Detecting a disc bending wave in a barred-spiral galaxy at redshift 4.4

Abstract

ABSTRACT The recent discovery of barred spiral galaxies in the early Universe (z > 2) poses questions of how these structures form and how they influence galaxy evolution in the early Universe. In this study, we investigate the morphology and kinematics of the far-infrared (FIR) continuum and [C ii] emission in BRI1335-0417 at z ≈ 4.4 from ALMA observations. The variations in position angle and ellipticity of the isophotes show the characteristic signature of a barred galaxy. The bar, $3.3^{+0.2}_{-0.2}$ kpc long in radius and bridging the previously identified two-armed spiral, is evident in both [C ii] and FIR images, driving the galaxy’s rapid evolution by channelling gas towards the nucleus. Fourier analysis of the [C ii] velocity field reveals an unambiguous kinematic m = 2 mode with a line-of-sight velocity amplitude of up to ∼30–40 km s−1; a plausible explanation is the disc’s vertical bending mode triggered by external perturbation, which presumably induced the high star formation rate and the bar/spiral structure. The bar identified in [C ii] and FIR images of the gas-rich disc galaxy (≳ 70 per cent of the total mass within radius R ≈ 2.2 disc scale lengths) suggests a new perspective of early bar formation in high redshift gas-rich galaxies – a gravitationally unstable gas-rich disc creating a star-forming gaseous bar, rather than a stellar bar emerging from a pre-existing stellar disc. This may explain the prevalent bar-like structures seen in FIR images of high-redshift submillimeter galaxies.