A Critical Assessment of Stellar Mass Measurement Methods

Abstract

This is the second paper in a series aimed at investigating the main sources of uncertainty in measuring the observable parameters in galaxies from their Spectral Energy Distributions (SEDs). In the first paper we presented a detailed account of the photometric redshift measurements and an error analysis of this process. In this paper we perform a comprehensive study of the main sources of random and systematic error in stellar mass estimates for galaxies, and their relative contributions to the associated error budget. Since there is no prior knowledge of the stellar mass of galaxies (unlike their photometric redshifts), we use mock galaxy catalogs with simulated multiwaveband photometry and known redshift, stellar mass, age and extinction for individual galaxies. The multi-waveband photometry for the simulated galaxies were generated in 13 filters spanning from U-band to mid-infrared wavelengths. Given different parameters affecting stellar mass measurement (photometric S/N ratios, SED fitting errors and systematic effects), the inherent degeneracies and correlated errors, we formulated different simulated galaxy catalogs to quantify these effects individually. For comparison, we also generated catalogs based on observed photometric data of real galaxies in the GOODS-South field, spanning the same passbands. The simulated and observed catalogs were provided to a number of teams within the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) collaboration to estimate the stellar masses for individual galaxies. A total of eleven teams participated, with different combinations of stellar mass measurement codes/methods, population synthesis models, star formation histories, extinction and age. For each simulated galaxy, the differences between the input stellar masses, M input , and those estimated by each team, M est , is defined as ∆ log(M ) ≡ log(M estimated ) − log(M input ), and used to identify the most fundamental parameters affecting stellar mass estimate in galaxies, with the following results: (1). no significant bias in ∆log(M ) was found among different codes, with all having comparable scatter (σ(∆log(M )) = 0.136 dex). The estimated stellar mass values are seriously affected by low photometric S/N ratios, with the rms scatter increasing for galaxies with H AB > 26 mag.; (2). A source of error contributing to the scatter in ∆log(M ) is found to be due to photometric uncertainties (0.136 dex) and low resolution in age and extinction grids when generating the SED templates;(3). The median of stellar masses among different methods provides a stable measure of the mass associated with any given galaxy (σ(∆log(M )) = 0.142 dex); (4). The ∆log(M ) values are strongly correlate with deviations in age (defined as the difference between the estimated and expected values), with a weaker correlation with extinction; (5). the rms scatter in the estimated stellar masses due to free parameters (after fixing redshifts and IMF) are quantified and found to be σ(∆log(M )) = 0.110 dex; (6). Using the...

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