Determining the stellar masses of submillimetre galaxies: the critical importance of star formation histories

Abstract

Submillimetre (submm) galaxies are among the most rapidly star-forming and most massive high-redshift galaxies; thus, their properties provide important constraints on galaxy evolution models. However, there is still a debate about their stellar masses and their nature in the context of the general galaxy population. To test the reliability of their stellar mass determinations, we used a sample of simulated submm galaxies for which we created synthetic photometry. The photometry were used to derived their stellar masses via spectral energy distribution (SED) modelling, as is generally done with real observations. We used various SED codes (Grasil, Magphys, Hyperz, and LePhare) and various alternative assumed star formation histories (SFHs). We found that the assumption of SFHs with two independent components enables the SED modelling codes to most accurately recover the true stellar masses of the simulated submm galaxies. Exponentially declining SFHs (tau models) lead to lower masses (albeit still formally consistent with the true stellar masses), while the assumption of single-burst SFHs results in a significant underestimation of the stellar masses. Thus, we conclude that studies based on the higher masses inferred from fitting the SEDs of real submm galaxies with double SFHs are most likely to be correct, implying that submm galaxies lie on the high-mass end of the main sequence of star-forming galaxies. This conclusion appears robust to assumptions of whether submm galaxies are driven by major mergers, since the suite of simulated galaxies modelled here contains examples of both merging and isolated galaxies. We identified discrepancies between the true and inferred stellar ages (rather than the dust attenuation) as the primary determinant of the success or failure of the mass recovery. Regardless of the choice of SFH, the SED-derived stellar masses exhibit a factor of ∼2 scatter around the true value, and this scatter is an inherent limitation of the SED modelling due to simplified assumptions (regarding, e.g., the SFH, detailed galaxy geometry and wavelength dependence of the dust attenuation). Finally, we found that the contribution of active galactic nuclei (<60% at the K-band in these simulations) does not have any significant impact on the derived stellar masses.

Should we believe the results of ultraviolet–millimetre galaxy spectral energy distribution modelling?

“…Consequently, it is not subject to the potential systematic errors that are associated with single-component SFHs (e.g. Michałowski et al 2012Michałowski et al , 2014. 3.…”

“…Thus, it would be worthwhile to perform similar tests for other commonly used SED modelling codes. As a first step, Michałowski et al (2014) tested the ability of multiple SED modelling codes to recover the stellar masses of simulated submm galaxies (SMGs). They found that as long as a singlecomponent SFH was not used, all of the codes were able to accurately recover the stellar masses, albeit with a factor of ∼ 2 uncertainty.…”

“…18 Note that the lack of MIR photometry may be partially responsible for the robustness of the results to AGN contamination. However, in a similar test, Michałowski et al (2014) included mock MIR photometry and also found that the stellar masses were robust to AGN contamination.…”

“…SED modelling codes typically assume relatively simple SFHs, and changing the assumed form can significantly affect the results of the modelling (e.g. Michałowski et al 2012Michałowski et al , 2014. Furthermore, the models must necessarily assume simple geometries.…”

“…Lee et al 2009;Trager & Somerville 2009;Pforr, Maraston, & Tonini 2012Mitchell et al 2013) or simulations (e.g. Wuyts et al 2009b;Lanz et al 2014;Michałowski et al 2014;Torrey et al, submitted) as a type of controlled numerical experiment. Wuyts et al (2009b) were the first to perform such tests using hydrodynamical simulations.…”