[Paper Review] ALMA as a Redshift Machine: Using [CII] to Efficiently Confirm Galaxies in the Epoch of Reionization

Sander Schouws, R. J. Bouwens+12 more|arXiv (Cornell University)|Feb 8, 2022

Astronomy and Astrophysical Research4 citations

TL;DR

This paper demonstrates that ALMA spectral scans targeting the [CII] 158μm line efficiently confirm high-redshift galaxies at z~7, detecting [CII] emission in 50% of six luminous Lyman Break Galaxies with photometric redshifts. The detected [CII] luminosities (5.6–8.8×10⁸ L☉) align with the local [CII]-SFR relation, and kinematic diversity reveals rotation, mergers, and clumpy star formation.

ABSTRACT

The [CII]$_{158μm}$ line has long been proposed as a promising line to spectroscopically confirm galaxies in the epoch of reionization. In this paper we present the results of new ALMA observations spectral scanning for [CII] in six particularly luminous Lyman Break Galaxies at $z\sim7$. The six sources were drawn from a sample of bright $z\sim7$ galaxies identified using the wide-area optical, near-IR, and Spitzer/IRAC data over the COSMOS/UltraVISTA field and were targeted on the basis of tight constraints on their redshifts from their IRAC [3.6]-[4.5] colors. We detect significant ($>9σ$) [CII] lines in three of our six targets ($50\%$) co-spatial with the rest-$UV$ emission from the ground/space-based near-IR imaging. The luminosities of the [CII] lines lie in the range $5.6$ to $8.8 imes10^{8}L_{\odot}$, consistent with the local [CII]-SFR relation. Meanwhile, their [CII]/$L_{IR}\sim1-3 imes10^{-3}$ ratios are slightly elevated compared to local (U)LIRGS. This could be due to lower dust-to-gas or dust-to-metal ratios. We also find that our sources display a large kinematic diversity, with one source showing signs of rotation, one source a likely major merger and one dispersion dominated source that might contain a bright star-forming clump. Our results highlight the effectiveness of spectral scans with ALMA in spectroscopically confirming luminous galaxies in the epoch of reionization, something that is being be applied on a significantly larger sample in the on-going REBELS large program.

Motivation & Objective

• To test the effectiveness of ALMA spectral scans in spectroscopically confirming high-redshift galaxies at z~7 using the [CII] 158μm line.
• To assess whether [CII] emission in luminous z~7 galaxies aligns with the local [CII]-SFR relation and dust-to-gas ratios.
• To investigate the kinematic structures of high-redshift galaxies through [CII] line morphology and velocity profiles.
• To evaluate the impact of selection bias in prior ALMA surveys, which favored Lyα-emitting galaxies with lower [CII] luminosities.
• To validate the use of IRAC [3.6]-[4.5] colors as a proxy for accurate photometric redshifts in selecting high-[CII] luminosity candidates.

Proposed method

• Targeted six luminous z~7 Lyman Break Galaxies selected from COSMOS/UltraVISTA and Spitzer/IRAC data with tight photometric redshifts from IRAC [3.6]-[4.5] colors.
• Conducted deep ALMA spectral scans centered on the [CII] 158μm line to detect redshifted emission at z~7.
• Used 9σ significance threshold to define secure [CII] detections, cross-validated with spatial co-location to rest-UV emission from HST/Spitzer imaging.
• Measured [CII] luminosities and compared them to the local [CII]-SFR relation to assess star formation efficiency.
• Calibrated [CII]/LIR ratios to evaluate dust-to-gas and dust-to-metal ratios relative to local (U)LIRGs.
• Analyzed [CII] line profiles to infer kinematic structures: rotation, mergers, and dispersion-dominated components.

Experimental results

Research questions

• RQ1 Can ALMA spectral scans successfully detect [CII] emission in luminous z~7 galaxies selected via IRAC [3.6]-[4.5] colors?
• RQ2 Do the [CII] luminosities of z~7 galaxies with high SFRs align with the local [CII]-SFR relation?
• RQ3 Are [CII]/LIR ratios in z~7 galaxies consistent with those in local (U)LIRGs, or do they indicate lower dust-to-gas or dust-to-metal ratios?
• RQ4 What kinematic structures (e.g., rotation, mergers, clumps) are revealed by [CII] line profiles in high-redshift galaxies?
• RQ5 Does targeting galaxies with strong IRAC colors rather than Lyα emission improve [CII] detection efficiency at z~7?

Key findings

• ALMA detected significant (>9σ) [CII] emission in three of six targeted z~7 galaxies, confirming a 50% detection rate in this sample.
• The [CII] luminosities ranged from 5.6×10⁸ to 8.8×10⁸ L☉, consistent with the local [CII]-SFR relation, indicating normal star formation efficiency.
• The [CII]/LIR ratios were 1–3×10⁻³, slightly elevated compared to local (U)LIRGs, suggesting lower dust-to-gas or dust-to-metal ratios.
• Kinematic analysis revealed diverse structures: one source showed signs of rotation, one exhibited a major merger signature, and one was dispersion-dominated, possibly hosting a bright star-forming clump.
• The success of [CII] detection in this sample supports the strategy of targeting luminous, photometrically well-constrained galaxies rather than Lyα emitters to maximize detection yield.
• The results validate the use of ALMA spectral scans as an efficient method for confirming high-redshift galaxies, now being applied in the larger REBELS survey.