Slicing COSMOS with SC4K: the evolution of typical Ly α emitters and the Ly α escape fraction from z ∼ 2 to 6

Abstract

We present and explore deep narrow-and medium-band data obtained with the Subaru and the Isaac Newton telescopes in the ∼ 2 deg 2 COSMOS field. We use these data as an extremely wide, low-resolution (R ∼ 20 − 80) IFU survey to slice through the COSMOS field and obtain a large sample of ∼ 4000 Lyα emitters (LAEs) from z ∼ 2 to z ∼ 6 in 16 redshift slices (SC4K). We present new Lyα luminosity functions (LFs) covering a co-moving volume of ∼ 10 8 Mpc 3 . SC4K extensively complements ultradeep surveys, jointly covering over 4 dex in Lyα luminosity and revealing a global (2.5 < z < 6) synergy LF with α = −1.93 +0.12 −0.12 , log 10 Φ * Lyα = −3.45 +0.22 −0.29 Mpc −3 and log 10 L * Lyα = 42.93 +0.15 −0.11 erg s −1 . The Schechter component of the Lyα LF reveals a factor ∼ 5 rise in L * Lyα and a ∼ 7× decline in Φ * Lyα from z ∼ 2 to z ∼ 6. The data reveal an extra power-law (or Schechter) component above L Lyα ≈ 10 43.3 erg s −1 at z ∼ 2.2 − 3.5 and we show that it is partially driven by X-ray and radio AGN, as their Lyα LF resembles the excess. The power-law component vanishes and/or is below our detection limits above z > 3.5, likely linked with the evolution of the AGN population. The Lyα luminosity density rises by a factor ∼ 2 from z ∼ 2 to z ∼ 3 but is then found to be roughly constant (1.1 +0.2 −0.2 × 10 40 erg s −1 Mpc −3 ) to z ∼ 6, despite the ∼ 0.7 dex drop in UV luminosity density. The Lyα/UV luminosity density ratio rises from 4±1% to 30 ± 6% from z ∼ 2.2 to z ∼ 6. Our results imply a rise of a factor of ≈ 2 in the global ionisation efficiency (ξ ion ) and a factor ≈ 4 ± 1 in the Lyα escape fraction from z ∼ 2 to z ∼ 6, hinting for evolution in both the typical burstiness/stellar populations and even more so in the typical ISM conditions allowing Lyα photons to escape.

Rest-frame UV properties of luminous strong gravitationally lensed Lyα emitters from the BELLS GALLERY Survey

“…Our results demonstrate that LAEs at z ∼ 2 − 3 can be as luminous as L Lyα 10 43.5 erg s −1 (or 5 × L * Lyα ) and M UV −23 AB (or 10 × L * UV ) without invoking an AGN component, in contrast to the previous findings. It is worth noting that, while luminous LAEs at z ∼ 2 − 3 appear scarce, as they have not been previously detected in these 1 − 4 deg 2 wide surveys (Ouchi et al 2008;Zheng et al 2016;Sobral et al 2018a), a few UV-luminous (M UV ∼ −23 AB) nonactive LBGs at similar redshifts have been already discovered (e.g., Allam et al 2007;Lee et al 2013;Le Fèvre et al 2013;Marques-Chaves et al 2018). However, UV-luminous LBGs present on average a strongly suppressed Lyα line (see panel b of Figure 10) consistent with large neutral gas column densities (N(H i)>10 20 cm −2 , Dessauges-Zavadsky et al 2010;Marques-Chaves et al 2018).…”

“…Despite the large UV luminosities, our LAEs show prominent Lyα emission lines with EW 0 up to 50Å. Green and red symbols mark the measurements of other LAEs (Ouchi et al 2008;Sobral et al 2018a) and UV-bright LBGs ; c) relation between the velocity offset of the ISM absorption lines relative the UV absolute magnitude. Red diamonds mark the measurements of other lensed LBGs (Pettini et al 2000;Quider et al 2009Quider et al , 2010Dessauges-Zavadsky et al 2010;Patrício et al 2016;Marques-Chaves et al 2018).…”

“…The relationship of rest-frame UV absolute magnitudes of our BELLS GALLERY LAEs (blue circles) on other properties: a) relation between rest-frame UV and Lyα luminosities. Horizontal and vertical dashed lines mark respectively the Lyα luminosity and UV absolute magnitude of typical z ∼ 2.5 LAEs (Sobral et al 2018b) and LBGs (Reddy & Steidel 2009). Our LAEs span a wide range of luminosities, both in Lyα ((0.2 − 5.4) × L * Lyα ) and UV ((0.5 − 10) × L * UV ).…”

“…This clearly suggests that the most luminous star-forming galaxies experience strong outflows caused by the large mechanical energy input from supernova explosions and stellar winds; and d) UV slope (β UV ) as a function of the intrinsic absolute magnitudes of our LAEs (blue). Other symbols mark these measurements in other z = 2 − 3 LAEs (green; Sobral et al 2018a;Santos et al 2019) and z = 2 − 4 LBGs (orange; Shapley et al 2003;Bouwens et al 2009;Lee et al 2011 nosities and star-formation rates on the kinematics of the gas. In panel c) of Figure 10 we compare the velocity offset of the ISM absorption lines relative to the systemic redshift and the absolute magnitudes of our LAEs.…”

“…Multi-wavelength follow-up imaging and spectroscopy have revealed that typical (L * ) LAEs have relatively low stellar masses (M * 10 7 − 10 9 M ; e.g., Gawiser et al 2007;Ono et al 2010b,a;Guaita et al 2011;Kusakabe et al 2018), show modest star-formation rates (SFR ∼ 10 M yr −1 , e.g., Nakajima et al 2012;Sobral et al 2018a), and present, on average, low gas-phase metallicity (e.g., Finkelstein et al 2011;Nakajima et al 2012Nakajima et al , 2013Amorín et al 2017;Kojima et al 2017). They show very compact and clumpy morphologies (effective radius r eff 1 kpc) with little to no evolution with redshift (e.g., Bond et al 2009;Taniguchi et al 2009;Bond et al 2012;Hernán-Caballero et al 2017;Cornachione et al 2018;Paulino-Afonso et al 2018;Shibuya et al 2019).…”