579 research outputs found
Chemical and kinematical evolution in nearby dwarf spheroidal galaxies
No full textDwarf spheroidal (dSph) galaxies are the least massive and luminous objects known to exist. These galaxies are
often considered as the fossil building blocks of massive systems predicted by some cosmological models. Nonetheless,
evidence has been mounting that this idea of hierarchical assembly may be too simplistic a picture, since a number of
the dSphs' characteristics, such as the [alpha]-element abundance patterns, stand in contradiction to the properties of stars
in the Galactic halo. Further, yet unsolved, puzzles include the missing satellite problem, the influence of feedback
and reionization, and the nature of dark matter. How then do the dSphs form, to what extent do they contribute to
the build-up of massive galaxies, what can we say about their dark matter content and how can one characterize their
role in cosmology and galactic evolution? These systems' intriguing properties, such as the omnipresence of old stellar
populations, their gas deficiency, their high velocity dispersion and flat dispersion profiles provide stringent tests of
the paradigm of galaxy formation and render these systems important benchmarks for studying galactic evolution
from the earliest epochs on. In particular, the proximity of the dSphs in the Local Group (LG) allows us to resolve
their stellar populations and to pursue near-field cosmology on the smallest scales.
In this Thesis I address several evolutionary aspects of these galaxies by concentrating on three Galactic satellites
and additionally investigating the global satellite galaxy system of the Andromeda galaxy, M31.
In this context, I spectroscopically analyzed the Carina dSph, which stands out among the LG dSphs because of
its unusual, episodic star formation (SF) history. Carina bears evidence of at least three prominent stellar populations.
Hence, I aimed at studying the metallicity spread of such systems, investigating potential age-metallicity relations,
searching for spatial gradients and exploring its evolutionary history accounting for chemical enrichment. This was
achieved by obtaining medium-resolution spectroscopy of ~1200 targets in Carina. Based on the near-infrared calcium
triplet as a well established metallicity indicator, I was able to compile the metallicity distribution function (MDF)
of this galaxy from a large sample of stars. Despite the wide spread in stellar ages present in Carina, originating
from its episodic SF, it exhibits a remarkably narrow red giant branch (RGB). On the other hand, I found a wide
spread in the metallicities, reaching from -3 dex to near-solar. Hence, I could show that age and metallicity conspire
to produce old, metal poor stars at the same locus on the RGB as young, metal rich ones. This manifestation of
an age-metallicity degeneracy generically explains the observed narrowness of Carina's RGB. In addition, I could
also derive the age of each single red giant from isochrone fits. The resultant age distribution indicates the presence
of three major peaks, with a prevailing intermediate-age population. These populations may in fact be related to
Carina's three well established SF episodes. By correlating metallicities and spatial information, I could confirm a
radial population gradient in the metallicities, in the sense that more metal rich stars are found towards the center of
the galaxy. This phenomenon points to a deep central potential well in the dSphs, where gas is longer retained for SF
and enrichment. In order to analyze the shape of the observed MDF, several simple models of chemical evolution were
calculated, which support the view that the galaxy's early SF must have occurred from pre-enriched gas. Moreover,
all the models in use tend to overestimate the number of metal poor stars, i.e., there is a persistent G-dwarf problem.
This medium-resolution study was complemented by measurements of iron- and [alpha]-element abundance ratios
from high-resolution spectra of ten stars in Carina. These elements are important tracers of SF and thus reflect the
evolutionary status of any stellar system. It could be shown that the calibration of the metallicity [Fe/H] via the
calcium triplet reproduces the \true" stellar iron abundance well for moderately metal-poor stars, but the calibration
tends to fail towards the most metal-poor populations. Carina's [alpha/Fe]-ratios are well consistent with those measured
in other dSphs of the LG and confirm that they are systematically lower than those in Galactic halo stars of comparable
metallicities. The overall abundance patterns are not inconsistent with an episodic SF, but the accuracy and small
number statistics of such measurements impedes quantifying the underlying evolution. It is, however, safe from the
present data to say that also Carina inheres the typical characteristics of other dSphs in terms of a low SF efficiency
and the occurrence of strong galactic winds.
