Extensive Lensing Survey of Optical and Near-Infrared Dark Objects (El Sonido): HST H-Faint Galaxies behind 101 Lensing Clusters

We present a Spitzer/IRAC survey of H-faint (H₁₆₀ 26. 4, <5) sources in 101 lensing cluster fields. Across a CANDELS/Wide-like survey area of 648 arcmin² (effectively 221 arcmin² in the source plane), we have securely discovered 53 sources in the IRAC Channel-2 band (CH2, 4. 5 m; median CH2=22. 460. 11 AB mag) that lack robust HST/WFC3-IR F160W counterparts. The most remarkable source in our sample, namely ES-009 in the field of Abell 2813, is the brightest H-faint galaxy at 4. 5 m known so far (CH2=20. 480. 03 AB mag). We show that the H-faint sources in our sample are massive (median Mₛtar = 10^10. 3 0. 3 M_), star-forming (median star formation rate =100-₄₀^+60 M_yr^-1) and dust-obscured (AV=2. 60. 3) galaxies around a median photometric redshift of z=3. 90. 4. The stellar continua of 14 H-faint galaxies can be resolved in the CH2 band, suggesting a median circularized effective radius (Rₑ, circ; lensing corrected) of 1. 90. 2 kpc and <1. 5 kpc for the resolved and whole samples, respectively. This is consistent with the sizes of massive unobscured galaxies at z4, indicating that H-faint galaxies represent the dusty tail of the distribution of a wider galaxy population. Comparing with the ALMA dust continuum sizes of similar galaxies reported previously, we conclude that the heavy dust obscuration in H-faint galaxies is related to the compactness of both stellar and dust continua (Rₑ, circ 1 kpc). These H-faint galaxies make up 16-₇^+13% of the galaxies in the stellar mass range of 10^10-10^11. 2 M_ at z=35, contributing to 8-₄^+8% of the cosmic star formation rate density in this epoch and likely tracing the early phase of massive galaxy formation.