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.

We observe the radio galaxy PKS 1934-63 (at z = 0.1825) using MUSE (Multi Unit Spectroscopic Explorer) on the Very Large Telescope (VLT). The radio source is GigaHertz Peaked Spectrum and compact (0.13 kpc), implying an early stage of evolution (≤ 10 4 yr). Our data show an interacting pair of galaxies, projected separation 9.1 kpc, velocity difference ∆(v) = 216 km s −1 . The larger galaxy is a M * 10 11 M spheroidal with the emission-line spectrum of a high-excitation young radio AGN, e.g. strong [OI]6300 and [OIII]5007. Emission-line ratios indicate a large contribution to the line luminosity from high-velocity shocks ( 550 km s −1 ) . The companion is a non-AGN disk galaxy, with extended Hα emission from which its star-formation rate is estimated as 0.61 M yr −1 . Both galaxies show rotational velocity gradients in Hα and other lines, with the interaction being prograde-prograde. The SE-NW velocity gradient of the AGN host is misaligned from the E-W radio axis, but aligned with a previously discovered central ultraviolet source, and a factor 2 greater in amplitude in Hα than in other (forbidden) lines (e.g. [OIII]5007). This could be produced by a fast rotating (100-150 km s −1 ) disk with circumnuclear star-formation. We also identify a broad component of [OIII]5007 emission, blueshifted with a velocity gradient aligned with the radio jets, and associated with outflow. However, the broad component of [OI]6300 is redshifted. In spectral fits, both galaxies have old stellar populations plus ∼ 0.1% of very young stars, consistent with the galaxies undergoing first perigalacticon, triggering infall and star-formation from ∼ 40 Myr ago followed by the radio outburst.

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...

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.

Extreme emission line galaxies (EELGs) are a notable galaxy genus, ultimately being regarded as local prototypes of early galaxies at the cosmic noon. Robust characterisation of their stellar content, however, is hindered by the exceptionally high nebular emission present in their optical spectroscopic data. This study is dedicated into recovering the stellar properties of a sample of 414 EELGs as observed by the SDSS Survey. Such is achieved by means of the spectral synthesis code FADO, which self-consistently considers the stellar and nebular emission in an optical spectrum. Additionally, a comparative analysis was carried on, by further processing the EELGs sample with the purely stellar spectral synthesis code STARLIGHT, and by extending the analysis to a sample of 697 normal star-forming galaxies, expected to be less affected by nebular contribution. We find that, for both galaxy samples, stellar mass and mean age estimates by STARLIGHT are systematically biased towards higher values, and that an adequate determination of the physical and evolutionary properties of EELGs via spectral synthesis is only possible when nebular continuum emission is taken into account. Moreover, the differences between the two population synthesis codes can be ascribed to the degree of star-formation activity through the specific star-formation rate and the sum of the flux of the most prominent emission lines. As expected, on the basis of the theoretical framework, our results emphasise the importance of considering the nebular emission while performing spectral synthesis, even for galaxies hosting typical levels of star-formation activity.

Various lines of evidence suggest that the cores of a large portion of early-type galaxies (ETGs) are virtually evacuated of warm ionised gas. This implies that the Lyman-continuum (LyC) radiation produced by an assumed active galactic nucleus (AGN) can escape from the nuclei of these systems without being locally reprocessed into nebular emission, which would prevent their reliable spectroscopic classification as Seyfert galaxies with standard diagnostic emission-line ratios. The spectral energy distribution (SED) of these ETGs would then lack nebular emission and be essentially composed of an old stellar component and the featureless power-law (PL) continuum from the AGN. A question that arises in this context is whether the AGN component can be detected with current spectral population synthesis in the optical, specifically, whether these techniques effectively place an AGN detection threshold in LyC-leaking galaxies. To quantitatively address this question, we took a combined approach that involves spectral fitting with Starlight of synthetic SEDs composed of stellar emission that characterises a 10 Gyr old ETG and an AGN power-law component that contributes a fraction 0 ≤ x AGN < 1 of the monochromatic luminosity at λ 0 = 4020 Å. In addition to a set of fits for PL distributions F ν ∝ ν −α with the canonical α = 1.5, we used a base of multiple PLs with 0.5 ≤ α ≤ 2 for a grid of synthetic SEDs with a signal-to-noise ratio of 5-10 3 . Our analysis indicates an effective AGN detection threshold at x AGN 0.26, which suggests that a considerable fraction of ETGs hosting significant accretion-powered nuclear activity may be missing in the AGN demographics.