The MUSE Extremely Deep Field: The cosmic web in emission at high redshift
Roland Bacon, David Mary, Thibault Garel, J. Blaizot, M. V. Maseda, Joop Schaye, L. Wisotzki, Simon Conseil, J. Brinchmann, Floriane Leclercq, Valentina Abril-Melgarejo, Leindert Boogaard, N. Bouché, T. Contini, A. Feltre, B. Guiderdoni, C. Herenz, W. Kollatschny, Haruka Kusakabe, Jorryt Matthee, L. Michel-Dansac, Themiya Nanayakkara, Johan Richard, Martin M. Roth, K. B. Schmidt, Matthias Steinmetz, L. Tresse, T. Urrutia, Anne Verhamme, Peter M. Weilbacher, Johannes Zabl, Sebastiaan L. Zoutendijk
Abstract
We report the discovery of diffuse extended Ly α 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 α 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 α 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 Ly α luminosity from these filaments comes from beyond the circumgalactic medium of any identified Ly α emitter. Fluorescent Ly α 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 Ly α emission of a large population of ultra low-luminosity Ly α emitters (< 10 40 erg s −1 ), provided that the faint end of the Ly α luminosity function is steep ( α ⪅ −1.8), it extends down to luminosities lower than 10 38 − 10 37 erg s −1 , and the clustering of these Ly α emitters is significant (filling factor < 1/6). If these Ly α 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 α emission in typical filamentary environments and the first observational clue indicating the existence of a large population of ultra low-luminosity Ly α emitters at high redshift.