Fitting spectral energy distributions of FMOS-COSMOS emission-line galaxies at z ∼ 1.6: Star formation rates, dust attenuation, and [OIII]λ5007 emission-line luminosities

Abstract

We perform a spectral energy distribution fitting analysis on a COSMOS photometric sample covering the ultra-violet up to the far-infrared wavelengths and including emission lines from the Fiber Multi-Object Spectrograph survey. The sample consists of 182 objects with Hα and [OIII]λ5007 emission line measurements lying in a redshift range of 1.40 < z < 1.68. We obtain robust estimates of the stellar mass and star-formation rate spanning over a range of 109.5 − 1011.5 M⊙ and 101 − 103 M⊙ yr−1 from the Bayesian analysis performed with CIGALE and using continuum photometry and Hα fluxes. Combining photometry and spectroscopy gives secure estimations of the amount of dust attenuation for both continuum and line emissions. We obtain a median attenuation of AHα = 1.16 ± 0.19 mag and A[OIII] = 1.41 ± 0.22 mag. Hα and [OIII]λ5007 attenuations are found to increase with stellar mass, confirming previous findings with Hα. A difference of 57% in the attenuation experienced by emission lines and continuum is found to be in agreement with the emission lines being more attenuated than the continuum emission. Implementation of new CLOUDY HII-region models in CIGALE enables good fits of the Hα, Hβ, [OIII]λ5007 emission lines with discrepancies smaller than 0.2 dex in the predicted fluxes. Fitting the [NII]λ6584 line is found challenging due to well-known discrepancies in the locus of galaxies in the [NII]-BPT diagram at intermediate and high redshifts. We find a positive correlation between SFR and L[OIII]λ5007 after correcting for dust attenuation and we derive the linear relation log10(SFR/M⊙ yr−1)=log10(L[OIII]/ergs s−1)−(41.20 ± 0.02). Leaving the slope as a free parameter leads to log10(SFR/M⊙ yr−1)=(0.83 ± 0.06)log10(L[OIII]/ergs s−1)−(34.01 ± 2.63). The spread in the relation is driven by differences in the gas-phase metallicity and ionization parameter accounting for a 0.24 dex and 1.1 dex of the dispersion, respectively. We report an average value of log U ≈ −2.85 for this sample of galaxies. Including HII-region models to fit simultaneously photometric data and emission line fluxes is paramount to analyses of upcoming data sets from large spectroscopic surveys of the future, such as MOONS and PFS.