[Paper Review] The ALPINE-ALMA [CII] survey: Small Lya-[CII] velocity offsets in main-sequence galaxies at 4.4 < z < 6

P. Cassata, L. Morselli+31 more|arXiv (Cornell University)|Feb 3, 2020

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy115 references15 citations

TL;DR

This study analyzes 53 main-sequence galaxies at 4.4 < z < 6 using ALMA [CII] and optical spectroscopy to measure Lyα-[CII] velocity offsets. It finds that 90% of galaxies have small offsets (0 < Δv < 400 km s⁻¹), which correlate strongly with high Lyα equivalent width and escape fraction, indicating that low neutral hydrogen column density and low covering fraction—rather than fast outflows—drive enhanced Lyα escape at high redshift.

ABSTRACT

The Lya line in the UV and the [CII] line in the FIR are widely used tools to identify galaxies and to obtain insights into ISM properties in the early Universe. By combining data obtained with ALMA in band 7 at ~ 320 GHz as part of the ALMA Large Program to INvestigate [CII] at Early Times (ALPINE) with spectroscopic data from DEIMOS at Keck, VIMOS and FORS2 at the VLT, we assembled a unique sample of 53 main-sequence star-forming galaxies at 4.4 < z < 6 in which we detect both the Lya line and the [CII]. We used [CII], observed with ALMA, as a tracer of the systemic velocity of the galaxies, and we find that 90% of the selected objects have Lya-[CII] velocity offsets in the range 0 < Dv_Lya-[CII] < 400 km/s, in line with the few measurements available so far in the early Universe, and significantly smaller than those observed at lower z. We observe ISM-[CII] offsets in the range -500 < Dv_ISM-[CII] < 0 km/s, in line with values at all redshifts. We find significant anticorrelations between Dv_Lya-[CII] and the Lya rest-frame equivalent width EW0(Lya) (or equivalently, the Lya escape fraction f_esc(Lya)). According to available models for the radiative transfer of Lya photons, the escape of Lya photons would be favored in galaxies with high outflow velocities, in agreement with our observations. The uniform shell model would also predict that the Lya escape in galaxies with slow outflows (0 < v_out < 300 km/s) is mainly determined by the neutral hydrogen column density (NHI), while the alternative model by Steidel+10 would favor a combination of NHI and covering fraction as driver of the Lya escape. We suggest that the observed increase in Lya escape that is observed between z~2 and z~6 is not due to a higher incidence of fast outflows at high redshift, but rather to a decrease in average NHI along the line of sight, or alternatively, a decrease in HI covering fraction. [abridged]

Motivation & Objective

• To constrain the kinematic relationship between Lyα emission and systemic velocity in high-redshift main-sequence galaxies.
• To investigate the physical drivers behind the observed increase in Lyα escape fraction from z ~2 to z ~6.
• To determine whether fast outflows or reduced neutral hydrogen column density governs enhanced Lyα emission at high redshift.
• To use [CII] as a robust systemic velocity tracer to measure accurate Lyα and ISM velocity offsets in 53 galaxies at 4.4 < z < 6.
• To test radiative transfer models of Lyα escape by comparing observed velocity offsets with predictions from outflow and clumpy ISM scenarios.

Proposed method

• Used ALMA Band 7 observations to measure [CII] 158 μm emission as a systemic velocity tracer in 53 galaxies at 4.4 < z < 6.
• Combined with optical spectroscopy from Keck/DEIMOS, VLT/VIMOS, and FORS2 to measure Lyα and ISM line velocities.
• Calculated Lyα-[CII] and ISM-[CII] velocity offsets by comparing line redshifts relative to the [CII] peak velocity.
• Classified morpho-kinematic structures (rotators, mergers, dispersion-dominated) to assess spatial and kinematic coherence.
• Re-extracted [CII] spectra from spatially matched regions to confirm that velocity peaks were not biased by multiple components.
• Applied radiative transfer models (uniform shell and Steidel et al. 2010) to interpret the observed offset-escape fraction correlations.

Experimental results

Research questions

• RQ1 What is the distribution of Lyα-[CII] velocity offsets in main-sequence galaxies at 4.4 < z < 6, and how does it compare to lower redshift systems?
• RQ2 How do Lyα velocity offsets correlate with Lyα rest-frame equivalent width and escape fraction in high-redshift galaxies?
• RQ3 What physical ISM properties—such as outflow velocity, neutral hydrogen column density (NHI), or covering fraction—best explain the observed Lyα escape trends?
• RQ4 Is the increase in Lyα escape fraction from z ~2 to z ~6 driven by higher incidence of fast outflows or by changes in ISM geometry and density?
• RQ5 To what extent do [CII] line profiles and spatial morphology affect the reliability of [CII] as a systemic velocity tracer in high-redshift galaxies?

Key findings

• 90% of the 53 galaxies exhibit Lyα-[CII] velocity offsets in the range 0 < ΔvLyα−[CII] < 400 km s⁻¹, significantly smaller than observed at lower redshifts.
• A strong anticorrelation is found between ΔvLyα−[CII] and both Lyα rest-frame equivalent width (EW0(Lyα)) and escape fraction fesc(Lyα), such that smaller offsets correlate with higher Lyα emission and escape.
• ISM-[CII] velocity offsets span −500 < ΔvISM−[CII] < 0 km s⁻¹, consistent with outflows in these galaxies, indicating net redshifted ISM kinematics relative to [CII].
• The observed correlation between small Lyα-[CII] offsets and high Lyα escape is best explained by low neutral hydrogen column density (NHI) and low covering fraction along the line of sight, not by fast outflows.
• The increase in Lyα escape fraction from z ~2 to z ~6 is not driven by a higher incidence of fast outflows at high redshift, but rather by reduced NHI or covering fraction.
• The data favor the Steidel et al. (2010) model, which emphasizes the role of NHI at systemic velocity and covering fraction in regulating Lyα escape, over models dominated by outflow velocity alone.