Abstract
We present results on the physical conditions of the interstellar medium of
star-forming galaxies at 1.4<~z<~1.7 from the FMOS-COSMOS survey. We use a
sample of 701 galaxies, for which the H$\alpha$ emission line is detected, that
represents the star-forming population over the stellar mass range
10^9.6<~M*/Msun<~10^11.6 with those at M*>10^11Msun being well sampled. The
excitation state and chemical enrichment of the ionized gas are investigated
using diagnostic diagrams based on the ratios of emission line strengths,
including Alpha, NII6584, SII6717,6731, Hbeta, and OIII5007. Our data
confirm an offset of the star-forming sequence on the BPT diagram (OIII/Hbeta
vs. NII/Halpha), primarily towards higher OIII/Hbeta, compared with local
star-forming galaxies. Based on the SII ratio, we measure an electron density
(n_e=222^+172_-128 cm^-3), higher than that of local galaxies. Overall,
these changes in emission-line properties are due to a higher ionization
parameter in high redshift galaxies as demonstrated by a lower than expected
SII/Halpha ratio and a comparison to theoretical models. These results likely
rule out an offset in the BPT diagram caused by a harder radiation field or AGN
as assessed with Chandra. Finally, we revisit the mass-metallicity relation by
using a conventional NII/Halpha-based indicator and a new calibration based
on the NII/SII ratio introduced by Dopita et al. Consistent with our
previous results, the most massive galaxies (M*>~10^11Msun) are fully enriched,
while lower mass galaxies have metallicities considerably lower than local
galaxies, in agreement with many studies. In addition, we show that the FMOS
sample is inconsistent with a "fundamental metallicity relation", though
well-fit with a physically-motivated model of galaxy evolution with gas
regulation.
Description
[1604.06802] The FMOS-COSMOS survey of star-forming galaxies at z~1.6. IV: Excitation state and chemical enrichment of HII regions
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