180 research outputs found
Resolving stellar populations with crowded field 3D spectroscopy
(Abridged) We describe a new method to extract spectra of stars from
observations of crowded stellar fields with integral field spectroscopy (IFS).
Our approach extends the well-established concept of crowded field photometry
in images into the domain of 3-dimensional spectroscopic datacubes. The main
features of our algorithm are: (1) We assume that a high-fidelity input source
catalogue already exists and that it is not needed to perform sophisticated
source detection in the IFS data. (2) Source positions and properties of the
point spread function (PSF) vary smoothly between spectral layers of the
datacube, and these variations can be described by simple fitting functions.
(3) The shape of the PSF can be adequately described by an analytical function.
Even without isolated PSF calibrator stars we can therefore estimate the PSF by
a model fit to the full ensemble of stars visible within the field of view. (4)
By using sparse matrices to describe the sources, the problem of extracting the
spectra of many stars simultaneously becomes computationally tractable. We
present extensive performance and validation tests of our algorithm using
realistic simulated datacubes that closely reproduce actual IFS observations of
the central regions of Galactic globular clusters. We investigate the quality
of the extracted spectra under the effects of crowding. The main effect of
blending between two nearby stars is a decrease in the S/N in their spectra.
The effect increases with the crowding in the field in a way that the maximum
number of stars with useful spectra is always ~0.2 per spatial resolution
element. This balance breaks down when exceeding a total source density of ~1
significantly detected star per resolution element. We close with an outlook by
applying our method to a simulated globular cluster observation with the
upcoming MUSE instrument at the ESO-VLT.Comment: accepted for publication in A&A, 19 pages, 19 figure
The central dynamics of M3, M13, and M92: Stringent limits on the masses of intermediate-mass black holes
We used the PMAS integral field spectrograph to obtain large sets of radial
velocities in the central regions of three northern Galactic globular clusters:
M3, M13, and M92. By applying the novel technique of crowded field 3D
spectroscopy, we measured radial velocities for about 80 stars within the
central ~ 10 arcsec of each cluster. These are by far the largest spectroscopic
datasets obtained in the innermost parts of these clusters up to now. To obtain
kinematical data across the whole extent of the clusters, we complement our
data with measurements available in the literature. We combine our velocity
measurements with surface brightness profiles to analyse the internal dynamics
of each cluster using spherical Jeans models, and investigate whether our data
provide evidence for an intermediate-mass black hole in any of the clusters.
The surface brightness profiles reveal that all three clusters are consistent
with a core profile, although shallow cusps cannot be excluded. We find that
spherical Jeans models with a constant mass-to-light ratio provide a good
overall representation of the kinematical data. A massive black hole is
required in none of the three clusters to explain the observed kinematics. Our
1sigma (3sigma) upper limits are 5300 M_sun (12000 M_sun) for M3, 8600 M_sun
(13000 M_sun) for M13, and 980 M_sun (2700 M_sun) for M92. A puzzling
circumstance is the existence of several potential high velocity stars in M3
and M13, as their presence can account for the majority of the discrepancies
that we find in our mass limits compared to M92.Comment: accepted for publication in A&A, 20 pages, 15 figures, tables D1 to
D6 only available at CD
Towards DIB mapping in galaxies beyond 100 Mpc. A radial profile of the 5780.5 diffuse interstellar band in AM 1353-272 B
Diffuse Interstellar Bands (DIBs) are non-stellar weak absorption features of
unknown origin found in the spectra of stars viewed through one or several
clouds of Interstellar Medium (ISM). Research of DIBs outside the Milky Way is
currently very limited. Specifically spatially resolved investigations of DIBs
outside of the Local Group is, to our knowledge, inexistent. Here, we explore
the capability of the high sensitivity Integral Field Spectrograph, MUSE, as a
tool to map diffuse interstellar bands at distances larger than 100 Mpc. We use
MUSE commissioning data for AM 1353-272 B, the member with highest extinction
of the "The Dentist's Chair", an interacting system of two spiral galaxies.
High signal-to-noise spectra were created by co-adding the signal of many
spatial elements distributed in a geometry of concentric elliptical half-rings.
We derived decreasing radial profiles for the equivalent width of the
5780.5 DIB both in the receding and approaching side of the companion
galaxy up to distances of 4.6 kpc from the center of the galaxy.
Likewise, interstellar extinction, as derived from the Halpha/Hbeta line ratio
displays a similar trend, with decreasing values towards the external parts.
