient accretion. In fact merging may play a significant role as it is the starburst galaxies at all epochs, which have the more positive metallicity gradients. Our results may help to explain the origin of the fundamental metallicity relation, in which galaxies at a fixed mass are observed to have lower metallicities at higher star formation rates, especially if the metallicity is measured in an aperture encompassing only the central regions of the galaxy. Finally, we note that this study demonstrates the power of KMOS as an efficient instrument for large scale resolved galaxy surveys.
    Monthly Notices of the Royal Astronomical Society 07/2014; 443(3). DOI:10.1093/mnras/stu1343 · 5.23 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The N2 index ([N II]6584/Ha) is used to determine emission line galaxy metallicities at all redshifts, including high redshift, where galaxies tend to be metal-poor. The initial aim of the work was to improve the calibrations used to infer oxygen abundance from N2 employing updated low-metallicity galaxy databases. We compare N2 and the metallicity determined using the direct method for the set of extremely metal-poor galaxies compiled by Morales-Luis et al. (2011). To our surprise, the oxygen abundance presents a tendency to be constant with N2, with a very large scatter. Consequently, we find that the existing N2 calibrators overstimate the oxygen abundance for most low metallicity galaxies, and then they can be used only to set upper limits to the true metallicity in low-metallicity galaxies. An explicit expression for this limit is given. In addition, we try to explain the observed scatter using photoionization models. It is mostly due to the different evolutionary state of the HII regions producing the emission lines, but it also arises due to differences of N/O among the galaxies.
    The Astrophysical Journal 10/2014; 797(2). DOI:10.1088/0004-637X/797/2/81 · 6.28 Impact Factor