On the importance of using appropriate spectral models to derive physical properties of galaxies at 0.7 < z < 2.8

Interpreting observations of distant galaxies in terms of constraints on physical parameters such as stellarmass (M star), star formation rate (SFR) and dust optical depth ((tau) over cap (V)) -requires spectral synthesis modelling. We analyse the reliability of these physical parameters as determined under commonly adopted 'classical' assumptions: star formation histories assumed to be exponentially declining functions of time, a simple dust law and no emission-line contribution. Improved modelling techniques and data quality now allow us to use a more sophisticated approach, including realistic star formation histories, combined with modern prescriptions for dust attenuation and nebular emission. We present a Bayesian analysis of the spectra and multiwavelength photometry of 1048 galaxies from the 3D-HST survey in the redshift range 0.7 < z < 2.8 and in the stellar mass range 9 less than or similar to log (M star/M-circle dot) less than or similar to 12. We find that, using the classical spectral library, stellar masses are systematically overestimated (similar to 0.1 dex) and SFRs are systematically underestimated (similar to 0.6 dex) relative to our more sophisticated approach. We also find that the simultaneous fit of photometric fluxes and emission-line equivalent widths helps break a degeneracy between SFR and (tau) over capV, reducing the uncertainties on these parameters. Finally, we show how the biases of classical approaches can affect the correlation between M star and SFR for star-forming galaxies (the 'star-formation main sequence'). We conclude that the normalization, slope and scatter of this relation strongly depend on the adopted approach and demonstrate that the classical, oversimplified approach cannot recover the true distribution of M star and SFR.