Unveiling dust and gas in Lyman–α emitters across cosmic time

Rana, Rahul and Wardlow, Julie (2025) Unveiling dust and gas in Lyman–α emitters across cosmic time. PhD thesis, Lancaster University.

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Abstract

To understand the complete picture of galaxy evolution, it is important to investigate the total star formation, interstellar medium (ISM) conditions, and gas reservoirs of early galaxies. Since dust can absorb and scatter UV and optical light, it can heavily obscure regions of active star formation. This makes UV/optical measurements incomplete and they require corrections or alternative observations to reveal hidden star-forming activity. Furthermore, early galaxies are expected to be rich in gas, but estimating the amount of molecular hydrogen in these distant galaxies remains challenging due to the difficulty in detecting faint hydrogen tracers at high redshift. As a result, understanding the full picture of star formation and gas content in the early Universe requires alternative observations. In this thesis, we use far-infrared data to investigate the dust and gas content of early, star-forming galaxies selected from their Lyα emission, known as Lyα emitters (LAEs). Analysing the dust and gas emission enables the measurements of the obscured star formation rates, molecular gas content and ISM conditions in these young galaxies. In this thesis, we start by examining single-dish JCMT/SCUBA-2, Herschel/SPIRE and Herschel/PACS observations to investigate the far-infrared continuum emission from∼4000 LAEs at z = 2.2 – 6 from SC4K and measure their Lyα escape fraction (fesc(Lyα)). Five of the∼4000 LAEs contain enough dust to be individually detected at 850 µm, with fluxes of S850µm = 3.69–5.50 mJy; all of these systems host AGN, suggesting AGN heating might be enhancing dust visibility. To probe the average dust emission from individually-undetected LAEs we use stacking to show that the average LAE has S100, S160, S250, S350, S500 and S850 below the 0.24, 0.45, 0.27, 0.36, 0.36, and 0.09 mJy level (3σ), respectively. We also break the sample into bins of redshift, stellar mass, Lyα luminosity and AGN status. Most subsets are undetected, but stacks of the 298 LAEs with AGN, and the 669 most massive LAEs (M∗= 1010−12 M⊙) are detected at most wavelengths, including with S850 = 0.43 ±0.11 mJy and S850 = 0.29 ±0.07 mJy, respectively. Even when AGN are excluded the stack of most massive LAEs are still detected at the 4.5σ level at 250, 350 and 500 µm, indicating that the presence of dust in LAEs can be driven by stellar mass and not only AGN. The five individually-detected LAEs are found to have fesc(Lyα) = 1–7%, while detected stacks have fesc(Lyα) at the 4–5% level, and undetected stacks having higher limits (typically fesc(Lyα) ≳ 10%). The stack of all SC4K LAEs gives fesc(Lyα) >21% on average. We next utilize archival ALMA observations to probe deeper and detect faint SC4K LAEs that were missed by the single-dish observations. 375 SC4K LAEs were observed by ALMA and continuum emission is detected (≥ 4.5σ) in 15 of them, including 12 AGN. These LAEs have a wide range of fesc(Lyα), from 2% to 54%. Thus, deeper ALMA data are able to detect LAEs with higher fesc(Lyα) – i.e., LAEs with lower dust content compared to single-dish observations. Our results suggest that LAEs that contain AGN and those that are more massive, typically have lower fesc(Lyα) than the pure star-forming and lower mass systems. Examination of the relationship between infrared excess (IRX = LIR/LUV) and the UV slope (βUV) for the LAEs using single-dish observations, limited by depth, suggests that LAEs have higher IRX than typical star-forming galaxies at similar redshifts, often above the Calzetti curve. However, deeper ALMA observations show that LAEs actually follow a similar IRX–βUV relation to other star-forming galaxies like Lyman-break galaxies (LBGs). Furthermore, we examine the SFRIR–M∗ relation for LAEs and find a potential offset of∼2 dex when compared to the SFRLyα–M∗ relation. This suggests that significant amount of star formation in LAEs is obscured by the dust. To constrain the molecular gas content of LAEs, we use the ALMA archive to identify 34 LAEs that have ALMA observations at frequencies consistent with CO and [C i] 609 µm in their observed frame. Among these observations, we identify 12 potential line candidates, which result in six confirmed and two tentative detections of CO(3–2), CO(4–3), and CO(5–4) transitions in six LAEs. These detected LAEs have a median line luminosity of the order of∼1010 K km s−1 pc2, corresponding to molecular gas masses of∼7 ×1010 M⊙. We derive upper limits on molecular gas masses of∼(0.5–16) ×1010 M⊙for LAEs for which CO frequencies were observed but without being detected. The molecular gas fractions in LAEs are significantly higher than the other star-forming galaxies at similar redshifts, with a median of 0.92 ±0.25 for detections. Moreover, we observe that LAE gas fraction decreases with increasing stellar mass, suggesting that LAEs may evolve into more typical star-forming galaxies as they grow.