Impact of an AGN featureless continuum on estimation of stellar population properties

Abstract

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.

The continuous rise of bulges out of galactic disks

“…1). In terms of standard SFH parametrizations, the superposition of these processes might be empirically approximated by "delayed" exponentially declining SFR models (e.g., Cardoso et al 2017) with an e-folding timescale and a delay of the SFR peak scaling inversely to the present-day M ,T . We note that the evidence from this study is qualitatively consistent with insights gained from Lick index studies by, for example, Thomas & Davies (2006), which indicate that the smallest bulges show the lowest lightweighted ages and a late enrichment in iron from SNe Ia.…”

Stellar Populations of a Sample of Optically Selected AGN-host Dwarf Galaxies

“…As shown in Figures 7 and 8 in Cardoso et al (2017), the neglect of the AGN FC will cause an overestimation of ∼0.05−0.3 dex (depending on the shape of the FC) for the light-weighted stellar age when log t ⋆ L = 7, and an underestimation of ∼0.4 − 0.6 dex when log t ⋆ L = 10, for an intrinsic x AGN = 0.2. For log t ⋆ M , it will lead to an overestimation of 0.1 − 0.5 dex when log t ⋆ M = 9 to <0.1 dex when log t ⋆ M = 10.…”

“…However, these galaxies should have AGN contributions to the optical continuum though it could be intrinsically small. To have a rough estimation on how this biases our mean stellar age, we assume that we did not include an AGN FC in the input library, and then compare our results with those from Cardoso et al (2017), who have quantitatively examined the effect of an AGN FC on the estimation of SFHs of galaxies.…”

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

“…This allows the investigation of galaxies with complex SFHs: sudden bursts due to a merger, active galactic nuclei (AGNs) triggering, or the abrupt decline of star formation due to ram pressure stripping (e.g. Cardoso et al 2017;Pappalardo et al 2010).…”