CO and Fine-structure Lines Reveal Low Metallicity in a Stellar-mass-rich Galaxy at z ∼ 1?

Abstract

Abstract We present detections of the CO(4–3) and [C i] 609 μm spectral lines, as well as the dust continuum at 480.5 GHz (rest frame), in 3C 368, a Fanaroff–Riley class II (FR-II) galaxy at redshift (z) 1.131. 3C 368 has a large stellar mass, ∼ 3.6 × 1011 M ⊙, and is undergoing an episode of vigorous star formation, at a rate of ∼ 350 M ⊙ yr−1, and active galactic nucleus activity, with radio-emitting lobes extended over ∼ 73 kpc. Our observations allow us to inventory the molecular-gas reservoirs in 3C 368 by applying three independent methods: (1) using the CO(4–3)-line luminosity, excitation state of the gas, and an α CO conversion factor, (2) scaling from the [C i]-line luminosity, and (3) adopting a gas-to-dust conversion factor. We also present gas-phase metallicity estimates in this source, both using far-infrared fine-structure lines together with radio free–free continuum emission and independently employing the optical [O iii] 5007 Å and [O ii] 3727 Å lines (R 23 method). Both methods agree on a subsolar gas-phase metallicity of ∼ 0.3 Z ⊙. Intriguingly, comparing the molecular-gas mass estimated using this subsolar metallicity, M gas ∼ 6.4 × 1010 M ⊙, to dust-mass estimates from multicomponent spectral energy distribution modeling, M dust ∼ 1.4 × 108 M ⊙, yields a gas-to-dust ratio within ∼ 15% of the accepted value for a metallicity of 0.3 Z ⊙. The derived gas mass puts 3C 368 on a par with other galaxies at z ∼ 1 in terms of specific star formation rate and gas fraction. However, it does not explain how a galaxy can amass such a large stellar population while maintaining such a low gas-phase metallicity. Perhaps 3C 368 has recently undergone a merger, accreting pristine molecular gas from an external source.