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 (H ₁₆₀ ≳ 26.4, < 5σ) 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 μm; median CH2 = 22.46 ± 0.11 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 μm known so far (CH2 = 20.48 ± 0.03 AB mag). We show that the H-faint sources in our sample are massive (median M _star = 10^10.3±0.3 M _⊙), star-forming (median star formation rate $={100}_{-40}^{+60}$ M _⊙ yr^-1), and dust-obscured (A _V = 2.6 ± 0.3) galaxies around a median photometric redshift of z = 3.9 ± 0.4. The stellar continua of 14 H-faint galaxies can be resolved in the CH2 band, suggesting a median circularized effective radius (R _e,circ; lensing corrected) of 1.9 ± 0.2 kpc and <1.5 kpc for the resolved and whole samples, respectively. This is consistent with the sizes of massive unobscured galaxies at z ~ 4, 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 (R _e,circ ~ 1 kpc). These H-faint galaxies make up ${16}_{-7}^{+13}$ % of the galaxies in the stellar-mass range of 10¹⁰ - 10^11.2 M _⊙ at z = 3 ~ 5, 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.