7,144 research outputs found
Photoionization models of the CALIFA HII regions. I. Hybrid models
Photoionization models of HII regions require as input a description of the
ionizing SED and of the gas distribution, in terms of ionization parameter U
and chemical abundances (e.g. O/H and N/O). A strong degeneracy exists between
the hardness of the SED and U, which in turn leads to high uncertainties in the
determination of the other parameters, including abundances. One way to resolve
the degeneracy is to fix one of the parameters using additional information.
For each of the ~ 20000 sources of the CALIFA HII regions catalog, a grid of
photoionization models is computed assuming the ionizing SED being described by
the underlying stellar population obtained from spectral synthesis modeling.
The ionizing SED is then defined as the sum of various stellar bursts of
different ages and metallicities. This solves the degeneracy between the shape
of the ionizing SED and U. The nebular metallicity (associated to O/H) is
defined using the classical strong line method O3N2 (which gives to our models
the status of "hybrids"). The remaining free parameters are the abundance ratio
N/O and the ionization parameter U, which are determined by looking for the
model fitting [NII]/Ha and [OIII]/Hb. The models are also selected to fit
[OII]/Hb. This process leads to a set of ~ 3200 models that reproduce
simultaneously the three observations.
We find that the regions associated to young stellar bursts suffer leaking of
the ionizing photons, the proportion of escaping photons having a median of
80\%. The set of photoionization models satisfactorily reproduces the electron
temperature derived from the [OIII]4363/5007 line ratio. We determine new
relations between the ionization parameter U and the [OII]/[OIII] or
[SII]/[SIII] line ratios. New relations between N/O and O/H and between U and
O/H are also determined.
All the models are publicly available on the 3MdB database.Comment: Accepted for publication in A&
Entwicklung eines DNA-"Chip" zur Detektion somatischer Mutationen in Brustkrebszellen
Der Blickpunkt prädiktiver Molekulardiagnostik richtet sich immer stärker auf die Entwicklung innovativer genetischer Analysverfahren, mit denen sicher und im hohen Durchsatz de novo Mutationen bei einem Überschuss an "Wildtyp"-Sequenzen detektiert werden können. Im Rahmen dieser Arbeit wurde ein DNA-"chip" zur Detektion somatischer Mutationen auf der Grundlage eines modifizierten DNA-"mismatch"-bindenden Proteins entwickelt. Die genetischen Analysen zeigten, dass das modifizierte Protein myc-MutS sowohl über eine spezifische Basenfehlpaarungen erkennende DNA-Bindungsaffinität, als auch über eine unspezifische homologe DNA-Bindungsaffinität verfügt. Die spezifische Mutationserkennung wurde insbesondere durch die hohe Affinität des myc-MutS für einzelsträngige DNA behindert, weshalb die gewählte Strategie mit hoher Wahrscheinlichkeit nicht dazu geeignet ist, ein zuverlässiges und kostengünstiges "screening"-Verfahren für die Detektion somatischer Mutationen zu ermöglichen
Resolving galaxies in time and space: II: Uncertainties in the spectral synthesis of datacubes
In a companion paper we have presented many products derived from the
application of the spectral synthesis code STARLIGHT to datacubes from the
CALIFA survey, including 2D maps of stellar population properties and 1D
averages in the temporal and spatial dimensions. Here we evaluate the
uncertainties in these products. Uncertainties due to noise and spectral shape
calibration errors and to the synthesis method are investigated by means of a
suite of simulations based on 1638 CALIFA spectra for NGC 2916, with
perturbations amplitudes gauged in terms of the expected errors. A separate
study was conducted to assess uncertainties related to the choice of
evolutionary synthesis models. We compare results obtained with the Bruzual &
Charlot models, a preliminary update of them, and a combination of spectra
derived from the Granada and MILES models. About 100k CALIFA spectra are used
in this comparison.
Noise and shape-related errors at the level expected for CALIFA propagate to
0.10-0.15 dex uncertainties in stellar masses, mean ages and metallicities.
Uncertainties in A_V increase from 0.06 mag in the case of random noise to 0.16
mag for shape errors. Higher order products such as SFHs are more uncertain,
but still relatively stable. Due to the large number statistics of datacubes,
spatial averaging reduces uncertainties while preserving information on the
history and structure of stellar populations. Radial profiles of global
properties, as well as SFHs averaged over different regions are much more
stable than for individual spaxels. Uncertainties related to the choice of base
models are larger than those associated with data and method. Differences in
mean age, mass and metallicity are ~ 0.15 to 0.25 dex, and 0.1 mag in A_V.
