Abstract
We report the discovery of diffuse extended Lyα emission from redshift 3.1 to 4.5, tracing cosmic web filaments on scales of 2.5−4 cMpc. These structures have been observed in overdensities of Lyα emitters in the MUSE Extremely Deep Field, a 140 h deep MUSE observation located in the Hubble Ultra-Deep Field. Among the 22 overdense regions identified, five are likely to harbor very extended Lyα emission at high significance with an average surface brightness of 5 × 10−20 erg s−1 cm−2 arcsec−2. Remarkably, 70% of the total Lyα luminosity from these filaments comes from beyond the circumgalactic medium of any identified Lyα emitter. Fluorescent Lyα emission powered by the cosmic UV background can only account for less than 34% of this emission at z ≈ 3 and for not more than 10% at higher redshift. We find that the bulk of this diffuse emission can be reproduced by the unresolved Lyα emission of a large population of ultra low-luminosity Lyα emitters (< 1040 erg s−1), provided that the faint end of the Lyα luminosity function is steep (α ⪅ −1.8), it extends down to luminosities lower than 1038 − 1037 erg s−1, and the clustering of these Lyα emitters is significant (filling factor < 1/6). If these Lyα emitters are powered by star formation, then this implies their luminosity function needs to extend down to star formation rates < 10−4 M⊙ yr−1. These observations provide the first detection of the cosmic web in Lyα emission in typical filamentary environments and the first observational clue indicating the existence of a large population of ultra low-luminosity Lyα emitters at high redshift.
Highlights
The current paradigm of structure formation predicts that most of the gas in the intergalactic medium (IGM) is organized in a “cosmic web” composed of nonuniform gaseous filaments connecting galaxies on scales of megaparsecs (e.g., White et al 1987; Bond et al 1996)
We report the discovery of diffuse extended Lyα emission from redshift 3.1 to 4.5, tracing cosmic web filaments on scales of 2.5−4 cMpc
To understand how our model is affected by the uncertainty on how faint the power law behavior of the Lyα luminosity function (LF) extends, we show in Fig. 14 the results of our model when we ignore the contribution of galaxies fainter than LLyα = 1038 erg s−1 and 1037 erg s−1
Summary
The current paradigm of structure formation predicts that most of the gas in the intergalactic medium (IGM) is organized in a “cosmic web” composed of nonuniform gaseous filaments connecting galaxies on scales of megaparsecs (e.g., White et al 1987; Bond et al 1996). In recent years we have used a fraction of our MUSE guaranteed observing time to perform deep field observations in the Hubble Ultra-Deep Field (HUDF, Beckwith et al 2006) at 10 and 30 h depths (Bacon et al 2017) These observations have increased the amount of available spectroscopic information in the HUDF by more than an order of magnitude (Inami et al 2017) and enabled several breakthroughs in our understanding of the high redshift universe, notably the discovery of ubiquitous extended Lyα emission from the CGMs around individual galaxies at z > 3 (Wisotzki et al 2016; Leclercq et al 2017, 2020). All distances are given in comoving scale (cMpc)
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