Metallicity evolution, metallicity gradients, and gas fractions atz~ 3.4

Abstract

We used near-infrared integral field spectroscopic observations from the AMAZE and LSD ESO programs to constrain the metallicity in a sample of 40 star-forming galaxies at 3 < z < 5 (most of which are at z ∼ 3.4). We measured metallicities by exploiting strong emission-line diagnostics. We found that a significant fraction of star-forming galaxies at z ∼ 3.4 deviate from the fundamental metallicity relation (FMR), with a metallicity of up to a factor of ten lower than expected according to the FMR. This deviation does not correlate with the dynamical properties of the galaxy or with the presence of interactions. To investigate the origin of the metallicity deviation in more detail, we also inferred information on the gas content by inverting the Schmidt-Kennicutt relation, assuming that the latter does not evolve out to z ∼ 3.4. In agreement with recent CO observational data, we found that in contrast with the steeply rising trend at 0 < z < 2, the gas fraction in massive galaxies remains constant, with an indication of a marginal decline at 2 < z < 3.5. When combined with the metallicity information, we infer that to explain the low metallicity and gas content in z ∼ 3.4 galaxies, both prominent outflows and massive pristine gas inflows are needed. In ten galaxies we can also spatially resolve the metallicity distribution. We found that the metallicity generally anticorrelates with the distribution of star formation and with the gas surface density. We discuss these findings in terms of pristine gas inflows toward the center, and outflows of metal-rich gas from the center toward the external regions.

On the universality of luminosity–metallicity and mass–metallicity relations for compact star-forming galaxies at redshifts 0 < z < 3

“…To verify this conclusion with our sample we show in Fig. 9 star-forming galaxies at z > 2 by Cullen et al (2014) (stars), star-forming galaxies at 2.0 < z < 2.6 by Steidel et al (2014) (filled circles), star-forming galaxies at redshifts z ∼ 3.5 by Maiolino et al (2008) (filled squares), star-forming galaxies with z ∼ 2 by Maier et al (2014) (crosses), and star-forming galaxies with z ∼ 3.4 by Troncoso et al (2014) (asterisks). The relation for galaxies with z ∼ 2.3 by Sanders et al (2015) is shown by thick yellow solid line.…”

“…The location of star-forming galaxies with redshifts 2.0 < z < 2.6 by Steidel et al (2014) and with redshifts z > 2 by Cullen et al (2014) is shown by grey rectangles. Star-forming galaxies with redshifts z ∼ 3.4 by Troncoso et al (2014) are indicated by dark asterisks. (2004) is shown by thin dashed lines and dot-dashed lines are 1σ dispersions.…”

“…Star-forming galaxies at z ∼ 3.5 by Maiolino et al (2008) and at z ∼ 2 by Maier et al (2014) are shown by dark-grey filled squares and crosses, respectively. Thirty star-forming galaxies with redshifts z ∼ 3.4 by Troncoso et al (2014) are indicated by asterisks. The relation for z ∼ 2.3 by Sanders et al (2015) is shown by thick yellow solid line.…”

“…They derived oxygen abundances by using strong-line O3N 2 calibration, similar to that applied for our sample. To derive metallicities Cullen et al (2014) and Troncoso et al (2014) used empirical calibrations by Maiolino et al (2008), which also were calibrated with the use of galaxy samples, where oxygen abundances were derived by the Temethod. However, for consistency reasons we selected only high-z galaxies with [O iii] λ4959/Hβ 1 and recalculated their oxygen abundances using Eqs.…”

Star formation sustained by gas accretion

“…The parameterization of the FMR by Mannucci et al (2010), seems to hold until z < 2, and then it changes substantially at higher redshift (e.g., Mannucci et al 2010;Yabe et al 2012;Ly et al 2014;Troncoso et al 2014;Cullen et al 2014). Variations are expected if the relationship is caused by cosmic gas accretion since accretion rates and IGM metallicities change over time.…”