Spectral residuals are ~ 1% on average, but with systematic features of up to
4%. The origin of these features is discussed. (Abridged)Comment: A&A, accepte
Integral field spectroscopy of type-II QSOs at z=0.3-0.4
We present and analyse integral-field observations of six type-II QSOs with
z=0.3-0.4, selected from the Sloan Digital Sky Survey (SDSS). Two of our sample
are found to be surrounded by a nebula of warm ionized gas, with the largest
nebula extending across 8" (40 kpc). Some regions of the extended nebulae show
kinematics that are consistent with gravitational motion, while other regions
show relatively perturbed kinematics: velocity shifts and line widths too large
to be readily explained by gravitational motion. We propose that a ~20 kpc x20
kpc outflow is present in one of the galaxies. Possible mechanisms for
triggering the outflow are discussed. In this object, we also find evidence for
ionization both by shocks and the radiation field of the AGN.Comment: 6 pages, 5 figures, accepted by MNRAS Letter
RBS1423 - a new QSO with relativistic reflection from an ionised disk
We present the analysis and results of a 20 ks XMM-Newton observation of
RBS1423. X-ray spectral analysis is used to establish a significantly broadened
relativistic iron K-alpha line from a highly ionised disk. A QSO at z=2.262 was
considered to be the optical counterpart of this ROSAT Bright Survey X-ray
source. Based on the improved XMM-Newton source position we identified a
z=0.208 QSO as optical counterpart to RBS1423. The 0.2-12 keV X-ray luminosity
of this radio-quiet QSO is 6x10^{44} erg/s. The XMM-EPIC spectra are well
described by a power law with a significantly broadened iron K-alpha line. Disk
line models for both Schwarzschild and Kerr black holes require hydrogen-like
iron ions to fit the measured line profile. Significant ionisation of the
reflection disk is confirmed by model fits with ionised disk models, resulting
in an ionisation parameter xi~2000.Comment: 7 pages, 8 figures, accepted for publication in A&A (2 April 2007
The nature of LINER galaxies: Ubiquitous hot old stars and rare accreting black holes
Galaxies, which often contain ionised gas, sometimes also exhibit a so-called
low-ionisation nuclear emission line region (LINER). For 30 years this was
attributed to a central mass-accreting supermassive black hole (AGN) of low
luminosity, making LINER galaxies the largest AGN-sub-population, dominating in
numbers over higher luminosity Seyfert galaxies and quasars. This, however,
poses a serious problem. While the inferred energy balance is plausible, many
LINERs clearly do not contain any other independent signatures of an AGN. Using
integral field spectroscopic data from the CALIFA survey, we aim at comparing
the observed radial surface brightness profiles with what is expected from
illumination by an AGN. Essential for this analysis is a proper extraction of
emission-lines, especially weak lines such as the Balmer Hb line which is
superposed on an absorption trough. To accomplish this, we use the GANDALF code
which simultaneously fits the underlying stellar continuum and emission lines.
We show for 48 galaxies with LINER-like emission, that the radial emission-line
surface brightness profiles are inconsistent with ionisation by a central
point-source and hence cannot be due to an AGN alone. The most probable
explanation for the excess LINER-like emission is ionisation by evolved stars
during the short but very hot and energetic phase known as post-AGB. This leads
us to an entirely new interpretation. Post-AGB stars are ubiquitous and their
ionising effect should be potentially observable in every galaxy with gas
present and stars older than ~1 Gyr, unless a stronger radiation field from
young hot stars or an AGN outshines them. This means that galaxies with
LINER-like emission are in fact not a class defined by a property, but rather
by the absence of a property. It also explains why LINER emission is observed
mostly in massive galaxies with old stars and little star formation.Comment: 8 pages, 7 figure
Stellar Population gradients in galaxy discs from the CALIFA survey
While studies of gas-phase metallicity gradients in disc galaxies are common,
very little has been done in the acquisition of stellar abundance gradients in
the same regions. We present here a comparative study of the stellar
metallicity and age distributions in a sample of 62 nearly face-on, spiral
galaxies with and without bars, using data from the CALIFA survey. We measure
the slopes of the gradients and study their relation with other properties of
the galaxies. We find that the mean stellar age and metallicity gradients in
the disc are shallow and negative. Furthermore, when normalized to the
effective radius of the disc, the slope of the stellar population gradients
does not correlate with the mass or with the morphological type of the
galaxies. Contrary to this, the values of both age and metallicity at 2.5
scale-lengths correlate with the central velocity dispersion in a similar
manner to the central values of the bulges, although bulges show, on average,
older ages and higher metallicities than the discs. One of the goals of the
present paper is to test the theoretical prediction that non-linear coupling
between the bar and the spiral arms is an efficient mechanism for producing
radial migrations across significant distances within discs. The process of
radial migration should flatten the stellar metallicity gradient with time and,
therefore, we would expect flatter stellar metallicity gradients in barred
galaxies. However, we do not find any difference in the metallicity or age
gradients in galaxies with without bars. We discuss possible scenarios that can
lead to this absence of difference.Comment: 24 pages, 17 figures, accepted for publication in A&
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