292 research outputs found
Strangers in the night: Discovery of a dwarf spheroidal galaxy on its first Local Group infall
We present spectroscopic observations of the AndXII dwarf spheroidal galaxy
using DEIMOS/Keck-II, showing it to be moving rapidly through the Local Group
(-556 km/s heliocentric velocity, -281 km/s relative to Andromeda from the MW),
falling into the Local Group from ~115 kpc beyond Andromeda's nucleus. AndXII
therefore represents a dwarf galaxy plausibly falling into the Local Group for
the first time, and never having experienced a dense galactic environment. From
Green Bank Telescope observations, a limit on the H{I} gas mass of <3000 Msun
suggests that AndXII's gas could have been removed prior to experiencing the
tides of the Local Group galaxies. Orbit models suggest the dwarf is close to
the escape velocity of M31 for published mass models. AndXII is our best direct
evidence for the late infall of satellite galaxies, a prediction of
cosmological simulations.Comment: 4 pages 5 figures 1 table, accepted in ApJ, july issu
Analysis and calibration of Ca ii triplet spectroscopy of red giant branch stars from VLT/FLAMES observations
We demonstrate that low-resolution Ca ii triplet (CaT) spectroscopic estimates of the overall metallicity ([Fe/H]) of individual red giant branch (RGB) stars in two nearby dwarf spheroidal galaxies (dSphs) agree to ±0.1-0.2 dex with detailed high-resolution spectroscopic determinations for the same stars over the range −2.5 < [Fe/H] < −0.5. For this study, we used a sample of 129 stars observed in low- and high-resolution modes with VLT/FLAMES in the Sculptor and Fornax dSphs. We also present the data-reduction steps we used in our low-resolution analysis and show that the typical accuracy of our velocity and CaT [Fe/H] measurement is ∼2 km s−1 and 0.1 dex, respectively. We conclude that CaT-[Fe/H] relations calibrated on globular clusters can be applied with confidence to RGB stars in composite stellar populations over the range −2.5 < [Fe/H] < −0.
VLT/FLAMES spectroscopy of red giant branch stars in the Fornax dwarf spheroidal galaxy
Fornax is one of the most massive dwarf spheroidal galaxies in the Local
Group. The Fornax field star population is dominated by intermediate age stars
but star formation was going on over almost its entire history. It has been
proposed that Fornax experienced a minor merger event. Despite recent progress,
only the high metallicity end of Fornax field stars ([Fe/H]>-1.2 dex) has been
sampled in larger number via high resolution spectroscopy. We want to better
understand the full chemical evolution of this galaxy by better sampling the
whole metallicity range, including more metal poor stars. We use the VLT-FLAMES
multi-fibre spectrograph in high-resolution mode to determine the abundances of
several alpha, iron-peak and neutron-capture elements in a sample of 47
individual Red Giant Branch stars in the Fornax dwarf spheroidal galaxy. We
combine these abundances with accurate age estimates derived from the age
probability distribution from the colour-magnitude diagram of Fornax. Similar
to other dwarf spheroidal galaxies, the old, metal-poor stars of Fornax are
typically alpha-rich while the young metal-rich stars are alpha-poor. In the
classical scenario of the time delay between SNe II and SNe Ia, we confirm that
SNe Ia started to contribute to the chemical enrichment at [Fe/H] between -2.0
and -1.8 dex. We find that the onset of SNe Ia took place between 12-10 Gyrs
ago. The high values of [Ba/Fe], [La/Fe] reflect the influence of SNe Ia and
AGB stars in the abundance pattern of the younger stellar population of Fornax.
Our findings of low [alpha/Fe] and enhanced [Eu/Mg] are compatible with an
initial mass function that lacks the most massive stars and with star formation
that kept going on throughout the whole history of Fornax. We find that massive
stars kept enriching the interstellar medium in alpha-elements, although they
were not the main contributor to the iron enrichment.Comment: Resubmitted to A&A (18/09/2014) after Referee's comment
Chemical Composition of Extremely Metal-Poor Stars in the Sextans Dwarf Spheroidal Galaxy
Chemical abundances of six extremely metal-poor ([Fe/H]<-2.5) stars in the
Sextans dwarf spheroidal galaxy are determined based on high resolution
spectroscopy (R=40,000) with the Subaru Telescope High Dispersion Spectrograph.