This translates into an intrinsic correlation between the strength of the DIB
and the extinction within AM 1353-272 B consistent with the current existing
global trend between these quantities when using measurements for both Galactic
and extragalactic sight lines. Mapping of DIB strength in the Local Universe as
up to now only done for the Milky Way seems feasible. This offers a new
approach to study the relationship between DIBs and other characteristics and
species of the ISM in different conditions as those found in our Galaxy to the
use of galaxies in the Local Group and/or single sightlines towards supernovae,
quasars and galaxies outside the Local Group.Comment: 4 pages, 4 figures, accepted for publication as a Letter in Astronomy
and Astrophysics; Received 10 February 2015 / Accepted 20 February 2015 ;
English corrections include
Photoexcitation of valley-orbit currents in (111)-oriented silicon metal-oxide-semiconductor field-effect transistors
We demonstrate the injection of pure valley-orbit currents in multivalley semiconductors and present the phenomenological theory of this effect. We studied photoinduced transport in (111)-oriented silicon metaloxide-semiconductor ïŹeld effect transistors at room temperature. By shining circularly polarized light on exact oriented structures with six equivalent valleys, nonzero electron ïŹuxes within each valley are generated, which
compensate each other and do not yield a net electric current. By disturbing the balance between the valley ïŹuxes, we demonstrate that the pure valley-orbit currents can be converted into a measurable electric current
Discovery of an old nova remnant in the Galactic globular cluster M 22
A nova is a cataclysmic event on the surface of a white dwarf in a binary
system that increases the overall brightness by several orders of magnitude.
Although binary systems with a white dwarf are expected to be overabundant in
globular clusters (GCs) compared to the Galaxy, only two novae from Galactic
globular clusters have been observed. We present the discovery of an emission
nebula in the Galactic globular cluster M 22 (NGC 6656) in observations made
with the integral-field spectrograph MUSE. We extract the spectrum of the
nebula and use the radial velocity determined from the emission lines to
confirm that the nebula is part of NGC 6656. Emission-line ratios are used to
determine the electron temperature and density. It is estimated to have a mass
of 1 to solar masses. This mass and the emission-line
ratios indicate that the nebula is a nova remnant. Its position coincides with
the reported location of a 'guest star', an ancient Chinese term for
transients, observed in May 48 BCE. With this discovery, this nova may be one
of the oldest confirmed extrasolar events recorded in human history.Comment: 7 pages, 3 figures; accepted for publication in Astronomy &
Astrophysic
Classical ratchet effects in heterostructures with a lateral periodic potential
We study terahertz radiation induced ratchet currents in low dimensional
semiconductor structures with a superimposed one-dimensional lateral periodic
potential. The periodic potential is produced by etching a grating into the
sample surface or depositing metal stripes periodically on the sample top.
Microscopically, the photocurrent generation is based on the combined action of
the lateral periodic potential, verified by transport measurements, and the
in-plane modulated pumping caused by the lateral superlattice. We show that a
substantial part of the total current is caused by the polarization-independent
Seebeck ratchet effect. In addition, polarization-dependent photocurrents
occur, which we interpret in terms of their underlying microscopical
mechanisms. As a result, the class of ratchet systems needs to be extended by
linear and circular ratchets, sensitive to linear and circular polarizations of
the driving electro-magnetic force.Comment: 11 pages, 9 figures, 2 column
A MUSE map of the central Orion Nebula (M 42)
We present a new integral-field spectroscopic dataset of the central part of
the Orion Nebula (M 42), observed with the MUSE instrument at the ESO VLT. We
reduced the data with the public MUSE pipeline. The output products are two
FITS cubes with a spatial size of ~5.9'x4.9' (corresponding to ~0.76 pc x 0.63
pc) and a contiguous wavelength coverage of 4595...9366 Angstrom, spatially
sampled at 0.2". We provide two versions with a sampling of 1.25 Angstrom and
0.85 Angstrom in dispersion direction. Together with variance cubes these files
have a size of 75 and 110 GiB on disk. They represent one of the largest
integral field mosaics to date in terms of information content. We make them
available for use in the community. To validate this dataset, we compare world
coordinates, reconstructed magnitudes, velocities, and absolute and relative
emission line fluxes to the literature and find excellent agreement. We derive
a two-dimensional map of extinction and present de-reddened flux maps of
several individual emission lines and of diagnostic line ratios. We estimate
physical properties of the Orion Nebula, using the emission line ratios [N II]
and [S III] (for the electron temperature ) and [S II] and [Cl III] (for
the electron density ), and show two-dimensional images of the velocity
measured from several bright emission lines.Comment: Resubmitted to A&A after incorporating referee comments; access to
full dataset via http://muse-vlt.eu/science/data-release
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