The REBELS ALMA Survey: efficient Ly α transmission of UV-bright z ≃ 7 galaxies from large velocity offsets and broad line widths

Endsley, R ORCID: 0000-0003-4564-2771, Stark, DP, Bouwens, RJ, Schouws, S, Smit, R ORCID: 0000-0001-8034-7802, Stefanon, M ORCID: 0000-0001-7768-5309, Inami, H ORCID: 0000-0003-4268-0393, Bowler, RAA ORCID: 0000-0003-3917-1678, Oesch, P, Gonzalez, V, Aravena, M, Da Cunha, E, Dayal, P ORCID: 0000-0002-3766-2265, Ferrara, A, Graziani, L ORCID: 0000-0002-9231-1505, Nanayakkara, T ORCID: 0000-0003-2804-0648, Pallottini, A ORCID: 0000-0002-7129-5761, Schneider, R ORCID: 0000-0001-9317-2888, Sommovigo, L ORCID: 0000-0002-2906-2200, Topping, M et al (2022) The REBELS ALMA Survey: efficient Ly α transmission of UV-bright z ≃ 7 galaxies from large velocity offsets and broad line widths. Monthly Notices of the Royal Astronomical Society, 517 (4). pp. 5642-5659. ISSN 0035-8711

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Abstract

Recent work has shown that UV-luminous reionization-era galaxies often exhibit strong Lyman-alpha emission despite being situated at redshifts where the IGM is thought to be substantially neutral. It has been argued that this enhanced Ly α transmission reflects the presence of massive galaxies in overdense regions which power large ionized bubbles. An alternative explanation is that massive galaxies shift more of their Ly α profile to large velocities (relative to the systemic redshift) where the IGM damping wing absorption is reduced. Such a mass-dependent trend is seen at lower redshifts, but whether one exists at z ∼7 remains unclear owing to the small number of existing systemic redshift measurements in the reionization era. This is now changing with the emergence of [C ii]-based redshifts from ALMA. Here, we report MMT/Binospec Ly α spectroscopy of eight UV-bright (MUV ∼-22) galaxies at z ≃ 7 selected from the ALMA REBELS survey. We detect Ly α in four of eight galaxies and use the [C ii] systemic redshifts to investigate the Ly α velocity profiles. The Ly α lines are significantly redshifted from systemic (average velocity offset = 223 km s-1) and broad (FWHM ≈ 300-650 km s-1), with two sources showing emission extending to ≈750 km s-1. We find that the broadest Ly α profiles are associated with the largest [C ii] line widths, suggesting a potential link between the Ly α FWHM and the dynamical mass. Since Ly α photons at high velocities transmit efficiently through the z = 7 IGM, our data suggest that velocity profiles play a significant role in boosting the Ly α visibility of the most UV-luminous reionization-era galaxies.

Item Type:	Article
Additional Information:	This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2022 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Uncontrolled Keywords:	0201 Astronomical and Space Sciences; Astronomy & Astrophysics
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	Oxford University Press (OUP)
Date of acceptance:	20 October 2022
Date of first compliant Open Access:	9 January 2023
Date Deposited:	09 Jan 2023 12:44
Last Modified:	05 Jul 2025 11:15
DOI or ID number:	10.1093/mnras/stac3064
URI:	https://researchonline.ljmu.ac.uk/id/eprint/18565

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