(1) The Fe abundances derived from the high resolution spectra are in good
agreement with the metallicity estimated from the Ca triplet lines in low
resolution spectra. The lack of stars with [Fe/H]=<-3 in Sextans, found by
previous estimates from the Ca triplet, is confirmed by our measurements,
although we note that high resolution spectroscopy for a larger sample of stars
will be necessary to estimate the true fraction of stars with such low
metallicity. (2) While one object shows an overabundance of Mg (similar to
Galactic halo stars), the Mg/Fe ratios of the remaining five stars are similar
to the solar value. This is the first time that low Mg/Fe ratios at such low
metallicities have been found in a dwarf spheroidal galaxy. No evidence for
over-abundances of Ca and Ti are found in these five stars, though the
measurements for these elements are less certain. Possible mechanisms to
produce low Mg/Fe ratios, with respect to that of Galactic halo stars, are
discussed. (3) Ba is under-abundant in four objects, while the remaining two
stars exhibit large and moderate excesses of this element. The abundance
distribution of Ba in this galaxy is similar to that in the Galactic halo,
indicating that the enrichment of heavy elements, probably by the r-process,
started at metallicities [Fe/H] < -2.5, as found in the Galactic halo.Comment: 15 pages, 6 figures, 6 tables, A&A, in pres
Analysis and calibration of CaII triplet spectroscopy of red giant branch stars from VLT/FLAMES observations
We demonstrate that low-resolution Ca II triplet (CaT) spectroscopic estimates of the overall metallicity ([Fe/H]) of individual red giant branch (RGB) stars in two nearby dwarf spheroidal galaxies (dSphs) agree to +/- 0.1-0.2 dex with detailed high-resolution spectroscopic determinations for the same stars over the range-2.5</p
Analysis of leukocyte membrane protein interactions using protein microarrays
BACKGROUND: Protein microarrays represent an emerging class of proteomic tools to investigate multiple protein-protein interactions in parallel. A sufficient proportion of immobilized proteins must maintain an active conformation and an orientation that allows for the sensitive and specific detection of antibody and ligand binding. In order to establish protein array technology for the characterization of the weak interactions between leukocyte membrane proteins, we selected the human leukocyte membrane protein CD200 (OX2) and its cell surface receptor (hCD200R) as a model system. As antibody-antigen reactions are generally of higher affinity than receptor-ligand binding, we first analyzed the reactivity of monoclonal antibodies (mAb) to normal and mutant forms of immobilized CD200R. RESULTS: Fluorescently labelled mAb DX147, DX136 and OX108 were specifically reactive with immobilized recombinant hCD200R extracellular region, over a range of 0.1–40 μg ml(-1 )corresponding to a limit of sensitivity of 0.01–0.05 femtomol per spot. Orientating hCD200R using capture antibodies, showed that DX147 reacts with an epitope spatially distinct from the more closely related DX136 and OX108 epitopes. A panel of soluble recombinant proteins with mutations in hCD200R domain 1 produced by transiently transfected cells, was arrayed directly without purification and screened for binding to the three mAb. Several showed decreased binding to the blocking mAb DX136 and OX108, suggesting close proximity of these epitopes to the CD200 binding site. Binding of hCD200 to directly immobilized rat, mouse, and hCD200R was achieved with multimeric ligands, in the form of biotinylated-hCD200 coupled to FITC-labelled avidin coated beads. CONCLUSION: We have achieved sensitive, specific and reproducible detection of immobilized CD200R with different antibodies and mapped antigenic epitopes for two mAb in the vicinity of the ligand binding site using protein microarrays. We also detected CD200 binding to its receptor, a low affinity interaction, using beads presenting multivalent ligands. Our results demonstrate the quantitative aspects of protein arrays and their potential use in detecting simultaneously multiple protein-protein interactions and in particular the weak interactions found between leukocyte membrane proteins
Nucleosynthesis And The Inhomogeneous Chemical Evolution Of The Carina Dwarf Galaxy
