204 research outputs found
Chromatic roots and minor-closed families of graphs
Given a minor-closed class of graphs , what is the infimum of
the non-trivial roots of the chromatic polynomial of ? When
is the class of all graphs, the answer is known to be . We
answer this question exactly for three minor-closed classes of graphs.
Furthermore, we conjecture precisely when the value is larger than .Comment: 18 pages, 5 figure
A zero-free interval for chromatic polynomials of graphs with 3-leaf spanning trees
It is proved that if is a graph containing a spanning tree with at most
three leaves, then the chromatic polynomial of has no roots in the interval
, where is the smallest real root of the
polynomial . We also construct a family of
graphs containing such spanning trees with chromatic roots converging to
from above. We employ the Whitney -switch operation to manage the analysis
of an infinite class of chromatic polynomials.Comment: 16 pages, 5 figure
Testing stellar population synthesis models with Sloan Digital Sky Survey colors of M31's globular clusters
Accurate stellar population synthesis models are vital in understanding the
properties and formation histories of galaxies. In order to calibrate and test
the reliability of these models, they are often compared with observations of
star clusters. However, relatively little work has compared these models in the
ugriz filters, despite the recent widespread use of this filter set. In this
paper, we compare the integrated colors of globular clusters in the Sloan
Digital Sky Survey (SDSS) with those predicted from commonly used simple
stellar population (SSP) models. The colors are based on SDSS observations of
M31's clusters and provide the largest population of star clusters with
accurate photometry available from the survey. As such, it is a unique sample
with which to compare SSP models with SDSS observations. From this work, we
identify a significant offset between the SSP models and the clusters' g-r
colors, with the models predicting colors which are too red by g-r\sim0.1. This
finding is consistent with previous observations of luminous red galaxies in
the SDSS, which show a similar discrepancy. The identification of this offset
in globular clusters suggests that it is very unlikely to be due to a minority
population of young stars. The recently updated SSP model of Maraston &
Stromback better represents the observed g-r colors. This model is based on the
empirical MILES stellar library, rather than theoretical libraries, suggesting
an explanation for the g-r discrepancy.Comment: 8 pages, 4 figures, accepted for publication in Ap
Spectroscopy of globular clusters in the low-luminosity spiral galaxy NGC 45
CONTEXT: Extragalactic globular clusters have been studied in elliptical
galaxies and in a few luminous spiral galaxies, but little is known about
globular clusters in low-luminosity spirals.
AIMS: Past observations with the ACS have shown that NGC 45 hosts a large
population of globular clusters (19), as well as several young star clusters.
In this work we aim to confirm the bona fide globular cluster status for 8 of
19 globular cluster candidates and to derive metallicities, ages, and
velocities.
METHODS: VLT/FORS2 multislit spectroscopy in combination with the Lick/IDS
system was used to derive velocities and to constrain metallicities and
[alpha/Fe] element ratio of the globular clusters.
RESULTS: We confirm the 8 globular clusters as bona fide globular clusters.
Their velocities indicate halo or bulge-like kinematics, with little or no
overall rotation. From absorption indices such as H_beta, H_gamma, and H_delta
and the combined [MgFe]' index, we found that the globular clusters are
metal-poor [Z/H]<=-0.33 dex and [alpha/Fe]<=0.0 element ratio. These results
argue in favor of a population of globular clusters formed during the
assembling of the galaxy.Comment: Accepted for publication in Astronomy and Astrophysics. 10 pages, 6
figures. Table 6 and Fig. 6 will only be published in the electronic edition
of the A&A journa
A six-parameter space to describe galaxy diversification
Galaxy diversification proceeds by transforming events like accretion,
interaction or mergers. These explain the formation and evolution of galaxies
that can now be described with many observables. Multivariate analyses are the
obvious tools to tackle the datasets and understand the differences between
different kinds of objects. However, depending on the method used,
redundancies, incompatibilities or subjective choices of the parameters can
void the usefulness of such analyses. The behaviour of the available parameters
should be analysed before an objective reduction of dimensionality and
subsequent clustering analyses can be undertaken, especially in an evolutionary
context. We study a sample of 424 early-type galaxies described by 25
parameters, ten of which are Lick indices, to identify the most structuring
parameters and determine an evolutionary classification of these objects. Four
independent statistical methods are used to investigate the discriminant
properties of the observables and the partitioning of the 424 galaxies:
Principal Component Analysis, K-means cluster analysis, Minimum Contradiction
Analysis and Cladistics. (abridged)Comment: Accepted for publicationin A\&
New Light on the Formation and Evolution of M31 and its Globular Cluster System
We present spectroscopic ages, metallicities, and [alpha/Fe] ratios for 70
globular clusters in M31 that were derived from Lick line-index measurements.
In addition to a population of old (>10 Gyr) globular clusters with a wide
range of metallicities, from about -2.0 dex to solar values, we find evidence
for a population of intermediate-age globular clusters with ages between ~5 and
8 Gyr and a mean metallicity around [Z/H]=-0.6. We also confirm the presence of
young M31 globular clusters that were recently identified by Beasley et al.
(2004), which have ages <1 Gyr and relatively high metallicities around -0.4
dex. The M31 globular cluster system has a clearly super-solar mean
[alpha/Fe]=0.14\pm0.04 dex. Intermediate-age and young objects show roughly
solar abundance ratios. We find evidence for an age-[alpha/Fe] relation in the
sense that younger clusters have smaller mean [alpha/Fe] ratios. From a
comparison of indices, mostly sensitive to carbon and/or nitrogen abundance,
with SSP model predictions for nitrogen-enhanced stellar populations, we find a
dichotomy in nitrogen enhancement between young and old M31 globular clusters.
The indices of objects older than 5 Gyr are consistent with a factor of three
or higher in nitrogen enhancement compared to their younger counterparts. Using
kinematical data from Morrison et al. (2004) we find that the globular cluster
sub-population with halo kinematics is old (>9 Gyr), has a bimodal metallicity
distribution, and super-solar [alpha/Fe]. Disk globular clusters have a wider
range of ages, are on average more metal-rich, and have a slightly smaller mean
[alpha/Fe] ratio. (abridged)Comment: 32 pages, 12 figures, accepted for publication in A&
An updated survey of globular clusters in M 31. III. A spectroscopic metallicity scale for the Revised Bologna Catalog
We present a new homogeneous set of metallicity estimates based on Lick
indices for 245 old globular clusters of the M31 galaxy comprised in the
Revised Bologna Catalog. The metallicity distribution of the M31 globular
clusters is briefly discussed and compared with that of the Milky Way. Simple
parametric statistics suggests that the [Fe/H] distribution is likely not
unimodal. The strong correlation between metallicity and kinematics found in
previous studies is confirmed. The most metal-rich GCs tend to be packed at the
center of the system and share the galactic rotation as traced by the HI disk.
Although the velocity dispersion around the curve increases with decreasing
metallicity, also clusters with [Fe/H]<-1.0 display a clear rotational pattern,
at odds with their Milky Way counterparts.Comment: 25 pages with 17 figures and 2 Appendices - To appear on Astronomy &
Astrophysic
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