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

TL;DR

This study reassesses the stellar masses of three optically dark, high-redshift galaxies (S1, S2, S3) using Prospector with flexible non-parametric SFHs and a two-component dust attenuation model, incorporating new S1 spectroscopic redshift and extended JWST/FRESCO/JADES data plus FIR constraints. It shows that adopting a rising SFH base prior generally reduces inferred stellar masses compared to a constant SFH base prior, and that a degeneracy between dust attenuation slope, total attenuation, and M/L can drive mass uncertainties up to about an order of magnitude when the attenuation law is varied. Despite these systematics, S2 and S3 remain among the most massive and actively star-forming systems at their redshifts, implying high star-formation efficiencies that are broadly compatible with ΛCDM when environment and merger activity are considered. The work underscores the critical role of SFH priors and attenuation-law choices in high-z SED fitting and demonstrates the value of FIR data for breaking degeneracies in dusty galaxies.

Abstract

JWST observations have suggested that some high-redshift galaxies may be ultra-massive, thereby challenging standard models of early galaxy formation and cosmology. We analyse the stellar masses using different modelling assumptions and with new data of three galaxies (S1, S2 and S3), whose NIRCam/grism redshifts were consistent with $z>5$. These three optically dark galaxies have previously been reported to host exceptionally high stellar masses and star-formation rates, implying extremely high star-formation efficiencies. Recent NIRSpec/IFU observations for S1 indicate a spectroscopic redshift of $z_{\rm spec}=3.2461^{+0.0001}_{-0.0002}$, which is lower than previously reported. Using the Bayesian spectral energy distribution (SED) modelling tool \texttt{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 prior), the dust attenuation law, and the treatment of emission line fluxes. Our analysis yields revised stellar masses of $\log(M_{\star}/M_{\odot}) \approx 10.36^{+0.47}_{-0.32}, 10.95^{+0.11}_{-0.10}$ 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. 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 of heavily dust obscured sources. 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.

Figures (10)

• Figure 1: NIRCam RGB images of S1, S2 and S3 from left to right. The red/green/blue colours correspond to F277W/F200W/F150W for S1 and F444W/F200W/F090W for S2 and S3. The scale bars indicate a projected physical distance of 10 kpc, computed at the respective galaxy redshift using the angular diameter distance. S1 has a faint second component in the south-west (toward the lower right). S2 and S3 are more extended and show multiple components, potentially indicative of ongoing or recent merger activity.
• Figure 2: Top: Best-fitting SED for S1 with the redshift fixed to the spectroscopic value $z_{\rm spec}=3.246$ (blue solid line for RSFH prior and dashed orange line for CSFH prior) and, for comparison, to the redshift adopted by xiao24, $z_{\text{Xiao}} = 5.579$ (grey dashed line). Black points show the observed photometric data, including the ALMA 1.1 mm measurement (see Table \ref{['tab:fluxes']}). Bottom: The normalized residuals ($\chi$), e.g., photometric residuals between the observed and model-predicted fluxes, normalised by the observational uncertainties. Blue and orange circles correspond to the fit using the new spectroscopic redshift $z_{\rm spec}=3.246$ for the RSFH and CSFH prior, respectively, while grey crosses show the residuals obtained when fixing the redshift to $z_{\text{Xiao}} = 5.579$. The photometry favours the new spectroscopic redshift, yielding a total $\chi^2 = 42.5$ and 37.7 for the RSFH and CSFH prior, respectively, compared to $\chi^2 = 53.7$ for the fit adopting the redshift from xiao24.
• Figure 3: Top: SED fits for galaxies S2 (left panel) and S3 (right panel). Black points show photometric measurements for several filters shown in Table \ref{['tab:fluxes']}, with black error bars indicating observational uncertainties. NOEMA data for S2 is also included in the fit. The blue solid and orange dashed lines show the best-fit spectra from Prospector using rising and constant SFH priors, respectively. Blue and orange squares mark the corresponding model-predicted photometry. Bottom: The normalized residuals ($\chi$), normalized by the observational uncertainties, are generally distributed around zero for both galaxies, indicating a good overall fit for both the rising and constant SFH models. However, for both S2 and S3, the residuals from the rising SFH fits (blue circles) appear slightly more tightly clustered and symmetric around $\chi \approx 0$ compared to those from the constant SFH fits (orange circles), suggesting that the rising SFH prior provides a marginally better fit to the observed photometry. This is supported by the total $\chi^2$ values: for S2, $\chi^2_{\text{rising}} = 64.7$ and $\chi^2_{\text{constant}} = 64.8$; and for S3, $\chi^2_{\text{rising}} = 23.5$ and $\chi^2_{\text{constant}} = 24.2$.
• Figure 4: Posterior distribution of key stellar population parameters: total stellar mass formed ($M_{\star}$), specific star formation rate averaged over 50 Myr (sSFR$_{50}$), mass-weighted stellar age ($t_{50}$), optical depth of the diffuse dust attenuation ($\tau_2$), dust attenuation curve index ($n_{\rm dust}$), and stellar metallicity ($Z_{\star}$). Results are shown for two SFH base priors: rising SFH (RSFH; blue) and constant SFH (CSFH; orange). Median values for each parameter under both priors are indicated. The top-right panel shows the resulting SFH, illustrating that the CSFH prior prefers a higher SFR at earlier times than the RSFH prior, which gives rise to a higher stellar mass and a lower sSFR.
• Figure 5: Same as Fig. \ref{['fig:corner_s1']}, but for S2. The CSFH prior converges on a slightly higher stellar mass solutions, with a lower sSFR and older stellar age than the RSFH prior.