Oral Presentation
On the importance of using appropriate spectral models to derive physical properties of galaxies at 0.7 < z < 2.8
Interpreting ultraviolet-to-infrared observations of distant galaxies in terms of constraints on physical parameters – such as stellar mass, star formation rate (SFR) and attenuation by dust – requires spectral synthesis modelling. We investigate how increasing the level of sophistication of standard simplifying assumptions of such models can improve estimates of galaxy physical parameters. To achieve this, we compile a sample of 1048 galaxies at redshifts 0.7 < z < 2.8 with accurate photometry at rest-frame ultraviolet to near-infrared wavelengths from the 3D-HST survey. Of these galaxies, 364 have strong enough optical-line emission to be measured with good signal-to-noise ratio from grism spectroscopy. We compare the spectral energy distributions of these galaxies with those from different model spectral libraries to derive bayesian estimates of stellar mass, SFR and optical depth of the dust. We find that spectral libraries including sophisticated descriptions of galaxy star formation histories (SFHs), inspired from galaxy formation simulations, and modern prescriptions for attenuation by dust and nebular emission provide a much better representation of the observations than ‘classical’ spectral libraries, in which galaxy SFHs are generally assumed to be exponentially declining functions of time, associated with a simple prescription for dust attenuation and the neglect of nebular emission. As a result, for the galaxies in our sample, stellar masses derived using classical spectral libraries tend to be systematically overestimated (by a median ∼ 0.1 dex), and SFRs systematically underestimated (by a median ∼ 0.6 dex), relative to the values derived adopting a more realistic spectral library, such as that recently published by Pacifici et al. (2012), which includes a sophisticated and versatile prescription for dust attenuation as well as a careful account for nebular emission. The omission of nebular emission alone, even when including sophisticated treatments of SFHs and attenuation by dust, can systematically bias SFR estimates upward (by a median ∼ 0.1 dex). We also find that the simultaneous fit of photometric broad-band fluxes and emission-line equivalent widths helps break the degeneracy between SFR and dust attenuation, reducing considerably the uncertainties in the estimates of these parameters. Finally, we show how the biases introduced by oversimplified spectral fitting approaches can affect the slope, normalization and scatter of the correlation between stellar mass and SFR for star-forming galaxies (the ‘star-formation main sequence’). We conclude that the more sophisticated approach considered here is required to reliably interpret this and other fundamental diagnostics of galaxy evolution.
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