Self-consistent population spectral synthesis with FADO

Abstract

Context. The field of galaxy evolution will make a great leap forward in the next decade as a consequence of the huge effort by the scientific community in multi-object spectroscopic facilities. Various future surveys will enormously increase the number of available galaxy spectra, providing new insights into unexplored areas of research. To maximise the impact of such incoming data, the analysis methods must also step up, extracting reliable information from the available spectra. It is therefore urgent to refine and test reliable analysis tools that are able to infer the properties of a galaxy from medium- or high-resolution spectra. Aims. In this paper we aim to investigate the limits and the reliability of different spectral synthesis methods in the estimation of the mean stellar age and metallicity. These two quantities are fundamental to determine the assembly history of a galaxy by providing key insights into its star formation history. The main question this work aims to address is which signal-to-noise ratios (S/N) are needed to reliably determine the mean stellar age and metallicity from a galaxy spectrum and how this depends on the tool used to model the spectra. Methods. To address this question we built a set of realistic simulated spectra containing stellar and nebular emission, reproducing the evolution of a galaxy in two limiting cases: a constant star formation rate and an exponentially declining star formation with a single initial burst. We degraded the synthetic spectra built from these two star formation histories (SFHs) to different S/N and analysed with three widely used spectral synthesis codes, namely FADO, STECKMAP, and STARLIGHT, assuming similar fitting set-ups and the same base of spectral templates. Results. For S/N ≤ 5 all the three tools show a large diversity in the results. The FADO and STARLIGHT tools find median differences in the light-weighted mean stellar age of ∼0.1 dex, while STECKMAP shows a higher value of ∼0.2 dex. For S/N > 50 the median differences in FADO are ∼0.03 dex (∼7%), a factor 3 and 4 lower than the 0.08 dex (∼20%) and 0.11 dex (∼30%) obtained from STARLIGHT and STECKMAP, respectively. Detailed investigations of the best-fit spectrum for galaxies with overestimated mass-weighted quantities point towards the inability of purely stellar models to fit the observed spectral energy distribution around the Balmer jump. Conclusions. Our results imply that when a galaxy enters a phase of high specific star formation rate (sSFR) the neglect of the nebular continuum emission in the fitting process has a strong impact on the estimation of its SFH when purely stellar fitting codes are used, even in presence of high S/N spectra. The median value of these differences are of the order of 7% (FADO), 20% (STARLIGHT), and 30% (STECKMAP) for light-weighted quantities, and 20% (FADO), 60% (STARLIGHT), and 20% (STECKMAP) for mass-weighted quantities. More specifically, for a continuous SFH both STECKMAP and STARLIGHT overestimate the stellar age by > 2 dex within the first ∼100 Myr even for high S/N spectra. This bias, which stems from the neglect of nebular continuum emission, obviously implies a severe overestimation of the mass-to-light ratio and stellar mass. But even in the presence of a mild contribution from nebular continuum, there is still the possibility to misinterpret the data as a consequence of the poor quality of the observations. Our work underlines once more the importance of a self-consistent treatment of nebular emission, as implemented in FADO, which, according to our analysis, is the only viable route towards a reliable determination of the assembly of any high-sSFR galaxy at high and low redshift.

Characterisation of the stellar content of SDSS EELGs through self-consistent spectral modelling

“…Categorically, the series of tests herein, in addition to the theoretical modelling by Cardoso, Gomes & Papaderos (2019) and Pappalardo et al (2021) soundly point to the nebular continuum contribution as the main reason for the observed discrepancies between Starlight and FADO, in addition to the non-negligible effect of the individual mathematical recipes of the two PSS codes.…”

“…iv. Considering the study by Pappalardo et al (2021), where it was established a consistent dependence of the quality of the fits on the signal-to-noise ratio (S/N), and the dependence of the inclusion/exclusion of the Balmer jump at 3646Å (and Paschen at 8207Å for galaxies at z 0.12) on the resulting best-fitting model, it was selected a sub-sample of galaxies with (S/N) > 8 (measured at the restframe emission-line-free pseudo-continuum between 6390Å and 6490Å) and a restframe wavelength starting at < 3640Å, listing 464 galaxies.…”

“…FADO, on the other hand, is able to recover the mean stellar properties (i.e., total mass, mean age, and mean metallicity) with high accuracy (∼0.2 dex). In addition, the theoretical work by Pappalardo et al (2021) demonstrates that the overestimation of mass-weighted quantities observed in purely stellar codes such as Starlight and STECKMAP (Ocvirk 2011) relates to the inability of purely stellar models to fit the observed SED in the vicinity of the Balmer jump.…”

“…For EELGs in particular, which register especially high nebular contributions, the use of conventional PSS tools would introduce significant biases, impeding the accurate estimation of starformation and chemical enrichment histories (SFH and CEH, respectively). In this respect, FADO (Gomes & Papaderos 2017;Cardoso, Gomes & Papaderos 2019;Pappalardo et al 2021), with its unique capability of self-consistently considering the nebular component whilst fitting the stellar templates to the observed optical spectrum, is the only PSS tool that is qualified to produce sensitive results (it was recently employed to characterise the stellar content of three GPGs in the work by Fernández et al 2021).…”

Characterisation of the stellar content of SDSS EELGs through self-consistent spectral modelling

“…For EELGs in particular, which register especially high nebular contributions, the use of conventional PSS tools would introduce significant biases, impeding the accurate estimation of star-formation and chemical enrichment histories (SFH and CEH, respectively). In this respect, Fado (Gomes & Papaderos 2017;Cardoso et al 2019;Pappalardo et al 2021), with its unique capability of selfconsistently considering the nebular component whilst fitting the stellar templates to the observed optical spectrum, is the only PSS tool that is qualified to produce sensitive results (it was recently employed to characterise the stellar content of three GPGs in the work by Fernández et al 2022).…”