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

We report the discovery of diffuse extended Ly alpha 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 Ly alpha 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 Ly alpha emission at high significance with an average surface brightness of 5x10(-20) erg s(-1) cm(-2) arcsec(-2). Remarkably, 70% of the total Ly alpha luminosity from these filaments comes from beyond the circumgalactic medium of any identified Ly alpha emitter. Fluorescent Ly alpha emission powered by the cosmic UV background can only account for less than 34% of this emission at z approximate to 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 Ly alpha emission of a large population of ultra low-luminosity Ly alpha emitters (< 10(40) erg s(-1)), provided that the faint end of the Ly luminosity function is steep (alpha?-1.8), it extends down to luminosities lower than 10(38)-10(37) erg s(-1), and the clustering of these Ly alpha emitters is significant (filling factor < 1/6). If these Ly 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 Ly alpha emission in typical filamentary environments and the first observational clue indicating the existence of a large population of ultra low-luminosity Ly alpha emitters at high redshift.