Stellar masses of optically dark galaxies: uncertainty introduced by the attenuation law and star-formation histories

Abstract Recent studies using JWST have suggested that some high-redshift galaxies may be ultra-massive, thereby challenging standard models of early galaxy formation. We analyse the stellar masses using different modelling assumptions and in conjunction with new data of three galaxies (S1, S2 and S3), whose photometric and NIRCam/grism redshifts were consistent with z > 5. These three “optically dark” galaxies were reported to host exceptionally high stellar masses (M⋆ ≳ 1011 M⊙) and star-formation rates (SFR ≳ 600 M⊙ yr−1), implying extremely high star-formation efficiencies. Recent NIRSpec/IFU observations for S1 indicate a spectroscopic redshift of zspec = 3.2439 ± 0.0002, revising the previous redshift. Using the Bayesian spectral energy distribution (SED) modelling tool Prospector, we investigate the impact of key model assumptions on stellar mass estimates, such as the choice of star-formation history (SFH) priors (constant versus rising SFH base for the non-parametric SFH prior), the dust attenuation law, and the treatment of emission line fluxes. Our analysis yields revised stellar masses of $\log _{10}(M_{\star }/M_{\odot }) \approx 10.36^{+0.47}_{-0.32}, 10.95^{+0.11}_{-0.10}$ and $10.31^{+0.24}_{-0.19}$ for S1, S2, and S3, respectively. We find that adopting a rising SFH base prior results in lower inferred stellar masses compared to a constant SFH base prior. Additionally, we identify a significant degeneracy between the dust attenuation curve slope, the amount of dust attenuation, and stellar mass. Our results highlight various systematics in SED modelling due to SFH priors and dust attenuation that can influence stellar mass estimates. Nevertheless, even with these revised stellar mass estimates, two of the three galaxies remain among the most massive and actively star-forming systems at their respective redshifts, implying high star-formation efficiencies.