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...

Radial age gradients hold the cumulative record of the multitude of physical processes driving the build-up of stellar populations and the ensuing star formation (SF) quenching process in galaxy bulges, therefore potentially sensitive discriminators between competing theoretical concepts on bulge formation and evolution. Based on spectral modeling of integral field spectroscopy (IFS) data from the CALIFA survey, we derive mass-and light-weighted stellar age gradients (∇(t ,B ) L,M ) within the photometrically determined bulge radius (R B ) of a representative sample of local face-on late-type galaxies that span 2.6 dex in stellar mass (8.9 ≤ logM ,T ≤ 11.5). Our analysis documents a trend for decreasing ∇(t ,B ) L,M with increasing M ,T , with high-mass bulges predominantly showing negative age gradients and vice versa. The inversion from positive to negative ∇(t ,B ) L,M occurs at logM ,T 10, which roughly coincides with the transition from lower-mass bulges whose gas excitation is powered by SF to bulges classified as Composite, LINER or Seyfert. We discuss two simple limiting cases for the origin of radial age gradients in massive LTG bulges. The first one assumes that the stellar age in the bulge is initially spatially uniform (∇(t ,B ) L,M ≈ 0), thus the observed age gradients (∼ -3 Gyr/R B ) arise from an inside-out SF quenching (ioSFQ) front that is radially expanding with a mean velocity v q . In this case, the age gradients for massive bulges translate into a slow (v q ∼1-2 km s −1 ) ioSFQ that lasts until z ∼ 2, suggesting mild negative feedback by SF or an AGN. If, on the other hand, negative age gradients in massive bulges are not due to ioSFQ but primarily due to their inside-out formation process, then the standard hypothesis of quasi-monolithic bulge formation has to be discarded in favor of a scenario that involves gradual buildup of stellar mass over 2-3 Gyr through, e.g., inside-out SF and inward migration of SF clumps from the disk. In this case, rapid ( 1 Gyr) AGN-driven ioSFQ cannot be ruled out. While the M ,T vs. ∇(t ,B ) L,M relation suggests that the assembly history of bulges is primarily regulated by galaxy mass, its large scatter (∼1.7 Gyr/R B ) reflects a considerable diversity that calls for an in-depth examination of the role of various processes (e.g., negative and positive AGN feedback, bar-driven gas inflows) with higher-quality IFS data in conjunction with advanced spectral modeling codes.

The effect of the featureless power-law (PL) continuum of an active galactic nucleus (AGN) on the estimation of physical properties of galaxies with optical population spectral synthesis (PSS) remains largely unknown. With the goal of a quantitative examination of this issue, we fit synthetic galaxy spectra representing a wide range of galaxy star formation histories (SFHs) and including distinct PL contributions of the form F ν ∝ ν −α with the PSS code Starlight to study to which extent various inferred quantities (e.g. stellar mass, mean age, and mean metallicity) match the input. The synthetic spectral energy distributions (SEDs) computed with our evolutionary spectral synthesis code include an AGN PL component with 0.5 ≤ α ≤ 2 and a fractional contribution 0.2 ≤ x AGN ≤ 0.8 to the monochromatic flux at 4020 Å. At the empirical AGN detection threshold x AGN 0.26 that we previously inferred in a pilot study on this subject, our results show that the neglect of a PL component in spectral fitting can lead to an overestimation by ∼2 dex in stellar mass and by up to ∼1 and ∼4 dex in the light-and mass-weighted mean stellar age, respectively, whereas the light-and mass-weighted mean stellar metallicity are underestimated by up to ∼0.3 and ∼0.6 dex, respectively. These biases, which become more severe with increasing x AGN , are essentially independent of the adopted SFH and show a complex behaviour with evolutionary time and α. Other fitting set-ups including either a single PL or multiple PLs in the base reveal, on average, much lower unsystematic uncertainties of the order of those typically found when fitting purely stellar SEDs with stellar templates, however, reaching locally up to ∼1, 3 and 0.4 dex in mass, age and metallicity, respectively. Our results underscore the importance of an accurate modelling of the AGN spectral contribution in PSS fits as a minimum requirement for the recovery of the physical and evolutionary properties of stellar populations in active galaxies. In particular, this study draws attention to the fact that the neglect of a PL in spectral modelling of these systems may lead to substantial overestimates in stellar mass and age, thereby leading to potentially significant biases in our understanding of the co-evolution of AGN with their galaxy hosts.