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 $_160$ ensuremathgtrsim 26.4, < 5ensuremathσ) sources in 101 lensing cluster fields. Across a CANDELS/Wide-like survey area of åisebox-0.5ex~648 arcmin$^2$ (effectively i̊sebox-0.5ex~221 arcmin$^2$ in the source plane), we have securely discovered 53 sources in the IRAC Channel-2 band (CH2, 4.5 ensuremathμm; median CH2 = 22.46 ensuremath± 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 ensuremathμm known so far (CH2 = 20.48 ensuremath± 0.03 AB mag). We show that the H-faint sources in our sample are massive (median M $_star$ = 10$^10.3ensuremath±0.3$ M $ensuremathødot$), star-forming (median star formation rate $=100-40^+60$ M $ensuremathødot$ yr$^-1$), and dust-obscured (A $ V $ = 2.6 ensuremath± 0.3) galaxies around a median photometric redshift of z = 3.9 ensuremath± 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 ensuremath± 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 rs̊ebox-0.5ex~ 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 $e,circ$ rae̊box-0.5ex~ 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 $ensuremathødot$ at z = 3 raib̊ox-0.5ex~ 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.