540 research outputs found
Continuous star cluster formation in the spiral NGC 45
We determined ages for 52 star clusters with masses < 10^6 solar masses in
the low surface brightness spiral galaxy NGC 45. Four of these candidates are
old globular clusters located in the bulge. The remaining ones span a large age
range. The cluster ages suggest a continuous star/cluster formation history
without evidence for bursts, consistent with the galaxy being located in a
relatively unperturbed environment in the outskirts of the Sculptor group.Comment: 4 pages, 3 figures. To appear in "Island Universes - Structure and
Evolution of Disk Galaxies", Terschelling (Netherlands), July 200
Accurate photometry of extended spherically symmetric sources
We present a new method to derive reliable photometry of extended spherically
symmetric sources from {\it HST} images (WFPC2, ACS/WFC and NICMOS/NIC2
cameras), extending existing studies of point sources and marginally resolved
sources. We develop a new approach to accurately determine intrinsic sizes of
extended spherically symmetric sources, such as star clusters in galaxies
beyond the Local Group (at distances <~ 20 Mpc), and provide a detailed
cookbook to perform aperture photometry on such sources, by determining
size-dependent aperture corrections (ACs) and taking sky oversubtraction as a
function of source size into account. In an extensive Appendix, we provide the
parameters of polynomial relations between the FWHM of various input profiles
and those obtained by fitting a Gaussian profile (which we have used for
reasons of computational robustness, although the exact model profile used is
irrelevant), and between the intrinsic and measured FWHM of the cluster and the
derived AC. Both relations are given for a number of physically relevant
cluster light profiles, intrinsic and observational parameters. AC relations
are provided for a wide range of apertures. Depending on the size of the source
and the annuli used for the photometry, the absolute magnitude of such extended
objects can be underestimated by up to 3 mag, corresponding to an error in mass
of a factor of 15. We carefully compare our results to those from the more
widely used DeltaMag method, and find an improvement of a factor of 3--40 in
both the size determination and the AC.Comment: The paper is accepted for publication in A&A, Section 13
(Observational Techniques, published electronically). The published version
contains one example table per appendix. A version of the paper containing
all tables as well as all data in electronical form are available
http://www.astro.physik.uni-goettingen.de/~galev/panders/Sizes_AC
The young star cluster system of the Antennae galaxies
âThe original publication is available at www.springerlink.comâ. Copyright Springer. DOI: 10.1007/s10509-009-0103-xThe study of young star cluster (YSC) systems, preferentially in starburst and merging galaxies, has seen great interest in the recent past, as it provides important input to models of star formation. However, even some basic properties (such as the luminosity function; LF) of YSC systems are still being debated. Here, we study the photometric properties of the YSC system in the nearest major merger system, the Antennae galaxies. We find evidence for the existence of a statistically significant turnover in the LF.Peer reviewe
Detecting metal-rich intermediate-age globular clusters in NGC4570 using K-band photometry
âThe original publication is available at www.springerlink.comâ. Copyright Springer. DOI: 10.1007/s10509-009-0093-8Globular cluster systems (GCSs) of most early-type galaxies feature two peaks in their optical colour distributions. Blue-peak globular clusters (GCs) are believed to be old and metal-poor, whereas the ages, metallicities, and the origin of the red-peak GCs are still being debated. We obtained deep K-band photometry and combined it with Hubble Space Telescope observations in g and z to yield a full spectral energy distribution from the optical to the near-infrared. This now allows us to break the ageâmetallicity degeneracy. We used our evolutionary synthesis models galev for star clusters to compute a large grid of models with different metallicities and a wide range of ages. Comparing these models to our observations revealed a large population of intermediate-age (1â3 Gyr) and metal-rich (âsolar-metallicity) GCs, that will give us further insights into the formation history of this galaxy.Peer reviewe
Dynamical mass of a star cluster in M83: a test of fibre-fed multi-object spectroscopy
(Abridged) Aims: We obtained VLT/FLAMES+UVES high-resolution, fibre-fed
spectroscopy (FFS) of five young massive clusters in M83 (NGC 5236). This forms
the basis of a pilot study testing the feasibility of using FFS to measure the
velocity dispersions of several clusters simultaneously, in order to determine
