A tentative $\sim$1000 km s${}^{-1}$ offset between the [CII] 158 $\mu$m and Ly$\alpha$ 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 [CII] line at $\approx 4\sigma$ significance. The integrated [CII] line emission is spatially offset about $\sim 4$ kpc from the rest-frame ultraviolet (UV) emission. The total [CII] luminosity ($L_{[\mathrm{CII}]}=2.7\times 10^8{L}_{\odot }$) is consistent with the relation between [CII] luminosity and star formation rate (SFR) observed in nearby and high-$z$ star forming galaxies. More interestingly, the [CII] line is blueshifted with respect to the Ly$\alpha$ line by $980\pm 10$ km s${}^{-1}$. If confirmed, this corresponds to the largest velocity offset reported to date between the Ly$\alpha$ line and a non-resonant line at $z\gtrsim 6$. According to trends observed in other high redshift galaxies, the large Ly$\alpha$ velocity offset in GN-108036 is consistent with its low Ly$\alpha$ equivalent width and high UV absolute magnitude. Based on Ly$\alpha$ radiative transfer models of expanding shells, the large Ly$\alpha$ 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 $v_{\mathrm{out}}\sim \Delta v_{\mathrm{Ly}\alpha }/2\sim 500$ km s${}^{-1}$. We also report the 3$\sigma$ detection of a potential galaxy companion located $\sim 30$ kpc east of GN-108036, at a similar systemic velocity, and with no counterpart rest-frame UV emission.

Accepted for publication in Astronomy & Astrophysics Letters, 8 pages, 6 figures

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