Article

# IMAGES IV: strong evolution of the oxygen abundance in gaseous phases of intermediate mass galaxies from z ~ 0.8

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(Impact Factor: 4.48). 12/2008; 492(2). DOI: 10.1051/0004-6361:200810435
Source: arXiv

ABSTRACT Context. Intermediate mass galaxies (> 10$^{10}$ $M_\odot$) at $z$ ~ 0.6 are the likeliest progenitors of the present-day, numerous population of spirals. There is growing evidence that they have evolved rapidly in the last 6 to 8 Gyr, and likely already have formed a significant fraction of their stellar mass, often showing perturbed morphologies and kinematics.Aims. We have gathered a representative sample of 88 such galaxies and have provided robust estimates of their gas phase metallicity.Methods. We used moderate spectral resolution spectroscopy at VLT/FORS2 with an unprecedentedly high $S/N$ allowing us to remove biases coming from interstellar absorption lines and extinction, to establish robust values of $R_{23}$ = ([OII]$\lambda$3727 + [OIII]$\lambda\lambda$4959, 5007)/H$\beta$.Results. We definitively confirm that the predominant population of $z$ ~ 0.6 starbursts and luminous IR galaxies (LIRGs) are on average two times less metal rich than the local galaxies at a given stellar mass. We do find that the metal abundance of the gaseous phase of galaxies evolves linearly with time, from $z = 1$ to $z = 0$ and after comparing with other studies, from $z = 3$ to $z = 0$. Combining our results with the reported evolution of the Tully Fisher relation, we find that such an evolution requires that ~30% of the stellar mass of local galaxies have been formed through an external supply of gas, thus excluding the closed box model. Distant starbursts & LIRGs have properties (metal abundance, star formation efficiency & morphologies) similar to those of local LIRGs. Their underlying physics is likely dominated by gas infall, probably through merging or interactions.Conclusions. Our study further supports the rapid evolution of $z$ ~ 0.4–1 galaxies. Gas exchange between galaxies is likely the main cause of this evolution.

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##### Article: The mass-metallicity-star formation rate relation at z ≳ 2 with 3D Hubble Space Telescope
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ABSTRACT: We present new accurate measurements of the mass, metallicity and star-formation rate of a statistically significant sample of 93 galaxies at z ≳ 2 using near-infrared spectroscopy taken as part of the 3D-Hubble Space Telescope survey. We derive a mass-metallicity relation (MZR) for our sample with metallicities based on the oxygen and Hβ nebular emission lines. We find the MZR derived from our data to have the same trend as previous determinations in the range 0 < z < 3 with metallicity decreasing with stellar mass. However, we find that our MZR is offset from a previous determination at z ≳ 2 which used metallicities derived from the [N II]/Hα ratio. Incorporating star formation rate information, we find that our galaxies are also offset from the fundamental metallicity relation (FMR) by ̃0.3 dex. Using the Baldwin-Phillips-Terlevich (BPT) diagram we argue that, if the physical conditions of star-forming regions evolve with redshift, metallicity indicators based on [N II] and Hα, calibrated in the local Universe, may not be consistent with the ones based on oxygen lines and Hβ. Our results thus suggest that the evolution of the FMR previously reported at z ̃ 2-3 may be an artefact of the differential evolution in metallicity indicators and caution against using locally calibrated empirical metallicity relations at high redshift, which do not account for evolution in the physical conditions of star-forming regions.
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##### Article: The Relationship between Stellar Mass, Gas Metallicity, and Star Formation Rate for Halpha-selected Galaxies at z~0.8 from the NewHalpha Survey
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ABSTRACT: Using a sample of 299 Ha-selected galaxies at z~0.8, we study the relationship between galaxy stellar mass, gas-phase metallicity, and star formation rate (SFR), and compare to previous results. We use deep optical spectra obtained with the IMACS spectrograph at the Magellan telescope to measure strong oxygen lines. We combine these spectra and metallicities with (1) rest-frame UV-to-optical imaging, which allows us to determine stellar masses and dust attenuation corrections, and (2) Ha narrowband imaging, which provides a robust measure of the instantaneous SFR. Our sample spans stellar masses of 10^9 to 6*10^11 solar masses, SFRs of 0.4 to 270 solar masses per year, and metal abundances of 12+log(O/H)~8.3-9.1 (~0.4-2.6 solar metallicity). The correlations that we find between the Ha-based SFR and stellar mass (i.e., the star-forming "main sequence"), and between the stellar mass and metallicity, are both consistent with previous z~1 studies of star-forming galaxies. We then study the relationship between the three properties using various plane-fitting techniques (Lara-Lopez et al.) and a curve-fitting projection (Mannucci et al.). In all cases, we exclude strong dependence of the M-Z relation on SFR, but are unable to distinguish between moderate and no dependence. Our results are consistent with previous mass-metallicity-SFR studies. We check whether dataset limitations may obscure a strong dependence on the SFR by using mock samples drawn from the SDSS. These experiments reveal that the adopted signal-to-noise cuts may have a significant effect on the measured dependence. Further work is needed to investigate these results, and to test whether a "fundamental metallicity relation" or a "fundamental plane" describes star-forming galaxies across cosmic time.