Abstract
Abstract The far-infrared fine-structure line [C ii] at 1900.5 GHz is known to be one of the brightest cooling lines in local galaxies, and therefore it has been suggested to be an efficient tracer for star formation in very high redshift galaxies. However, recent results for galaxies at z > 6 have yielded numerous non-detections in star-forming galaxies, except for quasars and submillimetre galaxies. We report the results of ALMA observations of two lensed, star-forming galaxies at z = 6.029 and z = 6.703. The galaxy A383-5.1 (star formation rate [SFR] of 3.2 M⊙ yr−1 and magnification of μ = 11.4 ± 1.9) shows a line detection with $L_{\rm [C\,\small {II}]} = 8.9\times 10^{6}$ L⊙, making it the lowest $L_{\rm [C\,\small {II}]}$ detection at z > 6. For MS0451-H (SFR = 0.4 M⊙ yr−1 and μ = 100 ± 20) we provide an upper limit of $L_{\rm [C\,\small {II}]} < 3\times 10^{5}$ L⊙, which is 1 dex below the local SFR–$L_{\rm [C\,\small {II}]}$ relations. The results are consistent with predictions for low-metallicity galaxies at z > 6; however, other effects could also play a role in terms of decreasing L[CII]. The detection of A383-5.1 is encouraging and suggests that detections are possible, but much fainter than initially predicted.
Highlights
During the past decade, the number of galaxies with measured redshifts z > 6 has increased significantly (e.g. Hu et al 2002; Iye et al 2006) with even a few spectroscopic redshifts of z > 7 (Vanzella et al 2011; Ono et al 2012; Schenker et al 2012; Shibuya et al 2012; Finkelstein et al 2013; Oesch et al 2015; Watson et al 2015; Zitrin et al 2015), demonstrating the tremendous potential for progress of our understanding of galaxy formation during the first billion years after the big bang
The resulting fit parameters are: central frequency ν = 270.4448 ± 0.0061 GHz, peak flux of Speak = 0.96 ± 0.19 mJy, and full width at halfmaximum linewidth of V = 100 ± 23 km s−1. This corresponds to an integrated line intensity I[C II] = 0.102 ± 0.032 Jy km s−1
We derive a redshift of z[C II] = 6.0274 ± 0.0002, which is in agreement with the optical/UV redshift of 6.029 ± 0.002 (Richard et al 2011) and the C III] redshift of 6.0265 ± 0.00013 (Stark et al 2015)
Summary
The number of galaxies with measured redshifts z > 6 has increased significantly (e.g. Hu et al 2002; Iye et al 2006) with even a few spectroscopic redshifts of z > 7 (Vanzella et al 2011; Ono et al 2012; Schenker et al 2012; Shibuya et al 2012; Finkelstein et al 2013; Oesch et al 2015; Watson et al 2015; Zitrin et al 2015), demonstrating the tremendous potential for progress of our understanding of galaxy formation during the first billion years after the big bang. (2015) and Willott et al (2015b) detected [C II] in bright Lymanbreak galaxies (LBG) at 5 < z < 6 and z ∼ 6.1, respectively, with the [C II] line luminosities and SFR following similar relations as those found for local star-forming galaxies. Maiolino et al (2015) targeted three LBGs within the redshift range z = 6.8–7.1, but did not detect the [C II] line despite the galaxies having estimated SFRs ∼5–15 M yr−1. They find a detection that is offset from one of the targets possibly explained by feedback and/or gas accretion. The rms in a 15.6 MHz channel near the redshifted line is 0.125 mJy beam−1 and 0.163 mJy beam−1 for A383-5.1 and MS0451-H, respectively
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