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Rotation in C ii-emitting gas in two galaxies at a redshift of 6.8
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Rotation in C ii-emitting gas in two galaxies at a redshift of 6.8
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Rotation in C ii-emitting gas in two galaxies at a redshift of 6.8
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Journal Article
Rotation in C ii-emitting gas in two galaxies at a redshift of 6.8
2018
Overview
The emission of singly ionized carbon is used to identify two galaxies with redshifts of nearly 7—corresponding to the Universe’s first billion years—and with velocity structures suggestive of rotation.
Rotation in two high-redshift galaxies
The forbidden emission line of singly ionized carbon [C ɪɪ] at a wavelength of 157.7 micrometres is one of the main lines for cooling gas in nearby star-forming galaxies, and has been expected, although not yet proved, to be bright in the early Universe. Renske Smit and collaborators report spectroscopic confirmation of the redshifts of two infrared-selected galaxies at redshifts of 6.85 and 6.81, using the [C ɪɪ] line. The galaxies are luminous, with velocity gradients across their surfaces. If those gradients represent rotation, then the galaxies have dynamical properties like those of Hα-bright galaxies two billion years later in the history of the Universe.
The earliest galaxies are thought to have emerged during the first billion years of cosmic history, initiating the ionization of the neutral hydrogen that pervaded the Universe at this time. Studying this ‘epoch of reionization’ involves looking for the spectral signatures of ancient galaxies that are, owing to the expansion of the Universe, now very distant from Earth and therefore exhibit large redshifts. However, finding these spectral fingerprints is challenging. One spectral characteristic of ancient and distant galaxies is strong hydrogen-emission lines (known as Lyman-α lines), but the neutral intergalactic medium that was present early in the epoch of reionization scatters such Lyman-α photons. Another potential spectral identifier is the line at wavelength 157.4 micrometres of the singly ionized state of carbon (the [C
ii
]
λ
= 157.74 μm line), which signifies cooling gas and is expected to have been bright in the early Universe. However, so far Lyman-α-emitting galaxies from the epoch of reionization have demonstrated much fainter [C
ii
] luminosities than would be expected from local scaling relations
1
,
2
,
3
,
4
,
5
, and searches for the [C
ii
] line in sources without Lyman-α emission but with photometric redshifts greater than 6 (corresponding to the first billion years of the Universe) have been unsuccessful. Here we identify [C
ii
]
λ
= 157.74 μm emission from two sources that we selected as high-redshift candidates on the basis of near-infrared photometry; we confirm that these sources are two galaxies at redshifts of
z
= 6.8540 ± 0.0003 and
z
= 6.8076 ± 0.0002. Notably, the luminosity of the [C
ii
] line from these galaxies is higher than that found previously in star-forming galaxies with redshifts greater than 6.5. The luminous and extended [C
ii
] lines reveal clear velocity gradients that, if interpreted as rotation, would indicate that these galaxies have similar dynamic properties to the turbulent yet rotation-dominated disks that have been observed in Hα-emitting galaxies two billion years later, at ‘cosmic noon’.
