844 research outputs found
Conformal field theory, boundary conditions and applications to string theory
This is an introduction to two-dimensional conformal field theory and its
applications in string theory. Modern concepts of conformal field theory are
explained, and it is outlined how they are used in recent studies of D-branes
in the strong curvature regime by means of CFT on surfaces with boundary.Comment: 45 pages, LaTeX2
About the morphology of dwarf spheroidal galaxies and their dark matter content
The morphological properties of the Carina, Sculptor and Fornax dwarfs are
investigated using new wide field data with a total area of 29 square degrees.
The stellar density maps are derived, hinting that Sculptor possesses tidal
tails indicating interaction with the Milky Way. Contrary to previous studies
we cannot find any sign of breaks in the density profiles for the Carina and
Fornax dwarfs. The possible existence of tidal tails in Sculptor and of King
limiting radii in Fornax and Carina are used to derive global M/L ratios,
without using kinematic data. By matching those M/L ratios to kinematically
derived values we are able to constrain the orbital parameters of the three
dwarfs. Fornax cannot have M/L smaller than 3 and must be close to its
perigalacticon now. The other extreme is Sculptor that needs to be on an orbit
with an eccentricity bigger than 0.5 to be able to form tidal tails despite its
kinematic M/L.Comment: 9 pages, 7 figures, accepted by A&
Abundance patterns in early-type galaxies: is there a 'knee' in the [Fe/H] vs. [alpha/Fe] relation?
Early-type galaxies (ETGs) are known to be enhanced in alpha elements, in
accordance with their old ages and short formation timescales. In this
contribution we aim to resolve the enrichment histories of ETGs. This means we
study the abundance of Fe ([Fe/H]) and the alpha-element groups ([alpha/Fe])
separately for stars older than 9.5 Gyr ([Fe/H]o, [alpha/Fe]o) and for stars
between 1.5 and 9.5 Gyr ([Fe/H]i, [alpha/Fe]i). Through extensive simulation we
show that we can indeed recover the enrichment history per galaxy. We then
analyze a spectroscopic sample of 2286 early-type galaxies from the SDSS
selected to be ETGs. We separate out those galaxies for which the abundance of
iron in stars grows throughout the lifetime of the galaxy, i.e. in which
[Fe/H]o < [Fe/H]i. We confirm earlier work where the [Fe/H] and [alpha/Fe]
parameters are correlated with the mass and velocity dispersion of ETGs. We
emphasize that the strongest relation is between [alpha/Fe] and age. This
relation falls into two regimes, one with a steep slope for old galaxies and
one with a shallow slope for younger ETGs. The vast majority of ETGs in our
sample do not show the 'knee' in the plot of [Fe/H] vs. [alpha/Fe] commonly
observed in local group galaxies. This implies that for the vast majority of
ETGs, the stars younger than 9.5 Gyrs are likely to have been accreted or
formed from accreted gas. The properties of the intermediate-age stars in
accretion-dominated ETGs indicate that mass growth through late (minor) mergers
in ETGs is dominated by galaxies with low [Fe/H] and low [alpha/Fe]. The method
of reconstructing the stellar enrichment histories of ETGs introduced in this
paper promises to constrain the star formation and mass assembly histories of
large samples of galaxies in a unique way.Comment: 22 pages, 25 figures, accepted for publication by A&
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Chaotic cascades for D-branes on singularities
We briefly review our work on the cascading renormalization group flows for gauge theories on D-branes probing Calabi-Yau singularities. Such RG flows are sometimes chaotic and exhibit duality walls. We construct supergravity solutions dual to logarithmic flows for these theories. We make new observations about a surface of conformal theories and more complicated supergravity solutions
Calculations for Mirror Symmetry with D-branes
We study normal functions capturing D-brane superpotentials on several one-
and two-parameter Calabi-Yau hypersurfaces and complete intersections in
weighted projective space. We calculate in the B-model and interpret the
results using mirror symmetry in the large volume regime, albeit without
identifying the precise A-model geometry in all cases. We identify new classes
of extensions of Picard-Fuchs equations, as well as a novel type of topology
changing phase transition involving quantum D-branes. A 4-d domain wall which
is obtained in one region of closed string moduli space from wrapping a
four-chain interpolating between two Lagrangian submanifolds is, for other
values of the parameters, represented by a disk ending on a single Lagrangian.Comment: 42 page
Differential stellar population models: how to reliably measure [Fe/H] and [alpha/Fe] in galaxies
We present differential stellar population models, which allow improved
determinations of the ages, iron and alpha-element abundances of old stellar
populations from spectral fitting. These new models are calibrated at solar
abundances using the predictions from classical, semi-empirical stellar
population models. We then use the predictive power of fully synthetic models
to compute predictions for different [Fe/H] and [alpha/Fe]. We show that these
new differential models provide remarkably accurate fits to the integrated
optical spectra of the bulge globular clusters NGC6528 and NGC6553, and that
the inferred [Fe/H] and [alpha/Fe] agree with values derived elsewhere from
stellar photometry and spectroscopy. The analysis of a small sample of SDSS
early-type galaxies further confirms that our alpha-enhanced models provide a
better fit to the spectra of massive ellipticals than the solar-scaled ones.
Our approach opens new opportunities for precision measurements of abundance
ratios in galaxies.Comment: 5 pages, 5 figures, MNRAS in pres
Large scale kinematics and dynamical modelling of the Milky Way nuclear star cluster
Within the central 10pc of our Galaxy lies a dense nuclear star cluster
(NSC), and similar NSCs are found in most nearby galaxies. Studying the
structure and kinematics of NSCs reveals the history of mass accretion of
galaxy nuclei. Because the Milky Way (MW) NSC is at a distance of only 8kpc, we
can spatially resolve the MWNSC on sub-pc scales. This makes the MWNSC a
reference object for understanding the formation of all NSCs. We have used the
NIR long-slit spectrograph ISAAC (VLT) in a drift-scan to construct an
integral-field spectroscopic map of the central 9.5 x 8pc of our Galaxy. We use
this data set to extract stellar kinematics both of individual stars and from
the unresolved integrated light spectrum. We present a velocity and dispersion
map from the integrated light and model these kinematics using kinemetry and
axisymmetric Jeans models. We also measure CO bandhead strengths of 1,375
spectra from individual stars. We find kinematic complexity in the NSCs radial
velocity map including a misalignment of the kinematic position angle by 9
degree counterclockwise relative to the Galactic plane, and indications for a
rotating substructure perpendicular to the Galactic plane at a radius of 20" or
0.8pc. We determine the mass of the NSC within r = 4.2pc to 1.4 x 10^7 Msun. We
also show that our kinematic data results in a significant underestimation of
the supermassive black hole (SMBH) mass. The kinematic substructure and
position angle misalignment may hint at distinct accretion events. This
indicates that the MWNSC grew at least partly by the mergers of massive star
clusters. Compared to other NSCs, the MWNSC is on the compact side of the r_eff
- M_NSC relation. The underestimation of the SMBH mass might be caused by the
kinematic misalignment and a stellar population gradient. But it is also
possible that there is a bias in SMBH mass measurements obtained with
integrated light.Comment: 20 pages, 19 Figures, Accepted for publication in A&
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