659 research outputs found
On the Origin of the Colour-Magnitude Relation in the Virgo Cluster
We explore the origin of the colour-magnitude relation (CMR) of early type
galaxies in the Virgo cluster using spectra of very high S/N ratio for six
elliptical galaxies selected along the CMR. The data are analysed using a new
evolutionary stellar population synthesis model to generate galaxy spectra at
the resolution given by their velocity dispersions. In particular we use a new
age indicator that is virtually free of the effects of metallicity. We find
that the luminosity weighted mean ages of Virgo ellipticals are greater than ~8
Gyr, and show no clear trend with galaxy luminosity. We also find a positive
correlation of metallicity with luminosity, colour and velocity dispersion. We
conclude that the CMR is driven primarily by a luminosity-metallicity
correlation. However, not all elements increase equally with the total
metallicity and we speculate that the CMR may be driven by both a total
metallicity increase and by a systematic departure from solar abundance ratios
of some elements along the CMR. A full understanding of the role played by the
total metallicity, abundance ratios and age in generating the CMR requires the
analysis of spectra of very high quality, such as those reported here, for a
larger number of galaxies in Virgo and other clusters.Comment: To appear in The Astrophysical Journal Letters, 2001 April 20 (551,
number 2). 5 pages and 4 figure
SAURON Observations of Disks in Early-Type Galaxies
We briefly describe the SAURON project, aimed at determining the intrinsic
shape and internal dynamics of spheroids. We focus here on the ability of
SAURON to identify gaseous and stellar disks and to measure their morphology
and kinematics. We illustrate some of our results with complete maps of
NGC3377, NGC3623, and NGC4365.Comment: 4 pages, 4 figures (newpasp.sty). To appear in ASP Conf. Series
"Galaxy Disks and Disk Galaxies", eds. J.G. Funes S.J. & E.M. Corsini.
Version with full resolution images available at
http://www.strw.leidenuniv.nl/~bureau/pub_list.htm
Comparison of spheroids formed by rat glioma stem cells and neural stem cells reveals differences in glucose metabolism and promising therapeutic applications
Cancer stem cells (CSCs) are thought to be partially responsible for cancer resistance to current therapies and tumor recurrence. Dichloroacetate (DCA), a compound capable of shifting metabolism from glycolysis to glucose oxidation, via an inhibition of pyruvate dehydrogenase kinase was used. We show that DCA is able to shift the pyruvate metabolism in rat glioma CSCs but has no effect in rat neural stem cells. DCA forces CSCs into oxidative phosphorylation but does not trigger the production of reactive oxygen species and consecutive anti-cancer apoptosis. However, DCA, associated with etoposide or irradiation, induced a Bax-dependent apoptosis in CSCs in vitro and decreased their proliferation in vivo. The former phenomenon is related to DCA-induced Foxo3 and p53 expression, resulting in the overexpression of BH3-only proteins (Bad, Noxa, and Puma), which in turn facilitates Bax-dependent apoptosis. Our results demonstrate that a small drug available for clinical studies potentiates the induction of apoptosis in glioma CSCs
Kinematic and Structural Evolution of Field and Cluster Spiral Galaxies
To understand the processes that build up galaxies we investigate the stellar
structure and gas kinematics of spiral and irregular galaxies out to redshift
1. We target 92 galaxies in four cluster (z = 0.3 & 0.5) fields to study the
environmental influence. Their stellar masses derived from multiband VLT/FORS
photometry are distributed around but mostly below the characteristic
Schechter-fit mass. From HST/ACS images we determine morphologies and
structural parameters like disk length, position angle and ellipticity.
Combining the spectra of three slit positions per galaxy using the MXU mode of
VLT/FORS2 we construct the two-dimensional velocity field from gas emission
lines for 16 cluster members and 33 field galaxies. The kinematic position
angle and flatness are derived by a Fourier expansion of elliptical velocity
profiles. To trace possible interaction processes, we define three irregularity
indicators based on an identical analysis of local galaxies from the SINGS
project. Our distant sample displays a higher fraction of disturbed velocity
fields with varying percentages (10%, 30% and 70%) because they trace different
features. While we find far fewer candidates for major mergers than the SINS
sample at z~2, our data are sensitive enough to trace less violent processes.
Most irregular signatures are related to star formation events and less massive
disks are affected more than Milky-Way type objects. We detect similarly high
fractions of irregular objects both for the distant field and cluster galaxies
with similar distributions. We conclude that we may witness the building-up of
disk galaxies still at redshifts z~0.5 via minor mergers and gas accretion,
while some cluster members may additionally experience stripping, evaporation
or harassment interactions.Comment: 4 pages, 2 colour figures, to appear in the ASP Conference Series
Proceedings of "Galaxies in Isolation: Exloring Nature vs. Nurture", Granada,
200
Approaching the Coverability Problem Continuously
The coverability problem for Petri nets plays a central role in the
verification of concurrent shared-memory programs. However, its high
EXPSPACE-complete complexity poses a challenge when encountered in real-world
instances. In this paper, we develop a new approach to this problem which is
primarily based on applying forward coverability in continuous Petri nets as a
pruning criterion inside a backward coverability framework. A cornerstone of
our approach is the efficient encoding of a recently developed polynomial-time
algorithm for reachability in continuous Petri nets into SMT. We demonstrate
the effectiveness of our approach on standard benchmarks from the literature,
which shows that our approach decides significantly more instances than any
existing tool and is in addition often much faster, in particular on large
instances.Comment: 18 pages, 4 figure
The present-day galaxy population in spiral galaxies
Although there are many more stellar population studies of elliptical and
lenticular galaxies, studies of spiral galaxies are catching up, due to higher
signal to noise data on one hand, and better analysis methods on the other.
Here I start by discussing some modern methods of analyzing integrated spectra
of spiral galaxies, and comparing them with traditional methods. I then discuss
some recent developments in our understanding of the stellar content of spiral
galaxies, and their associated dust content. I discuss star formation
histories, radial stellar population gradients, and stellar populations in
sigma drops.Comment: 8 pages, Proceedings of 'Probing Stellar Populations out to the
Distant Universe', Cefalu, Italy, Sep 7-19, 2008, AIP Conf. Proc. Series.
Higher resolution version available at
http://www.astro.rug.nl/~peletier/rfpcefalu.pd
Optical/NIR stellar absorption and emission-line indices from luminous infrared galaxies
We analyze a set of optical-to-near-infrared long-slit nuclear spectra of 16
infrared-luminous spiral galaxies. All of the studied sources present H
emission, which reflects the star-forming nature of our sample, and they
clearly display H I emission lines in the optical. Their continua contain many
strong stellar absorption lines, with the most common features due to Ca I, Ca
II, Fe I, Na I, Mg I, in addition to prominent absorption bands of TiO, VO,
ZrO, CN and CO. We report a homogeneous set of equivalent width (EW)
measurements for 45 indices, from optical to NIR species for the 16
star-forming galaxies as well as for 19 early type galaxies where we collected
the data from the literature. This selected set of emission and
absorption-feature measurements can be used to test predictions of the
forthcoming generations of stellar population models. We find correlations
among the different absorption features and propose here correlations between
optical and NIR indices, as well as among different NIR indices, and compare
them with model predictions. While for the optical absorption features the
models consistently agree with the observations,the NIR indices are much harder
to interpret. For early-type spirals the measurements agree roughly with the
models, while for star-forming objects they fail to predict the strengths of
these indices.Comment: accepted for publication in MNRA
- …