Semi-empirical AGN detection threshold in spectral synthesis studies of Lyman-continuum-leaking early-type galaxies

Cardoso, Leandro S. M.; Gomes, J. M.; Papaderos, P.

doi:10.1051/0004-6361/201629278

Cited by 5 publications

(1 citation statement)

References 28 publications

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“…Spectral modeling suggests that the region with the highest mass fraction of stars younger than 100 Myr (14%) is C, followed by A (9%) and B (3%). It should be noted, however, that the SFH of region B is somehow uncertain, not only because the 3D geometry of dust probably departs from a simple foreground screen model (as already indicated by the dust lane), but also due to a possible contribution of a non-thermal AGN continuum that could appreciably bias spectral modeling studies (Cardoso et al 2016). Nevertheless, taking the inferred SFH (Fig.…”

Section: As For a Nebmentioning

confidence: 93%

Bulgeless disks, dark galaxies, inverted color gradients, and other expected phenomena at higher z

Papaderos¹,

Östlin²,

Breda³

2023

A&A

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The spectral energy distribution (SED) of galaxies varies both between galaxies and within them. For instance, early-type spiral galaxies have a red bulge surrounded by a bluer star-forming disk with H II regions within. When observing redshifted galaxies, a given photometric filter probes light at a bluer rest frame, and in relating the observed magnitudes to the rest frame of the filter, so-called k corrections are commonly applied to account for the relative dimming or brightening in addition to the pure distance effect. The amount of correction depends on the shape of the spectrum (SED), so different k corrections apply to galaxies of different spectral types. This is, however, only part of the story, since any galaxy with a spatially non-homogeneous SED will experience a spatially varying relative dimming or brightening as a function of observed wavelength. Also, the morphological appearance of galaxies will therefore change with redshift. For instance, an early spiral galaxy observed in the V band would show a prominent bulge at z = 0, whereas, if at redshift z ∼ 1, the V filter probes emission in the rest-frame near-ultraviolet where the bulge is faint and the disk relatively brighter, thus the galaxy may appear as bulgeless. One popular way of studying spatial variations in the stellar population and dust content of galaxies is the use of color maps. For star-forming galaxies that have an appreciable contribution from nebular emission (lines and continuum), an additional effect is that the shifting of strong features in or out of filters will result in a non-monotonous color evolution with redshift. Hence, unlike the effects of distance, cosmological surface brightness dimming, and gravitational lensing, which are all achromatic, the fact that most galaxies have a spatially varying SED leads to a chromatic surface brightness modulation (CMOD) with redshift. While the CMOD effects are in principle easy to grasp, they affect multicolor imaging surveys and photometric properties derived from such surveys in a complex fashion. Properties such as the bulge-to-disk ratio, Sérsic exponent, light concentration, asymmetry index and effective radius, radial color gradients, and stellar mass determinations from SED fitting will depend on the redshift, the filters employed, and the rest-frame 2D SED patterns in a galaxy and will bias results inferred on galaxy evolution across cosmic time (e.g., the evolution of the mass-size, bulge-supermassive black hole, and Tully-Fisher relation), and potentially also weak lensing, if these effects are not properly taken into account. In this article we quantify the CMOD effects for idealized galaxies built from spectral synthesis models and from galaxies with observed integral field spectroscopy, and we show that they are significant and should be taken into account in studies of resolved galaxy properties and their evolution with redshift.

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Section: As For a Nebmentioning

confidence: 93%

Bulgeless disks, dark galaxies, inverted color gradients, and other expected phenomena at higher z

Papaderos¹,

Östlin²,

Breda³

2023

A&A

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Fitting Analysis using Differential evolution Optimization (FADO):

Gomes

Papaderos

2017

A&A

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The goal of population spectral synthesis (pss; also referred to as inverse, semi-empirical evolutionary-or fossil record approach) is to decipher from the spectrum of a galaxy the mass, age and metallicity of its constituent stellar populations. This technique, which is the reverse of but complementary to evolutionary synthesis, has been established as fundamental tool in extragalactic research. It has been extensively applied to large spectroscopic data sets, notably the SDSS, leading to important insights into the galaxy assembly history. However, despite significant improvements over the past decade, all current pss codes suffer from two major deficiencies that inhibit us from gaining sharp insights into the star-formation history (SFH) of galaxies and potentially introduce substantial biases in studies of their physical properties (e.g., stellar mass, mass-weighted stellar age and specific star formation rate). These are i) the neglect of nebular emission in spectral fits, consequently, ii) the lack of a mechanism that ensures consistency between the best-fitting SFH and the observed nebular emission characteristics of a star-forming (SF) galaxy (e.g., hydrogen Balmer-line luminosities and equivalent widths-EWs, shape of the continuum in the region around the Balmer and Paschen jump). In this article, we present FADO (Fitting Analysis using Differential evolution Optimization) -a conceptually novel, publicly available pss tool with the distinctive capability of permitting identification of the SFH that reproduces the observed nebular characteristics of a SF galaxy. This so-far unique selfconsistency concept allows us to significantly alleviate degeneracies in current spectral synthesis, thereby opening a new avenue to the exploration of the assembly history of galaxies. The innovative character of FADO is further augmented by its mathematical foundation: FADO is the first pss code employing genetic differential evolution optimization. This, in conjunction with various other currently unique elements in its mathematical concept and numerical realization (e.g., mid-analysis optimization of the spectral library using artificial intelligence, test for convergence through a procedure inspired by Markov chain Monte Carlo techniques, quasi-parallelization embedded within a modular architecture) results in key improvements with respect to computational efficiency and uniqueness of the best-fitting SFHs. Furthermore, FADO incorporates within a single code the entire chain of pre-processing, modeling, post-processing, storage and graphical representation of the relevant output from pss, including emission-line measurements and estimates of uncertainties for all primary and secondary products from spectral synthesis (e.g., mass contributions of individual stellar populations, mass-and luminosity-weighted stellar ages and metallicities). This integrated concept greatly simplifies and accelerates a lengthy sequence of individual time-consuming steps that are generally involved in pss modeling, further enhancing ...

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

Cardoso

Gomes

Papaderos

2017

A&A

Self Cite

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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.

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Semi-empirical AGN detection threshold in spectral synthesis studies of Lyman-continuum-leaking early-type galaxies

Cited by 5 publications

References 28 publications

Bulgeless disks, dark galaxies, inverted color gradients, and other expected phenomena at higher z

Bulgeless disks, dark galaxies, inverted color gradients, and other expected phenomena at higher z

Fitting Analysis using Differential evolution Optimization (FADO):

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

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