Mean Hα+[N II]+[S II] EW inferred for star-forming galaxies at z ∼ 5.1–5.4 using high-quality Spitzer/IRAC photometry

Abstract

Recent Spitzer/InfraRed Array Camera (IRAC) photometric observations have revealed that rest-frame optical emission lines contribute significantly to the broad-band fluxes of high-redshift galaxies. Specifically, in the narrow redshift range z ∼ 5.1–5.4 the [3.6]–[4.5] colour is expected to be very red, due to contamination of the 4.5 μm band by the dominant Hα line, while the 3.6 μm filter is free of nebular emission lines. We take advantage of new reductions of deep Spitzer/IRAC imaging over the Great Observatories Origins Deep Survey-North+South fields (Labbé et al. 2015) to obtain a clean measurement of the mean Hα equivalent width (EW) from the [3.6]–[4.5] colour in the redshift range z = 5.1–5.4. The selected sources either have measured spectroscopic redshifts (13 sources) or lie very confidently in the redshift range z = 5.1–5.4 based on the photometric redshift likelihood intervals (11 sources). Our z_(phot) = 5.1–5.4 sample and z_(spec) = 5.10–5.40 spectroscopic sample have a mean [3.6]–[4.5] colour of 0.31 ± 0.05 and 0.35 ± 0.07 mag, implying a rest-frame EW (Hα+[N II]+[S II]) of 665 ± 53 and 707 ± 74 Å, respectively, for sources in these samples. These values are consistent albeit slightly higher than derived by Stark et al. at z ∼ 4, suggesting an evolution to higher values of the Hα+[N ii]+[S II] EW at z > 2. Using the 3.6 μm band, which is free of emission line contamination, we perform robust spectral energy distribution fitting and find a median specific star formation rate of sSFR = 17^(+2)_(−5) Gyr−1, 7^(+1)_(−2)× higher than at z ∼ 2. We find no strong correlation (<2σ) between the Hα+[N II]+[S II] EW and the stellar mass of sources. Before the advent of JWST, improvements in these results will come through an expansion of current spectroscopic samples and deeper Spitzer/IRAC measurements.

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