The MUSE Deep Lensed Field on the Hubble Frontier Field MACS J0416. Star-forming complexes at cosmological distances

Context. A census of faint and tiny star forming complexes at high redshift is key to improving our understanding of reionizing sources, galaxy growth, and the formation of globular clusters. Aims: We present the MUSE Deep Lensed Field (MDLF) program, which is aimed at unveiling the very faint population of high redshift sources that are magnified by strong gravitational lensing and to significantly increase the number of constraints for the lens model. Methods: We describe Deep MUSE observations of 17.1 h of integration on a single pointing over the Hubble Frontier Field galaxy cluster MACS J0416, providing line flux limits down to 2 × 10-19erg s-1cm-2within 300 km s-1and continuum detection down to magnitude 26, both at the three sigma level at λ = 7000 Å. For point sources with a magnification (μ) greater than 2.5 (7.7), the MLDF depth is equivalent to integrating more than 100 (1000) h in blank fields, as well as complementing non-lensed studies of very faint high-z sources. The source-plane effective area of the MDLF with μ >6.3 is Results: We confirm spectroscopic redshifts for all 136 multiple images of 48 source galaxies at 0.9 -20erg s-1cm-2, after correction for lensing magnification. Ultraviolet high-ionization metal lines (and He IIλ1640) are detected with S/N >10 for individual objects down to de-lensed magnitudes between 28 and 30. The median stacked spectrum of 33 sources with a median MUV≃ -17 and ⟨z⟩ = 3.2 (1.7 Conclusions: Deep MUSE observations, in combination with existing HST imaging, allowed us to: (1) confirm redshifts for extremely faint high-z sources; (2) peer into their internal structure to unveil clumps down to 100 - 200 pc scale; (3) in some cases, break down such clumps into star-forming complexes matching the scales of bound star clusters (Full Table A.2 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/646/A57Based on observations collected at the European Southern Observatory for Astronomical research in the Southern Hemisphere under ESO programmes ID 0100.A-0763(A) (PI E. Vanzella), 094.A-0115B (PI J. Richard), 094.A-0525(A) (PI F.E. Bauer)