their dynamical masses; Methods: We adopted two methods for determining the
velocity dispersion of the star clusters: cross-correlating the cluster
spectrum with the template spectra and minimising a chi^2 value between the
cluster spectrum and the broadened template spectra. Cluster 805 in M83 was
chosen as a control to test the reliability of the method, through a comparison
with the results obtained from a standard echelle VLT/UVES spectrum obtained by
Larsen & Richtler; Results: We find no dependence of the velocity dispersions
measured for a cluster on the choice of red giant versus red supergiant
templates, nor on the method adopted. We measure a velocity dispersion of
sigma_los = 10.2+/-1.1 km/s for cluster 805 from our FFS. Our FLAMES+UVES
velocity dispersion measurement gives M_vir = (6.6+/-1.7)e5 M_sun, consistent
with previous results. This is a factor of ~3 greater than the cluster's
photometric mass, indicating a lack of virial equilibrium. However, based on
its effective star formation efficiency, the cluster is likely to virialise,
and may survive for a Hubble time, in the absence of external disruptive
forces; Conclusions: We find that reliable velocity dispersions can be
determined from FFS. The advantages of observing several clusters
simultaneously outweighs the difficulty of accurate galaxy background
subtraction, providing that the targets are chosen to provide sufficient S/N
ratios, and are much brighter than the galaxy background.Comment: 10 pages, 4 figures, accepted by A&
Super star clusters in Haro 11: Properties of a very young starburst and evidence for a near-infrared flux excess
We have used multi-band imaging to investigate the nature of the extreme
starburst environment in Haro 11 galaxy. The central starburst region has been
observed in 8 HST wavebands and at 2.16 micron at the ESO-VLT. We constructed
integrated spectral energy distributions (SEDs) for about 200 star clusters and
compared them with single stellar population models in order to derive ages,
masses and extinctions of thestar clusters. The present starburst has lasted
for 40 Myr, and shows a peak of cluster formation only 3.5 Myr old. With such
an extremely young cluster population, Haro 11 represents a unique opportunity
to investigate the youngest phase of the cluster formation process and
evolution in starburst systems. Extinction tends to diminish as function of the
cluster age, but the spread is large and for clusters in partial embedded
phases ( 10^4 Msun), very young (1-3 Myr)
clusters is missing, either because they are embedded inthe parental molecular
cloud and heavily extinguished, or because of blending. Almost half of the
cluster sample is affected by flux excesses at wavelengths 8000 \AA which
cannot be explained by simple stellar evolutionary models. Fitting SED models
over all wavebands leads to systematic overestimates of cluster ages and
incorrect masses for the stellar population supplying the light in these
clusters. We show that the red excess affects also the HST F814W filter, which
is typically used to constrain cluster physical properties. The clusters which
show the red excess are younger than 40 Myr; we propose possible physical
explanations for the phenomenon. Finally, we estimate that Haro 11 hasproduced
bound clusters at a rate almost a factor of 10 higher than the massive and
regular spirals, like the Milky Way. (Abriged)Comment: Accepted for publication in MNRAS. 23 pages, 23 figure
Superconducting Gatemon Qubit based on a Proximitized Two-Dimensional Electron Gas
The coherent tunnelling of Cooper pairs across Josephson junctions (JJs)
generates a nonlinear inductance that is used extensively in quantum
information processors based on superconducting circuits, from setting qubit
transition frequencies and interqubit coupling strengths, to the gain of
parametric amplifiers for quantum-limited readout. The inductance is either set
by tailoring the metal-oxide dimensions of single JJs, or magnetically tuned by
parallelizing multiple JJs in superconducting quantum interference devices
(SQUIDs) with local current-biased flux lines. JJs based on
superconductor-semiconductor hybrids represent a tantalizing all-electric
alternative. The gatemon is a recently developed transmon variant which employs
locally gated nanowire (NW) superconductor-semiconductor JJs for qubit control.
Here, we go beyond proof-of-concept and demonstrate that semiconducting
channels etched from a wafer-scale two-dimensional electron gas (2DEG) are a
suitable platform for building a scalable gatemon-based quantum computer. We
show 2DEG gatemons meet the requirements by performing voltage-controlled
single qubit rotations and two-qubit swap operations. We measure qubit
coherence times up to ~2 us, limited by dielectric loss in the 2DEG host
substrate
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