A tentative ∼1000 km s−1 offset between the [CII] 158 m and Lyα line emission in a star-forming galaxy at z = 7.2

Abstract

GN-108036 is a star-forming galaxy at z = 7.21, and one of the most distant known sources in the Northern hemisphere. Based on observations from the NOrthern Extended Millimeter Array (NOEMA), here we report the tentative detection of the [C II] line at ≈4σ significance. The integrated [C II] line emission is spatially offset about ∼4 kpc from the rest-frame ultraviolet (UV) emission. The total [C II] luminosity (L[CII] = 2.7 × 108 L⊙) is consistent with the relation between [C II] luminosity and star formation rate (SFR) observed in nearby and high-z star forming galaxies. More interestingly, the [C II] line is blueshifted with respect to the Lyα line by 980 ± 10 km s−1. If confirmed, this corresponds to the largest velocity offset reported to date between the Lyα line and a non-resonant line at z ≳ 6. According to trends observed in other high redshift galaxies, the large Lyα velocity offset in GN-108036 is consistent with its low Lyα equivalent width and high UV absolute magnitude. Based on Lyα radiative transfer models of expanding shells, the large Lyα velocity offset in GN-108036 could be interpreted as the presence of a large column density of hydrogen gas, and/or an outflow with a velocity of vout ∼ ΔvLyα/2 ∼ 500 km s−1. We also report the 3σ detection of a potential galaxy companion located ∼30 kpc east of GN-108036, at a similar systemic velocity, and with no counterpart rest-frame UV emission.