Abstract

--- - Context. Probing the faint end of the number counts at millimeter wavelengths is important in order to identify the origin of the extragalactic background light in this regime. Aided by strong gravitational lensing, ALMA observations toward massive galaxy clusters have opened a window to probe this origin, allowing us to resolve sub-milliJansky dusty star-forming galaxies. - Aims. We aim to derive number counts at 1.1mm down to flux densities fainter than 0.1 mJy based on ALMA observations toward five Hubble Frontier Fields (FF) galaxy clusters, following a statistical approach to correct for lensing effects. - Methods. We created a source catalog that includes ALMA 1.1mm continuum detections around two new FF galaxy clusters, together with the sources previously detected around three FF galaxy clusters, making a total of 29 detected sources down to a 4.5 sigma significance. ALMA 1.1mm mosaics used for our source extraction covered the inner approximate to 2' x 2' FF regions, reached rms depths of approximate to 55 71 mu Jy beam 1, and had synthesized beam sizes from approximate to 0".5-1".5 (natural weighting). We derived source intrinsic flux densities using public lensing models. We folded the uncertainties in both magnifications and source redshifts into the number counts through Monte Carlo simulations. - Results. Using the combination of all cluster fields, we derive cumulative number counts over two orders of magnitude down to approximate to 0 .01 mJy after correction for lensing effects. Cosmic variance estimates are all exceeded by uncertainties in our median combined cumulative counts that come from both our Monte Carlo simulations and Poisson statistics. Our number counts agree at a 1 sigma level with our previous estimates using ALMA observations of the first three FFs, exhibiting a similar flattening at faint flux densities. They are also consistent to 1 sigma with most recent ALMA estimates and galaxy evolution models. However, below approximate to 0.1 mJy, our cumulative number counts are lower by approximate to 0.4 dex compared to two deep ALMA studies (namely one that probes several blank fields plus one lensed galaxy cluster, and the initial ALMA Spectroscopic Survey in the Hubble Ultra Deep Field, ASPECS-Pilot), while remaining consistent with the ASPECS Large Program (ASPECS-LP) within 1 sigma. Importantly, the flattening found for our cumulative counts at.0.1 mJy also extends further to less than or similar to 0.01 mJy, that is, approximate to 0.4 dex fainter than ASPECS-LP, and remains in agreement with extrapolations of their number counts down to this flux limit. We find a median contribution to the extragalactic background light (EBL) of 14(-8)(+12) Jy deg(-2) resolved in our demagnified sources down to approximate to 0.01 mJy, representing 75-86% of Planck-derived extragalactic EBL estimates at 1.1 mm. - Conclusions. We estimate cumulative 1.1mm number counts down to approximate to 0.01 mJy along the line of sight of five galaxy clusters that benefit from having rich deep multiwavelength data. They bring further support to the flattening of the number counts reported previously by us and ASPECS-LP, which has been interpreted by a recent galaxy evolution model as a measurement of the "knee" of the infrared luminosity function at high redshift. Our estimates of the contribution to the EBL associated with 1.1mm galaxies in the FFs suggest that we may be resolving most of the EBL at this wavelength down to approximate to 0.01 mJy.