53 research outputs found
Fitting Analysis using Differential Evolution Optimization (FADO): Spectral population synthesis through genetic optimization under self-consistency boundary conditions
The goal of population spectral synthesis (PSS) is to decipher from the
spectrum of a galaxy the mass, age and metallicity of its constituent stellar
populations. This technique has been established as a 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. 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 self-consistency concept allows us to
significantly alleviate degeneracies in current spectral synthesis. 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 other unique elements in its
mathematical concept (e.g., optimization of the spectral library using
artificial intelligence, convergence test, quasi-parallelization) results in
key improvements with respect to computational efficiency and uniqueness of the
best-fitting SFHs.Comment: 25 pages, 12 figures, A&A accepte
Impact of an AGN featureless continuum on estimation of stellar population properties
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 this in mind,
we fit synthetic galaxy spectra representing a wide range of galaxy star
formation histories (SFHs) and including distinct PL contributions of the form
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 and a fractional contribution to the monochromatic flux at 4020 \AA. At the
empirical AGN detection threshold 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. 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.Comment: 18 pages, 22 figures, accepted for publication in A&
Bulgeless disks, dark galaxies, inverted color gradients, and other expected phenomena at higher z. The chromatic surface brightness modulation (CMOD) effect
Since the k correction depends on the spectral energy distribution (SED) of a
galaxy, any high-z galaxy with a spatially non-homogeneous SED will experience
a spatially varying relative dimming or brightening in addition to the pure
distance effect. The morphology of galaxies will therefore change with z. For
instance, an early spiral galaxy observed in the V band would show a prominent
bulge at z=0, whereas, if at z=1, the V filter probes the rest-frame near-UV
where the bulge is faint and the disk relatively brighter, thus the galaxy may
appear as bulgeless. For galaxies with strong nebular emission, an additional
effect is that the shifting of strong nebular features in or out of filters
will result in a non-monotonous color evolution with z. 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 z.
While the CMOD effects are in principle easy to grasp, they affect the
characterization of galaxies in a complex fashion. Properties such as the
bulge-to-disk ratio, Sersic exponent, effective radius, radial color gradients,
and stellar mass determinations from SED fitting will depend on z, 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-SMBH, and Tully-Fisher relation), 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 z. (abridged)Comment: 41 pages, 32 figures, accepted for publication in A&
Indications of the invalidity of the exponentiality of the disk within bulges of spiral galaxies
(abridged) A fundamental subject in Extragalactic Astronomy concerns the
formation and evolution of late-type galaxies (LTGs). The standard scenario
comprises the early assembly of the bulge followed by disk accretion. However,
recent observational evidence points to a joint formation and perpetual
co-evolution of these structural components. Our current knowledge on the
properties of bulge and disk is mostly founded on photometric decomposition
studies, which sensitively depend on the adopted methodology and enclosed
assumptions on the structure of LTGs. A critical assumption whose validity was
never questioned is that galactic disks conserve their exponential nature up to
the galactic center. This implies that bulge and disk co-exist without
significant dynamical interaction and mass exchange over nearly the entire
Hubble time. Our goal is to examine the validity of the standard assumption
that galactic disks preserve their exponential intensity profile inside the
bulge radius all the way to the galactic center. We developed a
spectrophotometric bulge-disk decomposition technique that provides an
estimation for the net spectrum of the bulge. A systematic application of our
spectrophotometric bulge-disk decomposition tool to a representative sample of
135 local LTGs from the CALIFA Survey yields a significant fraction (up to
~30%) of unphysical net-bulge spectra when a purely exponential intensity
profile is assumed for the disk. The obtained results suggest that, for a
significant fraction of LTGs, the disk component shows a down-bending beneath
the bulge. If proven to be true, such result will call for a substantial
revision of structural decomposition studies for LTGs and have far-reaching
implications in our understanding of the photometric properties of their
bulges.Comment: 13 pages, 7 figures, accepted for publication in A&
The resolved chemical composition of the starburst dwarf galaxy CGCG007-025: Direct method versus photoionization model fitting
This work focuses on the gas chemical composition of CGCG007-025. This
compact dwarf is undergoing a galaxy wide star forming burst, whose spatial
behaviour has been observed by VLT/MUSE. We present a new line measurement
library to treat almost 7800 voxels. The direct method chemical analysis is
limited to 484 voxels with good detection of the temperature
diagnostic line. The recombination fluxes are corrected for stellar absorption
via a population synthesis. Additionally, we discuss a new algorithm to fit
photoionization models via neural networks. The 8 ionic abundances analyzed
show a spatial normal distribution with a , where only half
this value can be explained by the uncertainty in the measurements. The oxygen
abundance distribution is . The and
are also normally distributed. However, in the central and brightest
region, the is almost thrice the mean galaxy value. This is also
reflected in the extinction measurements. The ionization parameter has a
distribution of . The parameter spatial behaviour
agrees with the map. Finally, the discrepancies between the
direct method and the photoionization model fitting are discussed. In the
latter technique, we find that mixing lines with uneven uncertainty magnitudes
can impact the accuracy of the results. In these fittings, we recommend
overestimating the minimum flux uncertainty one order below the maximum line
flux uncertainty. This provides a better match with the direct method.Comment: Minor revision of your manuscript is requested before it is
reconsidered for publication in MNRA
Spectrophotometric investigations of Blue Compact Dwarf Galaxies: Markarian 35
We present results from a detailed spectrophotometric analysis of the blue
compact dwarf galaxy Mrk 35 (Haro 3), based on deep optical (B,V,R,I) and
near-IR (J,H,K) imaging, Halpha narrow-band observations and long-slit
spectroscopy. The optical emission of the galaxy is dominated by a central
young starburst, with a bar-like shape, while an underlying component of stars,
with elliptical isophotes and red colors, extends more than 4 kpc from the
galaxy center. High resolution Halpha and color maps allow us to identify the
star-forming regions, to spatially discriminate them from the older stars, and
to recognize several dust patches. We derive colors and Halpha parameters for
all the identified star-forming knots. Observables derived for each knot are
corrected for the contribution of the underlying older stellar population, the
contribution by emission lines, and from interstellar extinction, and compared
with evolutionary synthesis models. We find that the contributions of these
three factors are by no means negligible and that they significantly vary
across the galaxy. Therefore, careful quantification and subtraction of
emission lines, galaxy host contribution, and interstellar reddening at every
galaxy position, are essential to derive the properties of the young stars in
BCDs. We find that we can reproduce the colors of all the knots with an
instantaneous burst of star formation and the Salpeter initial mass function
with an upper mass limit of 100 M_solar. In all cases the knots are just a few
Myr old. The underlying population of stars has colors consistent with being
several Gyr old.Comment: 21 pages, 13 figures. Accepted for publication in ApJ, tentatively
scheduled for the ApJ November 1, 2007 v669n1 issu
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