In an analoguous manner I determined metallicities in the remote Galactic satellite Leo II. The resulting MDF
also shows a deficiency in very metal-poor stars. Furthermore it turns out to be rather asymmetric, with a rapid
decline towards higher metallicities. By comparing my measurements with model predictions of LG dSphs I illustrated
that Leo II's MDF bears resemblence to the UMi and Scl dSphs, albeit none of the models succeed in reproducing all
features of the MDF simultaneously. By additionally determining the ages of the RGB stars in Leo II, I showed that
the age-metallicity relation in this galaxy is essentially flat over a long time interval, while there is evidence for an
enrichment during the last 2{4 Gyr. The overall wide spread in ages present in this dSph support earlier views that
Leo II is in fact a galaxy with a prominent old and a prevalent intermediate-age population. Contrary to Carina, I
could not detect any radial metallicity nor age gradient in Leo II.
Another important aspect of the nature and evolution of dSphs is the study of their kinematical properties. These
low-mass galaxies are believed to be the smallest cosmological structures containing dark matter. By measuring radial
velocities in the remote dSph Leo I I showed that the resulting velocity dispersion profile is essentially flat out to the
nominal tidal radius. The non-detection of any apparent velocity gradient across the galaxy supports the negligible
role of Galactic tides in the course of its whole evolution. The application of dynamical modeling under the assumption
of an isotropic velocity distribution then yielded mass and density profiles. Moreover, the behaviour of the velocity
anisotropy was analysed. The resulting high mass to light ratio of Leo I is supportive of the idea that all dSphs share
a common dark halo mass-scale of ~ 4 x 107M[sun], so that the pure velocity (dispersion) information of such a system
is a direct proxy for mass. All this argues in favour of a general dark matter dominance in the dSphs and renders the
hypothesis that these systems are of tidal origin less likely.
Lastly, information about the origin and evolution of the dSphs can be gleaned by examining their spatial distribution
around their host galaxies. It has often been reported that, in the Milky Way system, the dSphs are aligned
along one or more great circles or polar planes. Hence, I reconstructed the three-dimensional distribution of the entire
M31 satellite sample. By applying detailed statistical methods I could demonstrate that seven out of 16 satellites are
located within a thin polar sheet. One reason for this planar alignment can be the break up of a common progenitor,
which was orbiting M31. Also plausible is that the dSphs fell in along the filamentary dark matter structure of the
cosmic web, which is underscored by the fact that the plane extends in the direction of nearby galaxy groups.
All in all, my studies of the chemical and kinematical properties of a sample of nearby dSph galaxies do confirm
that these are dark matter dominated systems, which are governed by highly complex chemical enrichment processes
and thus warrant detailed investigations. These turn out to be invaluable for drawing a global picture of galaxy
formation in a cosmological context
Star clusters as age tracers of the age-metallicity relation of the small magellanic cloud
No full textIn my Thesis, I determined ages of Small Magellanic Cloud (SMC) star clusters that have formed
during the galaxys entire lifetime. The youngest cluster ages (�10 Myr<age<1 Gyr) were derived using
ground-based photometric data. For the six intermediate-age clusters Lindsay 1, Kron 3, NGC339,
NGC416, Lindsay 38, and NGC419 and the only old globular cluster (GC), NGC121, observations
obtained with the Hubble Space Telescope exists. This work was part of a ground-based and spacebased
program to uncover the age-metallicity evolution of the SMC. In the first three parts of my Thesis,
I presented accurate ages, distance estimates, and structural parameters for the seven intermediateage
and old SMC star clusters. The cluster ages were determined fitting di�erent isochrone models
to the observed color-magnitude diagrams (CMDs). The CMDs reach at least 3 mag below the mainsequence
turno�-points, which makes it the deepest available photometry for these clusters obtained
so far. Only for a few SMC clusters ages had been determined in previous studies using spacebased
data. The ground-based spectroscopy was obtained with the Very Large Telescope (VLT).
