ALMA Lensing Cluster Survey: An ALMA Galaxy Signposting a MUSE Galaxy Group at z = 4.3 Behind “El Gordo”

Abstract

We report the discovery of a Multi Unit Spectroscopic Explorer (MUSE) galaxy group at z = 4.32 lensed by the massive galaxy cluster ACT-CL J0102-4915 (aka El Gordo) at z = 0.87, associated with a 1.2 mm source that is at a 2.07 ± 0.88 kpc projected distance from one of the group galaxies. Three images of the whole system appear in the image plane. The 1.2 mm source has been detected within the Atacama Large Millimetre/submillimetre Array (ALMA) Lensing Cluster Survey (ALCS). As this ALMA source is undetected at wavelengths λ < 2 μm, its redshift cannot be independently determined, however, the three lensing components indicate that it belongs to the same galaxy group at z = 4.32. The four members of the MUSE galaxy group have low to intermediate stellar masses (∼10 7 -10 10 M e ) and star formation rates (SFRs) of 0.4-24 M e yr −1 , resulting in high specific SFRs (sSFRs) for two of them, which suggest that these galaxies are growing fast (with stellar mass doubling times of only ∼2 × 10 7 yr). This high incidence of starburst galaxies is likely a consequence of interactions within the galaxy group, which is compact and has high velocity dispersion. Based on the magnification-corrected sub-/ millimeter continuum flux density and estimated stellar mass, we infer that the ALMA source is classified as an ordinary ultra-luminous infrared galaxy (with associated dust-obscured SFR ∼ 200-300 M e yr −1 ) and lies on the star formation main sequence. This reported case of an ALMA/MUSE group association suggests that some presumably isolated ALMA sources are in fact signposts of richer star-forming environments at high redshifts.

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

“…We then derive an effective survey area of 490 arcmin 2 in the image plane. This implies that ∼24% of the H-faint galaxies will be missed in our survey due to the existence of nearby field sources, for example, the triply imaged H-faint ALMA-bright galaxy at z = 4.3 behind the cluster 0102-4915 (Caputi et al 2021).…”

“…These ALMA-detected H-faint galaxies, found in abundance (∼530 deg −2 by Wang et al 2019), are proposed as a critical tracer of the early phase of massive galaxy formation history. However, most of their physical properties still remain highly uncertain except for rare cases with spectroscopic confirmations (e.g., Walter et al 2012;Wang et al 2019;Umehata et al 2020;Zhou et al 2020;Caputi et al 2021;Mitsuhashi et al 2021) due to their non-detections and faintness at most wavelengths.…”

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

“…We also note that H-faint sources are often found with less-obscured companions detectable with HST (e.g., Simpson et al 2017a;Schreiber et al 2018b;Caputi et al 2021). Adopting the angular cross-correlation function between Hfaint and rest-frame-UV-selected galaxies at similar photometric redshifts reported in Wang et al (2019) and the surface density of H < 27.3 galaxies at z 3 − 5 within the GOODS field (Barro et al 2019), the surface density of UV-bright companion galaxies within 1 from H-faint sources is around 0.004 arcsec −2 , i.e., one order of magnitude lower than random field sources.…”

“…These ALMA-detected H-faint galaxies, found in abundance (∼ 530 deg −2 by Wang et al 2019), are proposed as a critical tracer of the early phase of massive galaxy formation history. However, most of their physical properties still remain highly uncertain except for rare cases with spectroscopic confirmations (e.g., Walter et al 2012;Wang et al 2019;Zhou et al 2020;Umehata et al 2020;Caputi et al 2021;Mitsuhashi et al 2021) due to their non-detections and faintness at most wavelengths.…”

High-redshift Lyman-alpha emitters in blank and lensing fields

“…Finally, we compared our data points to the M * -SFR relation found in Rin21 for both main sequence and starburst galaxies, and the lower envelope of the starburst cloud in Caputi et al (2021). We find that our sources lie both on the starburst and the main sequence relation, with a slight preference for the starburst lines.…”