Revisiting stellar properties of star-forming galaxies with stellar and nebular spectral modelling

Abstract

Context. Spectral synthesis is a powerful tool for interpreting the physical properties of galaxies by decomposing their spectral energy distributions (SEDs) into the main luminosity contributors (e.g. stellar populations of distinct age and metallicity or ionised gas). However, the impact nebular emission has on the inferred properties of star-forming (SF) galaxies has been largely overlooked over the years, with unknown ramifications to the current understanding of galaxy evolution. Aims. The objective of this work is to estimate the relations between stellar properties (e.g. total mass, mean age, and mean metallicity) of SF galaxies by simultaneously fitting the stellar and nebular continua and comparing them to the results derived through the more common purely stellar spectral synthesis approach. Methods. The main galaxy sample from SDSS DR7 was analysed with two distinct population synthesis codes: Fado, which estimates self-consistently both the stellar and nebular contributions to the SED, and the original version of Starlight, as representative of purely stellar population synthesis codes. Results. Differences between codes regarding average mass, mean age and mean metallicity values can go as high as ∼0.06 dex for the overall population of galaxies and ∼0.12 dex for SF galaxies (galaxies with EW(Hα)>3 Å), with the most prominent difference between both codes in the two populations being in the light-weighted mean stellar age. Fado presents a broader range of mean stellar ages and metallicities for SF galaxies than Starlight, with the latter code preferring metallicity solutions around the solar value (Z = 0.02). A closer look into the average light-and mass-weighted star formation histories of intensively SF galaxies (EW(Hα)>75 Å) reveals that the light contributions of simple stellar populations (SSPs) younger than ≤ 10 7 (10 9 ) years in Starlight are higher by ∼5.41% (9.11%) compared to Fado. Moreover, Fado presents higher light contributions from SSPs with metallicity ≤ Z /200 (Z /50) of around 8.05% (13.51%) when compared with Starlight. This suggests that Starlight is underestimating the average light-weighted age of intensively SF galaxies by up to ∼0.17 dex and overestimating the light-weighted metallicity by up to ∼0.13 dex compared to Fado (or vice versa). The comparison between the average stellar properties of passive, SF and intensively SF galaxy samples also reveals that differences between codes increase with increasing EW(Hα) and decreasing total stellar mass. Moreover, comparing SF results from Fado in a purely stellar mode with the previous results qualitatively suggests that differences between codes are primarily due to mathematical and statistical differences and secondarily due to the impact of the nebular continuum modelling approach (or lack thereof). However, it is challenging to adequately quantify the relative role of each factor since they are likely interconnected. Conclusions. This work finds indirect evidence that a purely stellar population synthesis approach negatively i...

An investigation of the star-forming main sequence considering the nebular continuum emission at low-z

“…The application of FADO to the SDSS-DR7 spectroscopic data was fully described in Cardoso et al (2022), and the spectrum pipeline analysis used to derive the data in the MPA-JHU catalogue was presented in Tremonti et al (2004). Here, we only summarise the procedures adopted in these two works.…”

“…Here, we only summarise the procedures adopted in these two works. Cardoso et al (2022) corrected the raw SDSS-DR7 spectra for the foreground extinction considering the dust maps from Schlegel et al (1998) and were then converted into the observer restframe. Next, the spectra were fitted with FADO in the wavelength range between 3400 and 8900 Å with a base library from Bruzual & Charlot (2003) composed of 150 simple stellar populations (SSPs), containing 25 ages (between 1 Myr and 15 Gyr) and six metallicities (1/200, 1/50, 1/5, 2/5, 1, and 2.5 Z ).…”

“…We considered the application of FADO to SDSS-DR7 carried out Cardoso et al (2022) 4 . We started by finding the galaxies in common in the FADO data and the galaxies available in the MPA-JHU catalogue.…”

“…The capabilities of FADO were tested in a series of benchmark publications (Cardoso et al 2019;Pappalardo et al 2021). Cardoso et al (2022) compared FADO with STARLIGHT (Cid Fernandes et al 2005 by applying these tools to a set of synthetic spectra of galaxies with different star formation histories (SFHs). The consideration of different SFHs allowed the comparison of the results between FADO and STARLIGHT as a function of the level of the nebular contribution.…”

“…The next step after these initial tests was to apply FADO to real spectra, and with this goal in mind, FADO was applied to the SDSS-DR7 (Cardoso et al 2022). Cardoso et al (2022) carried out a similar analysis to that of Cardoso et al (2019), but this time to real data. In Section 7, this work and the results obtained are discussed in more detail.…”