Observations of the Lyman-α Universe

Abstract

Hydrogen Lyman-α (Lyα) emission has been one of the major observational probes for the high-redshift Universe since the first discoveries of high- z Lyα-emitting galaxies in the late 1990s. Due to the strong Lyα emission originated by resonant scattering and recombination of the most abundant element, Lyα observations witness not only Hii regions of star formation and active galactic nuclei (AGNs) but also diffuse Hi gas in the circumgalactic medium (CGM) and the intergalactic medium (IGM). Here, we review Lyα sources and present theoretical interpretations reached to date. We conclude the following: ▪ A typical Lyα emitter (LAE) at z ≳ 2 with a L* Lyα luminosity is a high- z counterpart of a local dwarf galaxy, a compact metal-poor star-forming galaxy (SFG) with an approximate stellar (dark matter halo) mass and star-formation rate of 108−9M⊙ (1010−11M⊙) and 1–10 M⊙ year−1, respectively. ▪ High- z SFGs ubiquitously have a diffuse Lyα-emitting halo in the CGM extending to the halo virial radius and beyond. ▪ Remaining neutral hydrogen at the epoch of cosmic reionization makes a strong dimming of Lyα emission for galaxies at z > 6 that suggests the late reionization history. The next-generation large-telescope projects will combine Lyα emission data with Hi Lyα absorptions and 21-cm radio data that map out the majority of hydrogen (Hi+Hii) gas, uncovering the exchanges of ( a) matter by outflow and inflow and ( b) radiation, relevant to cosmic reionization, between galaxies and the CGM/IGM.

The MUSE Extremely Deep Field: The cosmic web in emission at high redshift

“…Narrowband Lyα emitter wide field surveys, followed-up by spectroscopic multi-object observations, have produced a wealth of information on LAE clustering and the large-scale structure of galaxies at high z (e.g., Francis et al 2004;Matsuda et al 2005;Zheng et al 2016;Ouchi et al 2020). They cover a large volume: for example 200 × 200 × 80 Mpc 3 for the SILVER-RUSH survey (Ouchi et al 2018;Shibuya et al 2017), but at the expense of a bright limiting luminosity (typically ∼10 43 erg s −1 ) and sparse sampling (e.g., 179 LAEs for the Harikane et al 2019 study at z = 5.7 and 6.6, that is 5.6 × 10 −5 Mpc −3 ).…”

The synchrony of production and escape: half the bright Lyα emitters at z ≈ 2 have Lyman continuum escape fractions ≈50

“…Stark et al 2010;Kusakabe et al 2020). At redshifts 𝑧 ∼ 6, the LAE fraction of bright 𝑀 UV −19 galaxies is about 40%, while it is about 10% at 𝑧 ≈ 3 (see Ouchi et al 2020 for a review). Thus, our findings about LAEs at 𝑧 ≈ 2 might apply to a significant fraction of the galaxy population at higher redshifts.…”

“…Perhaps most importantly, LyC constraints based on Ly𝛼 profiles can be extrapolated to higher redshifts with some confidence because LAEs at 𝑧 ≈ 2 − 6 are fundamentally similarin e.g., their sizes (e.g., Malhotra et al 2012;Paulino-Afonso et al 2018), UV slopes (e.g., , and Ly𝛼 line profiles corrected for IGM absorption (e.g., Hayes et al 2021). Further, Ly𝛼 LFs are almost unevolving across 𝑧 ≈ 2 − 6, therefore a luminositylimited survey at 𝑧 ≈ 2 would have a similar proportion of bright and faint LAEs as at higher redshifts (e.g., Sobral et al 2018;Herenz et al 2019;Ouchi et al 2020).…”

High-redshift Lyman-alpha emitters in blank and lensing fields

“…The only way we have of accurately recover the SFR of the galaxy using hydrogen emission lines is if we have the luminosity of one or more Balmer lines available. A useful review on the topic of Lyα modeling and observations that deserves mention is Ouchi et al (2020).…”

“…Putting all these processes together we get an interesting, if a bit muddled, picture of what is happening in the universe at the first few billion years of cosmic time (Shapley et al 2003;Dijkstra 2017;Hernán-Caballero et al 2017;Karman et al 2017;Vanzella et al 2018;Marchi et al 2019). A recent review on the state of the Lyα observations can be found in Ouchi et al (2020). Modeling the Lyα line profile is useful to investigate the physical conditions of the galaxy interstellar medium.…”