The detailed abundances of 23 chemical elements in nine bright red giant branch stars in the Carina dwarf spheroidal galaxy are presented based on high-resolution spectra gathered at the Very Large Telescope (VLT) and Magellan telescopes. A spherical model atmospheres analysis is applied using standard methods (local thermodynamic equilibrium and plane-parallel radiative transfer) to spectra ranging from 380 to 680 nm. Stellar parameters are found to be consistent between photometric and spectroscopic analyses, both at moderate and high resolution. The stars in this analysis range in metallicity from -2.9 < [Fe/H] < -1.3, and adopting the ages determined by Lemasle et al., we are able to examine the chemical evolution of Carina's old and intermediate-aged populations. One of the main results from this work is the evidence for inhomogeneous mixing in Carina and therefore for a poor statistical sampling of the supernova contributions when forming stars; a large dispersion in [Mg/Fe] indicates poor mixing in the old population, an offset in the [alpha/Fe] ratios between the old and intermediate-aged populations (when examined with previously published results) suggests that the second star formation event occurred in alpha-enriched gas, and one star, Car-612, seems to have formed in a pocket enhanced in SN Ia/II products. This latter star provides the first direct link between the formation of stars with enhanced SN Ia/II ratios in dwarf galaxies to those found in the outer Galactic halo (Ivans et al.). Another important result is the potential evidence for SNII driven winds. We show that the very metal-poor stars in Carina have not been enhanced in asymptotic giant branch or SN Ia products, and therefore their very low ratios of [Sr/Ba] suggests the loss of contributions from the early SNe II. Low ratios of [Na/Fe], [Mn/Fe], and [Cr/Fe] in two of these stars support this scenario, with additional evidence from the low [Zn/Fe] upper limit for one star. It is interesting that the chemistry of the metal-poor stars in Carina is not similar to those in the Galaxy, most of the other dwarf spheroidal galaxies, or the ultra faint dwarfs, and suggests that Carina may be at the critical mass where some chemical enrichments are lost through SN II driven winds.NSERCNSF AST 99-84073McDonald Observator
Two distinct ancient components in the Sculptor Dwarf Spheroidal Galaxy: First Results from DART
We have found evidence for the presence of two distinct ancient stellar
components (both geq 10 Gyr old) in the Sculptor dwarf spheroidal galaxy. We
used the ESO Wide Field Imager (WFI) in conjunction with the VLT/FLAMES
spectrograph to study the properties of the resolved stellar population of
Sculptor out to and beyond the tidal radius. We find that two components are
discernible in the spatial distribution of Horizontal Branch stars in our
imaging, and in the [Fe/H] and v_hel distributions for our large sample of
spectroscopic measurements. They can be generally described as a ``metal-poor''
component ([Fe/H] -1.7). The
metal-poor stars are more spatially extended than the metal-rich stars, and
they also appear to be kinematically distinct. These results provide an
important insight into the formation processes of small systems in the early
universe and the conditions found there. Even this simplest of galaxies appears
to have had a surprisingly complex early evolution.Comment: accepted for publication in ApJL 12 pages, 4 figure
Manganese in dwarf spheroidal galaxies
We provide manganese abundances (corrected for the effect of the hyperfine
structure) for a large number of stars in the dwarf spheroidal galaxies
Sculptor and Fornax, and for a smaller number in the Carina and Sextans dSph
galaxies. Abundances had already been determined for a number of other elements
in these galaxies, including alpha and iron-peak ones, which allowed us to
build [Mn/Fe] and [Mn/alpha] versus [Fe/H] diagrams. The Mn abundances imply
sub-solar [Mn/Fe] ratios for the stars in all four galaxies examined. In
Sculptor, [Mn/Fe] stays roughly constant between [Fe/H]\sim -1.8 and -1.4 and
decreases at higher iron abundance. In Fornax, [Mn/Fe] does not vary in any
significant way with [Fe/H]. The relation between [Mn/alpha] and [Fe/H] for the
dSph galaxies is clearly systematically offset from that for the Milky Way,
which reflects the different star formation histories of the respective
galaxies. The [Mn/alpha] behavior can be interpreted as a result of the
metal-dependent Mn yields of type II and type Ia supernovae. We also computed
chemical evolution models for star formation histories matching those
determined empirically for Sculptor, Fornax, and Carina, and for the Mn yields
of SNe Ia, which were assumed to be either constant or variable with
metallicity. The observed [Mn/Fe] versus [Fe/H] relation in Sculptor, Fornax,
and Carina can be reproduced only by the chemical evolution models that include
a metallicity-dependent Mn yield from the SNe Ia.Comment: 19 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
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