The MUSE Extremely Deep Field: the Cosmic Web in Emission at High Redshift

Abstract: 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 comoving Mpc. These structures have been observed in overdensities of Ly-alpha emitters in the MUSE Extremely Deep Field, a 140 hour deep MUSE observation located in the Hubble Ultra Deep Field. Among the 22 overdense regions identified, 5 are likely to harbor very extended Ly-alpha emission at high significance with an average surface brightness of $\mathrm{5 \times 10^{-20} erg s^{-1} cm^{-2} arcsec^{-2}}$. Remarkably, 70% of the total Ly-alpha luminosity from these filaments comes from beyond the circumgalactic medium of any identified Ly-alpha emitters. Fluorescent Ly-alpha emission powered by the cosmic UV background can only account for less than 34% of this emission at z$\approx$3 and for not more than 10% at higher redshift. We find that the bulk of this diffuse emission can be reproduced by the unresolved Ly-alpha emission of a large population of ultra low luminosity Ly-alpha emitters ($\mathrm{<10^{40} erg s^{-1}}$), provided that the faint end of the Ly-alpha luminosity function is steep ($\alpha \lessapprox -1.8$), it extends down to luminosities lower than $\mathrm{10^{38} - 10^{37} erg s^{-1}}$ and the clustering of these Ly-alpha emitters is significant (filling factor $< 1/6$). If these Ly-alpha emitters are powered by star formation, then this implies their luminosity function needs to extend down to star formation rates $\mathrm{< 10^{-4} M_\odot yr^{-1}}$. These observations provide the first detection of the cosmic web in Ly-alpha emission in typical filamentary environments and the first observational clue for the existence of a large population of ultra low luminosity Ly-alpha emitters at high redshift.

• The paper identifies diffuse extended Lyα emission tracing cosmic web filaments over 2.5–4 cMpc at redshifts 3.1–4.5.
• It employs 140-hour deep MUSE observations with adaptive optics to map 22 Lyα emitter overdensities and 5 significant filament regions.
• The study suggests that about 70% of Lyα luminosity originates from ultra-faint emitter populations beyond conventional galaxy halos.

The MUSE Extremely Deep Field: Tracing High-Redshift Cosmic Web Filaments Through Lyα Emission

The study presented in the paper "The MUSE Extremely Deep Field: the Cosmic Web in Emission at High Redshift" represents a significant observational effort to detect and analyze Lyα emission within large-scale cosmic web structures at high redshifts. Utilizing the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope, the authors have conducted exceptionally deep field observations amounting to a 140-hour exposure in the Hubble Ultra Deep Field.

Key Findings

The primary outcome of this research is the identification and characterization of diffuse extended Lyα emission detected within redshift regions spanning from 3.1 to 4.5. These emissions trace cosmic web filaments on scales ranging from 2.5 to 4 comoving megaparsecs (cMpc). The authors identified 22 regions of Lyα emitter overdensities within the MUSE Extremely Deep Field, out of which five regions displayed significant extended Lyα emission.

Methodology

The investigation capitalized on the depth and spatial resolution afforded by the MUSE instrument, enhanced with adaptive optics, to obtain three-dimensional insights into the intergalactic medium (IGM). The dataset extended beyond the circumgalactic medium (CGM) of any recognized galaxy entities, suggesting a genuine observation of the cosmic web.

Numerical simulations predicted the Lyα emissions to manifest along filaments that are projected to be 50 to 100 kpc wide and several megaparsecs in length. By employing stacking techniques and multiscale analysis, the authors effectively enhanced the signal-to-noise ratio for detecting extended emissions beyond the conventionally observed CGM around individual galaxies.

Interpretation and Implications

Among the novel findings, the study estimates that 70% of the total Lyα luminosity emanating from these filamental structures originates beyond the CGM of identified galaxies. Utilizing data on Lyα flux and its distribution across the detected structures, the authors propose that this diffuse emission may be attributed predominantly to an extensive population of ultra-low luminosity Lyα emitters. Such emitters would necessitate a steep faint-end slope of the Lyα luminosity function, potentially extending below luminosities of $10^{38} - 10^{37} \, \mathrm{erg \, s^{-1}}$.

Furthermore, the authors argue that known Lyα emission sources, such as fluorescence induced by the cosmic UV background, account for merely a fraction of this emission at high redshifts. They speculate on the potential existence of star-forming sources at exceedingly low star formation rates, positing a deep continuation of the Lyα luminosity function.

Future Directions

The paper underscores the critical need for further observational efforts to better resolve and characterize high-redshift filaments. This involves probing fainter luminosity thresholds and increasing field views to build a comprehensive inventory of the cosmic web on megaparsec scales. Such insights would profoundly inform models of structure formation and galaxy evolution, particularly in refining the characteristics of these hypothesized ultra-faint populations.

In conclusion, this study not only captures a snapshot of the cosmic web in its high-redshift state but also challenges preconceived notions of galaxy formation and the distribution of baryonic matter in the early universe. Sustained observation with next-generation instruments and telescopes may extend these findings, potentially unveiling new paradigms in cosmology and extragalactic astrophysics.