1,300 research outputs found
The non-universality of the low-mass end of the IMF is robust against the choice of SSP model
We perform a direct comparison of two state-of-the art single stellar
population (SSP) models that have been used to demonstrate the non-universality
of the low-mass end of the Initial Mass Function (IMF) slope. The two public
versions of the SSP models are restricted to either solar abundance patterns or
solar metallicity, too restrictive if one aims to disentangle elemental
enhancements, metallicity changes and IMF variations in massive early-type
galaxies (ETGs) with star formation histories different from the solar
neighborhood. We define response functions (to metallicity and
\alpha-abundance) to extend the parameter space of each set of models. We
compare these extended models with a sample of Sloan Digital Sky Survey (SDSS)
ETGs spectra with varying velocity dispersions. We measure equivalent widths of
optical IMF-sensitive stellar features to examine the effect of the underlying
model assumptions and ingredients, such as stellar libraries or isochrones, on
the inference of the IMF slope down to ~0.1 solar masses. We demonstrate that
the steepening of the low-mass end of the Initial Mass Function (IMF) based on
a non-degenerate set of spectroscopic optical indicators is robust against the
choice of the stellar population model. Although the models agree in a relative
sense (i.e. both imply more bottom-heavy IMFs for more massive systems), we
find non-negligible differences on the absolute values of the IMF slope
inferred at each velocity dispersion by using the two different models. In
particular, we find large inconsistency in the quantitative predictions of IMF
slope variations and abundance patterns when sodium lines are used. We
investigate the possible reasons for these inconsistencies.Comment: 16 pages, 9 figures, 2 tables, accepted for publication on Ap
Hierarchical Bayesian inference of the Initial Mass Function in Composite Stellar Populations
The initial mass function (IMF) is a key ingredient in many studies of galaxy
formation and evolution. Although the IMF is often assumed to be universal,
there is continuing evidence that it is not universal. Spectroscopic studies
that derive the IMF of the unresolved stellar populations of a galaxy often
assume that this spectrum can be described by a single stellar population
(SSP). To alleviate these limitations, in this paper we have developed a unique
hierarchical Bayesian framework for modelling composite stellar populations
(CSPs). Within this framework we use a parameterized IMF prior to regulate a
direct inference of the IMF. We use this new framework to determine the number
of SSPs that is required to fit a set of realistic CSP mock spectra. The CSP
mock spectra that we use are based on semi-analytic models and have an IMF that
varies as a function of stellar velocity dispersion of the galaxy. Our results
suggest that using a single SSP biases the determination of the IMF slope to a
higher value than the true slope, although the trend with stellar velocity
dispersion is overall recovered. If we include more SSPs in the fit, the
Bayesian evidence increases significantly and the inferred IMF slopes of our
mock spectra converge, within the errors, to their true values. Most of the
bias is already removed by using two SSPs instead of one. We show that we can
reconstruct the variable IMF of our mock spectra for signal-to-noise ratios
exceeding 75.Comment: Accepted for publication in MNRAS, 16 pages, 8 figure
The X-shooter Lens Survey - II. Sample presentation and spatially resolved kinematics
We present the X-shooter Lens Survey (XLENS) data. The main goal of XLENS is
to disentangle the stellar and dark matter content of massive early-type
galaxies (ETGs), through combined strong gravitational lensing, dynamics and
spectroscopic stellar population studies. The sample consists of 11 lens
galaxies covering the redshift range from to and having stellar
velocity dispersions between and . All
galaxies have multi-band, high-quality HST imaging. We have obtained long-slit
spectra of the lens galaxies with X-shooter on the VLT. We are able to
disentangle the dark and luminous mass components by combining lensing and
extended kinematics data-sets, and we are also able to precisely constrain
stellar mass-to-light ratios and infer the value of the low-mass cut-off of the
IMF, by adding spectroscopic stellar population information. Our goal is to
correlate these IMF parameters with ETG masses and investigate the relation
between baryonic and non-baryonic matter during the mass assembly and structure
formation processes. In this paper we provide an overview of the survey,
highlighting its scientific motivations, main goals and techniques. We present
the current sample, briefly describing the data reduction and analysis process,
and we present the first results on spatially resolved kinematics.Comment: Accepted for publication in MNRA
Evidence for intermediate-age stellar populations in early-type galaxies from K-band spectroscopy
The study of stellar populations in early-type galaxies in different
environments is a powerful tool for constraining their star formation
histories. This study has been traditionally restricted to the optical range,
where dwarfs around the turn-off and stars at the base of the RGB dominate the
integrated light at all ages. The near-infrared spectral range is especially
interesting since in the presence of an intermediate-age population, AGB stars
are the main contributors. In this letter, we measure the near-infrared indices
NaI and D for a sample of 12 early-type galaxies in low density
environments and compare them with the Fornax galaxy sample presented by Silva
et al. (2008). The analysis of these indices in combination with Lick/IDS
indices in the optical range reveals i) the NaI index is a metallicity
indicator as good as C4668 in the optical range, and ii) D is a
tracer of intermediate-age stellar populations. We find that low-mass galaxies
in low density environments show higher NaI and D than those located
