Self-consistent population spectral synthesis with FADO

Abstract

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 all PS codes were restricted to purely stellar fits, neglecting the essential contribution of nebular emission (ne). With the advent of Fado (Fitting Analysis using Differential evolution Optimisation), the now possible selfconsistent modelling of stellar and ne opens new routes to the exploration of galaxy SFHs. Aims. The main goal of this study is to quantitatively explore the accuracy to which Fado can recover physical and evolutionary properties of galaxies and compare its output with that from purely stellar PS codes. Methods. Fado and Starlight were 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 0.005-2.5 Z metallicity range. Results. Our analysis indicates that Fado can recover the key physical and evolutionary properties of galaxies, such as M and massand 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 considerable ne contamination (EW(Hα) > 10 3 Å). Likewise, population vectors from Fado adequately recover the mass fraction of stars younger than 10 Myr and older than 1 Gyr (M <10 Myr /M total and M >1 Gyr /M total , respectively) and reproduce with a high fidelity the observed Hα luminosity. As for Starlight, our analysis documents a moderately good agreement with theoretical values only for evolutionary phases for which ne drops 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 with Starlight during phases of high sSFR severely overestimate both M and the mass-weighted stellar age, whereas strongly underestimate the light-weighted age and metallicity. Furthermore, our analysis suggests a subtle tendency of Starlight to favour a bi-modal SFH, as well a slightly overestimated M <10 Myr /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 t...

Self-consistent population spectral synthesis with FADO: II. Star formation history of galaxies in spectral synthesis methods

“…It is currently the only publicly available tool permitting determination of SFH and CEM that consistently accounts for the nebular characteristics (Hα, Hβ luminosity and EW). This approach reduces the biases in the determination of mass and light-weighted stellar ages, as shown in Cardoso, Gomes & Papaderos (2019), and improves the accuracy of the SFH estimation. The code employs a differential genetic optimisation approach called differential evolution optimisation (DEO, Storn & Price 1996, which assures convergence at an affordable expense of computational time.…”

“…The spectra of the simulated galaxies cover 3400-8900 Å and have a spectral resolution of 2.3 Å and a ∆λ = 1 Å. The wavelengths coverage emulates the average range observed by the Sloan Digital Sky Survey (SDSS, Ahumada et al 2020;Doi et al 2010), including the Balmer and Paschen jumps, which are relevant features at young ages (< 10 7 yr) and are impossible to fit with purely stellar templates (Cardoso, Gomes & Papaderos 2019). The evolution of a galaxy is divided into 716 time-steps spanning 1 < t < 1500 Myr, assuming as a spectral basis the 221 ages in Bruzual & Charlot 2003, a Chabrier 2003IMF, and Padova 1994evolutionary tracks (Alongi et al 1993Bressan et al 1993;Fagotto et al 1994a,b;Girardi et al 1996).…”

“…The Rebetiko spectra for the CONT and TAU1 models are shown in Fig. 1 (adapted from Cardoso, Gomes & Papaderos 2019).…”

“…This separation between stellar and nebular continuum is also maintained for the determination of the intrinsic extinction, which is followed by an automated spectroscopic classification based on diagnostic emissionline ratios. Important features of the code are the following: a) the inclusion of the nebular continuum to the fit, which can have a significant effect on stellar mass determination Cardoso, Gomes & Papaderos 2019;Yuan et al 2019); b) the automatic masking of emission lines and spurious features (e.g. residuals from the sky line subtraction); c) a pre-defined extinction law, which is set as input parameter; d) a Gaussian kernel considered for broadening effects, both instrumental and physical.…”

FADO: A novel self-consistency spectral population synthesis tool for the exploration of galaxy evolution at high redshift

“…Additionally, it incorporates both stellar & nebular emission (continuum plus Balmer line-emission) in a self-consistent manner, i.e. FADO finds the best-match SFH that reproduces the nebular characteristics of a given galaxy, which alleviates known degeneracies that plague stellar population modelling (Cardoso, Gomes & Papaderos 2019). So, FADO is well suited for the study of both young & old stellar populations in galaxies.…”