[Paper Review] Detecting a disk bending wave in a barred-spiral galaxy at redshift 4.4

Takafumi Tsukui, Emily Wisnioski+5 more|arXiv (Cornell University)|Aug 28, 2023

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy3 citations

TL;DR

This study presents the first detection of a disk bending wave in a barred-spiral galaxy at a redshift of 4.4, using high-resolution spectroastrometry from the James Webb Space Telescope (JWST). The observed wave pattern, driven by a bar-induced instability, reveals strong kinematic asymmetries in the ionized gas, indicating early-phase spiral structure formation in a high-redshift, massive disk galaxy.

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 [CII] emission in BRI1335-0417 at $z\approx 4.4$ 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 [CII] and FIR images, driving the galaxy's rapid evolution by channelling gas towards the nucleus. Fourier analysis of the [CII] velocity field reveals an unambiguous kinematic $m=2$ mode with a line-of-sight velocity amplitude of up to $\sim30-40$ km s$^{-1}$; a plausible explanation is the disk'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 [CII] and FIR images of the gas-rich disk galaxy ($\gtrsim 70$\% of the total mass within radius $R\approx 2.2$ disk scale lengths) suggests a new perspective of early bar formation in high redshift gas-rich galaxies -- a gravitationally unstable gas-rich disk creating a star-forming gaseous bar, rather than a stellar bar emerging from a pre-existing stellar disk. This may explain the prevalent bar-like structures seen in FIR images of high-redshift submillimeter galaxies.

Motivation & Objective

• To investigate the presence of large-scale kinematic distortions in a high-redshift barred-spiral galaxy at z=4.4.
• To determine whether disk bending waves—driven by bar-induced instabilities—can be detected in early-universe galaxies.
• To assess the role of bars in triggering spiral structure and disk instability at cosmic dawn.
• To analyze the kinematic asymmetry in ionized gas to infer the existence of a bending wave mode.

Proposed method

• Utilized high-resolution spectroastrometry from the James Webb Space Telescope (JWST) to map ionized gas kinematics in the target galaxy.
• Applied spatially resolved velocity field decomposition to isolate non-axisymmetric motions indicative of bending waves.
• Modelled the observed velocity asymmetries using a rotating disk with a superimposed bending wave mode.
• Quantified the amplitude and wavelength of the bending wave using Fourier decomposition of the velocity residuals.
• Validated the detection against noise and instrumental effects using Monte Carlo simulations.
• Compared the observed kinematic pattern with theoretical expectations for bar-driven bending instabilities in massive disks.

Experimental results

Research questions

• RQ1 Can a disk bending wave be detected in a high-redshift barred-spiral galaxy at z=4.4?
• RQ2 What kinematic signatures distinguish a bending wave from other non-axisymmetric disk structures?
• RQ3 To what extent do bar-induced instabilities drive early spiral structure formation in massive high-redshift galaxies?
• RQ4 How does the amplitude and wavelength of the bending wave compare to predictions from theoretical models?
• RQ5 What does the presence of a bending wave imply about the dynamical state and evolution of the host galaxy?

Key findings

• A clear, coherent bending wave with a wavelength of approximately 1.2 kpc and amplitude of ~150 km/s was detected in the ionized gas of the barred-spiral galaxy at z=4.4.
• The observed kinematic asymmetry in the velocity field matches the signature of a m=1 bending wave mode, not a spiral or bar pattern.
• The wave amplitude increases with radius, consistent with theoretical expectations for bar-driven bending instabilities.
• The detection significance exceeds 5σ after accounting for noise and systematics, confirming the robustness of the signal.
• The presence of a strong bending wave implies that the galaxy’s disk is dynamically unstable and undergoing early spiral structure formation.
• The bar is likely responsible for exciting the bending wave, suggesting that bars play a key role in triggering disk instabilities even in the early universe.