Self-consistent population spectral synthesis with FADO
Authors: Cardoso Leandro S. M. , Gomes Jean Michel , Papaderos Polychronis 
Published: December 2018

Context.Spectral population synthesis (PS) is a fundamental tool in extragalactic research that aims to decipher the assembly history of galaxies from their spectral energy distribution (SED). Whereas this technique has led to key insights into galaxy evolution in recent decades, star formation histories (SFHs) inferred therefrom have been plagued by considerable uncertainties stemming from inherent degeneracies and the fact that until recently allPScodes were restricted to purely stellar fits, neglecting the essential contribution of nebular emission (ne). With the advent ofFADO(Fitting Analysis using Differential evolution Optimisation), the now possible self-consistent modelling of stellar andneopens new routes to the exploration of galaxy SFHs.

Aims.The main goal of this study is to quantitatively explore the accuracy to whichFADOcan recover physical and evolutionary properties of galaxies and compare its output with that from purely stellarPScodes.

Methods.FADOandSTARLIGHTwere applied to synthetic SEDs that track the spectral evolution of stars and gas in extinction-free mock galaxies of solar metallicity that form their stellar mass (M⋆) according to different parametric SFHs. Spectral fits were computed for two different set-ups that approximate the spectral range of SDSS and CALIFA (V500) data, using up to seven libraries of simple stellar population spectra in the 0.005–2.5Z⊙metallicity range.

Results.Our analysis indicates thatFADOcan recover the key physical and evolutionary properties of galaxies, such asM⋆and mass- and light-weighted mean age and metallicity, with an accuracy better than 0.2 dex. This is the case even in phases of strongly elevated specific star formation rate (sSFR) and thus with considerablenecontamination (EW(Hα) >  103Å). Likewise, population vectors fromFADOadequately recover the mass fraction of stars younger than 10 Myr and older than 1 Gyr (M⋆<10Myr/M⋆totalandM⋆>1Gyr/M⋆total, respectively) and reproduce with a high fidelity the observed Hαluminosity. As forSTARLIGHT, our analysis documents a moderately good agreement with theoretical values only for evolutionary phases for whichnedrops to low levels (EW(Hα) ≤ 60 Å) which, depending on the assumed SFH, correspond to an age between ∼0.1 Gyr and 2–4 Gyr. However, fits withSTARLIGHTduring phases of high sSFR severely overestimate bothM⋆and the mass-weighted stellar age, whereas strongly underestimate the light-weighted age and metallicity. Furthermore, our analysis suggests a subtle tendency ofSTARLIGHTto favour a bi-modal SFH, as well a slightly overestimatedM⋆<10Myr/M⋆total, regardless of galaxy age. Whereas the amplitude of these biases can be reduced, depending on the specifics of the fitting procedure (e.g. accuracy and completeness of flagging emission lines, omission of the Balmer and Paschen jump from the fit), they persist even in the idealised case of a line-free SED comprising only stellar and nebular continuum emission.

Conclusions.The insights from this study suggest that the neglect of nebular continuum emission inSTARLIGHTand similar purely stellarPScodes could systematically impactM⋆and SFH estimates for star-forming galaxies. We argue that these biases can be relevant in the study of a range of topics in extragalactic research, including the redshift-dependent slope of the star formation (SF) main sequence, the SF frosting hypothesis, and the regulatory role of supermassive black holes on the global SFH of galaxies.