in Fornax cluster, which points towards a late stage of star formation for the
galaxies in less dense environments, in agreement with results from other
studies using independent methods.Comment: 7 pages, 3 figures, accepted for publication in ApJ
(—)-3-(1-Phenylpropyl)-4-hydroxycoumarin
The title compound [generic name: (—)-phenprocoumon], C18H16O3, monoclinic, P21. Z = 4 with two molecules/asymmetric unit, a= 7·171 (1), b = 17·751 (5), c = 11·752 (2) Å, β = 92·58 (2)°, V = 1494 Å3, Dc = 1·28, d0 = 1·30 (2) g cm-3 is pseudoisostructural with the crystalline racemate as suggested by a similarity in cell constants and symmetry. Structural differences involve small translations and where the racemate crystal contains layers of (—) and layers of (+) enantiomers, the (—) crystal contains one layer nearly identical with the (—) layer in the racemate while the 01olecules in the other layer adopt a different conformation so that packing is similar to that in the (+) layer of the racemate. Already high thermal motion in the racemate is dramatically increased in the enantiomeric structure which has a larger cell volume. Hydrogen bonding occurs along the a direction with O···O distances of 2·617 (5) and 2·587 (6) Å. The final R is 0·094 on 3060 counter-collected data
A hierarchical Bayesian approach for reconstructing the initial mass function of single stellar populations
Recent studies based on the integrated light of distant galaxies suggest that
the initial mass function (IMF) might not be universal. Variations of the IMF
with galaxy type and/or formation time may have important consequences for our
understanding of galaxy evolution. We have developed a new stellar population
synthesis (SPS) code specifically designed to reconstruct the IMF. We implement
a novel approach combining regularization with hierarchical Bayesian inference.
Within this approach we use a parametrized IMF prior to regulate a direct
inference of the IMF. This direct inference gives more freedom to the IMF and
allows the model to deviate from parametrized models when demanded by the data.
We use Markov Chain Monte Carlo sampling techniques to reconstruct the best
parameters for the IMF prior, the age, and the metallicity of a single stellar
population. We present our code and apply our model to a number of mock single
stellar populations with different ages, metallicities, and IMFs. When
systematic uncertainties are not significant, we are able to reconstruct the
input parameters that were used to create the mock populations. Our results
show that if systematic uncertainties do play a role, this may introduce a bias
on the results. Therefore, it is important to objectively compare different
ingredients of SPS models. Through its Bayesian framework, our model is
well-suited for this.Comment: Accepted for publication in MNRAS, 30 pages, 28 figure
ESO 381-47, an early-type galaxy with extended HI and a star forming ring
ESO 381-47 is an early type galaxy with an extended HI disk. GALEX and very
deep optical images reveal a distinct stellar ring far outside the optical body
with a diameter of ~30 kpc, which has undergone recent star formation at 1.8 x
10^-4 Msun/yr/kpc^-2, consistent with other new results which detect low level
star formation below the traditional Kennicutt relation in the outer parts of
spiral galaxies. The morphology of this galaxy resembles the recently
identified class of ultraviolet objects called extended ultraviolet disks, or
XUV-disks. New HI observations of this galaxy taken at the ATCA and in the CnB
array at the VLA show that the cold gas lies in an extended (diameter ~90 kpc)
ring around the central S0 galaxy. The HI data cube can be well modeled by a
warped ring. The faint ionized gas in the inner parts of the galaxy is
kinematically decoupled from the stars and instead appears to exhibit
velocities consistent with the rotation of the HI ring at larger radius. The
peak of the stellar ring, as seen in the optical and UV, is slightly displaced
to the inside relative to the peak of the HI ring. We discuss the manner in
which this offset could be caused by the propagation of a radial density wave
through an existing stellar disk, perhaps triggered by a galaxy collision at
the center of the disk, or possibly due to a spiral density wave set up at
early times in a disk too hot to form a stellar bar. Gas accretion and
resonance effects due to a bar which has since dissolved are also considered to
explain the presence of the star forming ring seen in the GALEX and deep
optical data.Comment: 48 pages, 16 figures, 4 tables. Accepted for publication in the
Astronomical Journa
A Search for Environmental Effects on Type Ia Supernovae
We use integrated colors and B and V absolute magnitudes of Type Ia supernova
(SN) host galaxies in order to search for environmental effects on the SN
optical properties. With the new sample of 44 SNe we confirm the conclusion by
Hamuy et al. (1996a) that bright events occur preferentially in young stellar
environments. We find also that the brightest SNe occur in the least luminous
galaxies, a possible indication that metal-poorer neighbourhoods produce the
more luminous events. The interpretation of these results is made difficult,
however, due to the fact that galaxies with younger stellar populations are
also lower in luminosity. In an attempt to remove this ambiguity we use models
for the line strengths in the absorption spectrum of five early-type galaxies,
in order to estimate metallicities and ages of the SN host galaxies. With the
addition of abundance estimates from nebular analysis of the emission spectra
of three spiral galaxies, we find possible further evidence that luminous SNe
are produced in metal-poor neighborhoods. Further spectroscopic observations of
the SN host galaxies will be necessary to test these results and assist in
disentangling the age/metallicity effects on Type Ia SNe.Comment: 14 pages, 5 figures, to appear in the September 2000 issue of The
Astronomical Journa
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