The ALPINE–ALMA [C II] Survey: The Infrared–Radio Correlation and Active Galactic Nucleus Fraction of Star-forming Galaxies at z ∼ 4.4–5.9

Abstract

We present the radio properties of 66 spectroscopically confirmed normal star-forming galaxies (SFGs) at 4.4 < z < 5.9 in the COSMOS field that were [C II]-detected in the Atacama Large Millimeter/submillimeter Array Large Program to INvestigate [C II] at Early times (ALPINE). We separate these galaxies ("C II-detected-all") into lower-redshift ("C II-detected-lz"; 〈z〉 = 4.5) and higher-redshift ("C II-detected-hz"; 〈z〉 = 5.6) subsamples, and stack multiwavelength imaging for each subsample from X-ray to radio bands. A radio signal is detected in the stacked 3 GHz images of the C II-detected-all and lz samples at ≳3σ. We find that the infrared–radio correlation of our sample, quantified by q_TIR, is lower than the local relation for normal SFGs at a ∼3σ significance level, and is instead broadly consistent with that of bright submillimeter galaxies at 2 < z < 5. Neither of these samples show evidence of dominant active galactic nucleus activity in their stacked spectral energy distributions (SEDs), UV spectra, or stacked X-ray images. Although we cannot rule out the possible effects of the assumed spectral index and applied infrared SED templates in causing these differences, at least partially, the lower obscured fraction of star formation than at lower redshift can alleviate the tension between our stacked q_(TIR)s and those of local normal SFGs. It is possible that the dust buildup, which primarily governs the infrared emission, in addition to older stellar populations, has not had enough time to occur fully in these galaxies, whereas the radio emission can respond on a more rapid timescale. Therefore, we might expect a lower q_TIR to be a general property of high-redshift SFGs.

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