My photometric results are combined with these spectroscopic metallicity determinations to obtain a
well-sampled age-metallicity relation. I measured structural parameters of these seven star clusters
and extended the sample of known SMC clusters having accurate age measurements and structural
parameters enormously.
The SMC hosts a large number of intermediate-age and young star clusters, but only one ‘old’
GC, NGC121, for which I determined an age of �10.5 Gyr. Consequently, NGC121 is 2–3 Gyr
younger than the oldest GCs in the Large Magellanic Cloud (LMC) and the Milky Way (MW). For
comparison, the GC system of the MW exhibits a range of ages between �10.5 and 14 Gyr, similar
to the LMC, with the oldest populations belonging to the most ancient surviving stellar systems.
NGC121 is similar in age to the youngest GC in the Fornax dSph and to several of the young Galactic
halo clusters. On the other hand, NGC121 is not as young as some of the Sgr dwarf galaxy’s GCs or
the youngest Galactic GCs. With the SMC having no ‘truly’ old star cluster, it appears that the SMC
has a delayed cluster formation compared to its companions.
The isochrone that fitted best the CMD of NGC121 was �-enhanced. NGC121 is the only known
�-enhanced star cluster in the SMC, a property that it shares with many of the old outer Galactic
halo globulars and which indicates an early rapid chemical enrichment. In a subsequent project
additional SMC clusters will be analyzed for possible �-abundances and elements produced through
r-processes (rapid neutron capture). These �-elements and r-process elements are synthesized in
quickly evolving high-mass stars. The outcoming results can be compared to chemical evolution
models, which calculate chemical abundances in detail (e.g., Pagel & Tautvaiˇsien˙e 1998). With these
models the chemical evolution of the SMC can be further constrained and analyzed.
It is also intriguing that NGC121 is not as metal-poor as the oldest LMC andMWglobulars. The
SMC must have experienced substantial enrichment prior to the formation of NGC121. In the LMC,
two main epochs of the formation of compact populous clusters are observed. In the first epoch, GCs
with ages and metallicities similar to the oldest MW GCs are found, but also very old GCs having a
metallicity similar to NGC121, indicating very early chemical enrichment also in this galaxy. In the second epoch, numerous stellar populations with ages less than 3-4 Gyr developed. The two epochs
are separated by an ‘age gap’ of �4-9 Gyr in which no star cluster has formed. Also theMWcontains
old GCs that have similarly high metallicities as the younger NGC121. Evidently, the conditions
for and the e�ciency of star formation varied in these three galaxies at early epochs. The fact that
this cluster is younger than the Galactic mean, relatively metal-rich, and enhanced in �-elements has
interesting implications for the early development of the SMC.
The SMC is the only dwarf galaxy in the Local Group in which populous star clusters formed and
survived for most of its lifetime. The intermediate-age clusters in the SMC appear to be capable of
surviving a Hubble time, due to their high mass and the structure of the SMC (no bulge or disk to be
passed). After the formation of NGC121, there is a gap of �3 Gyr and thus likely in cluster formation
activity. The second oldest star cluster is Lindsay 1 for which I determined an age of �7.5 Gyr. Since
then compact populous star clusters formed fairly continuously until the present day in the SMC -
a contrast to both the LMC and the MW. For the youngest cluster in my sample, NGC419, me and
Elena Sabbi found indication for a multiple stellar population. A more detailed analysis is in progress
and the study will be published by Elena Sabbi. Only a few multiple stellar populations are known in
theMW, the LMC, and the SMC, but their number is increasing also due to the improved instruments.
Combining the newly derived ages with age and metallicity estimates adopted from di�erent sources
in the literature, it is possible to present a well-sampled age-metallicity relation (AMR) for the SMC,
which is fully based on space-based age determinations and spectroscopic metallicity measurements.
The SMC has experienced an early enrichment as can be seen in the relatively metal-rich oldest
SMC star cluster NGC121. The most striking feature in the AMR is the wide metallicity spread for
clusters with ages around 6 Gyr indicating that the SMC was not very well mixed in the past. The
mean metallicity, however, remains relatively constant for about 4 Gyr, but rises for star clusters that
have formed within the past 2–3 Gyr to a present day metallicity of [Fe/H]�-0.70.
