On the Interpretation of Far-infrared Spectral Energy Distributions. I: The 850 $\hat{I}¼$m Molecular Mass Estimator

Abstract

We use a suite of cosmological zoom galaxy formation simulations and dust radiative transfer calculations to explore the use of the monochromatic $850\hat{I}¼m$ luminosity (L${}_{\hat{\mathrm{I}}½,850}$) as a molecular gas mass (M${}_{\mathrm{mol}}$) estimator in galaxies between $0<z<9.5$ for a broad range of masses. For our fiducial simulations, where we assume the dust mass is linearly related to the metal mass, we find that empirical L${}_{\hat{\mathrm{I}}½,850}$-M${}_{\mathrm{mol}}$ calibrations accurately recover the molecular gas mass of our model galaxies, and that the L${}_{\hat{\mathrm{I}}½,850}$-dependent calibration is preferred. We argue the major driver of scatter in the L${}_{\hat{\mathrm{I}}½,850}$-M${}_{\mathrm{mol}}$ relation arises from variations in the molecular gas to dust mass ratio, rather than variations in the dust temperature, in agreement with the previous study of Liang et al. Emulating a realistic measurement strategy with ALMA observing bands that are dependent on the source redshift, we find that estimating S${}_{\hat{\mathrm{I}}½,850}$ from continuum emission at a different frequency contributes $10-20\%$ scatter to the L${}_{\hat{\mathrm{I}}½,850}$-M${}_{\mathrm{mol}}$ relation. This additional scatter arises from a combination of mismatches in assumed T${}_{dust}$ and ${\hat{I}}^2$ values, as well as the fact that the SEDs are not single-temperature blackbodies.Finally we explore the impact of a dust prescription in which the dust-to-metals ratio varies with metallicity. Though the resulting mean dust temperatures are $\sim 50\%$ higher, the dust mass is significantly decreased for low-metallicity halos. As a result, the observationally calibrated L${}_{\hat{\mathrm{I}}½,850}$-M${}_{\mathrm{mol}}$ relation holds for massive galaxies, independent of the dust model, but below L${}_{\hat{\mathrm{I}}½,850}\lesssim 10^{28}$ erg s${}^{-1}$ (metallicities ${\log }_{10}(\mathrm{Z}/{\mathrm{Z}}_{\odot })\lesssim -0.8$) we expect galaxies may deviate from literature observational calibrations by $\gtrsim 0.5$ dex.

23 pages and 12 figures including appendices, published in the Astrophysical Journal, abstract shortened due to arXiv restrictions