From the apparent magnitudes of the cluster’s red clumps, I provided an estimate of direct distances
for the clusters. Together with cluster distances from the literature that were obtained using
the same approach and that are based on space-based observations, I confirmed the large depth extent
of the SMC along the line-of-sight. The three oldest clusters (age>7 Gyr) are located in the
north-western part of the SMC. NGC361 is a candidate for having an age older than 8 Gyr, but the
age determination found in the literature is associated with large uncertainties and new space-based
photometry of this cluster is needed. The youngest clusters (age<1 Gyr) lie near the SMC main body
in active star forming regions.
The number of intermediate-age and old SMC clusters having accurate structural parameters
and reliable ages was extended enormously in this study. The galactic environment causes external
perturbations such as tidal shocking that occurs as star clusters cross the disk or pass near the bulge.
These processes tend to decrease the cluster mass and therefore change its structural parameters. I
confirmed previous findings (Mackey & Gilmore 2003a,b) that some of the older objects in LMC and
SMC have experienced a significant change in core radius, while for other old objects the core radii
apparently have almost remained unchanged. The core radii of SMC clusters show a trend of older
clusters having a larger spread in core radii than the younger population. Even though I extended
the sample with structural parameters from the literature, the sample is highly incomplete, because
only for a few intermediate-age clusters both reliable ages and corresponding profiles are available.
The analysis of structural parameters of additional SMC clusters is necessary. Clusters in the LMC
have experienced a similar evolution, even though the two galaxies show strong di�erences in various
other aspects. The two confirmed Sagittarius clusters as well as the five Fornax clusters show the same
spread in core radii. The oldest clusters in the MW, however, modified their original structure during
their lifetime and have developed small cores. The largest di�erence between GCs in the MW, the
LMC, and the SMC is that the MW GCs clusters are subject to much larger tidal e�ects. The biggest
dynamical influence on most MW globular halo clusters is the tidal shocking that occurs when they
cross the disk of the MW. Tidal shocking is likely much less e�ective in the LMC and probably even
less so in the SMC. Therefore, the main reason for the smaller core radii of MW globulars is the
di�erent morphology of the three galaxies. The di�erent morphologies might also be the reason for the di�erent flattening distributions of star
clusters in the SMC, the LMC, and theMW. SMC clusters are more flattened than clusters in theMW
and even more flattened than those in the LMC. I found that only NGC121 and Lindsay 38 exhibit a
significant flattening. Galactic GCs modify their original structure and become more spherical with
increasing age, while LMC and SMC clusters maintain their original shape. This might be explained
with the di�erent dynamical influence and therefore the varying strength of the tidal field of the parent
galaxy. The tidal fields of the LMC and SMC might not be strong enough to modify the shape of their
clusters significantly. No relation between cluster age, distance from the SMC center, and ellipticity
was found, but this point needs further analysis because only for a few SMC star clusters reliable
ages, ellipticities and distances are available.
Finally, I provided today’s largest catalog of young SMC star clusters containing ages and luminosities.
The catalog covers an age range between 10 Myr and 1 Gyr. Star clusters are claimed to
be produced through strong shock compressions induced by the collision of their host galaxies which
causes enhanced star formation during close encounters. The most recent model calculations (e.g.,
Bekki & Chiba 2005, Kallivayalil et al. 2006a,b) showed that the SMC, the LMC, and the MW have
only interacted long enough to produce the Magellanic Stream. The models predict the last close encounter
between LMC and SMC around 200 Myr ago due to which enhanced cluster formation can be
expected. The cluster age distribution combining my results with the cluster ages provided by Chiosi
et al. (2006) shows indeed evidence for episodic star formation. The second of two peaks in the age
distribution coincides with the model predicted closest approach of the LMC. The origin of the first
peak about 6.5 Myr ago might have been triggered by internal mechanisms. Looking at their spatial
distribution, the young clusters are assembled in the two large star forming HI super-shells and in the
inter-shell region. Their formation might have been triggered by the expansion of the shells through
gas compression. I found no indication of cluster dissolution. As mentioned above, SMC clusters
evolve di�erently from MW clusters. Due to the di�erent morphologies of the parent galaxies, the
tidal field of the SMC has no big influence on its star clusters. It is most likely that SMC clusters decrease their mass through stellar evolution with time until the clusters finally dissolve
Dr. E.K. Wright
No full textPhotograph - The first County Playdowns curling event, Athabasca, Alberta. Dr. E.K. Wright with the trophy. Feb 27, 196
Defining the semiclassical limit of the quantum Rabi Hamiltonian
No full textThe crossover from quantum to semiclassical behavior in the seminal Rabi
model of light-matter interaction still, surprisingly, lacks a complete and
rigorous understanding. A formalism for deriving the semiclassical model
directly from the quantum Hamiltonian is developed here. Working in a displaced
Fock-state basis , the semiclassical limit is
obtained by taking and the coupling to zero.
This resolves the discrepancy between coherent-state dynamics and semiclassical
Rabi oscillations in both standard and ultrastrong coupling/driving regimes.
Furthermore, it provides a framework for studying the quantum-to-classical
transition, with potential applications in quantum technologies.Comment: 7 pages, 1 figure; 7 pages Supplemental Material; v3: change of
author name from E.K. Irish to E.K. Twyeffort Iris
Kron 3: a fourth intermediate age cluster in the SMC with evidence of multiple populations
No full textWe present the results of a spectroscopic study of the intermediate age (approximately 6.5 Gyr) massive cluster Kron 3 in the Small Magellanic Cloud. We measure CN and CH band strengths (at 3839 and 4300 Angstroms respectively) using VLT FORS2 spectra of 16 cluster members and find a sub-population of 5 stars enriched in nitrogen. We conclude that this is evidence for multiple populations in Kron 3, the fourth intermediate age cluster, after Lindsay 1, NGC 416 and NGC 339 (ages 6-8 Gyr), to display this phenomenon originally thought to be a unique characteristic of old globular clusters. At 6.5 Gyr this is one of the youngest clusters with multiple populations, indicating that the mechanism responsible for their onset must operate until a redshift of at least 0.75, much later than the peak of globular cluster formation at redshift ~3
Satellites in the Local Group and Other Nearby Groups
No full textIn recent years the census of known satellites in our own Local Group and in nearby galaxy groups has increased substantially due to sensitive wide-area surveys. In the Local Group these surveys have more than doubled its galaxy content and extended the galaxy luminosity function to very faint total magnitudes. Deep ground-based imaging and spectroscopic observations as well as high-resolution imaging with the Hubble Space Telescope have revolutionized our understanding of the chemical evolution and star formation histories of the satellites. We often find long-lasting star formation episodes with low star formation efficiencies. There is evidence for localized, stochastic enrichment, and recent searches are now beginning to uncover even extremely metal-deficient stars. In many satellites evidence for two or more distinct stellar subpopulations is found. Differing fractions of old populations have been detected in all satellites studied in sufficient detail so far. Kinematic measurements support a picture in which satellites are dark-matter dominated, although recent results indicate that the proposed common mass scale may not hold for very low-mass satellites. When considering satellite ensembles, we find global morphology–distance and gas-content–distance relations in all groups studied thus far, but individual star formation histories seem to also strongly depend on a given satellite’s intrinsic properties
Thick disk kinematics from RAVE and the solar motion
No full textWe present a method to derive kinematic parameters for the Galactic thick disk based on stellar radial velocity data alone, without previous knowledge of distances. The prospects and limitations of this method are then explored with the aid of photometric distances. We introduce selection criteria in order to clean the observed radial velocities from the Galactic differential rotation and to take into account the partial sky coverage of RAVE. We deduce the components of the Solar motion relative to the Local Standard of Rest (LSR) in the radial and vertical directions as well as the components of the velocity dispersion tensor. The results extend the analysis already started on the velocity distribution function of the thin and thick disk with RAVE data
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