1,484 research outputs found
A comprehensive Maximum Likelihood analysis of the structural properties of faint Milky Way satellites
We derive the structural parameters of the recently discovered very low
luminosity Milky Way satellites through a Maximum Likelihood algorithm applied
to SDSS data. For each satellite, even when only a few tens of stars are
available down to the SDSS flux limit, the algorithm yields robust estimates
and errors for the centroid, position angle, ellipticity, exponential
half-light radius and number of member stars. This latter parameter is then
used in conjunction with stellar population models of the satellites to derive
their absolute magnitudes and stellar masses, accounting for `CMD shot-noise'.
We find that faint systems are somewhat more elliptical than initially found
and ascribe that to the previous use of smoothed maps which can be dominated by
the smoothing kernel. As a result, the faintest half of the Milky Way dwarf
galaxies (M_V>-7.5) is significantly (4-sigma) flatter (e=0.47+/-0.03) than its
brightest half (M_V<-7.5, e=0.32+/-0.02). From our best models, we also
investigate whether the seemingly distorted shape of the satellites, often
taken to be a sign of tidal distortion, can be quantified. We find that, except
for tentative evidence of distortion in CVnI and UMaII, these can be completely
accounted for by Poisson scatter in the sparsely sampled systems. We consider
three scenarios that could explain the rather elongated shape of faint
satellites: rotation supported systems, stars following the shape of more
triaxial dark matter subhalos, or elongation due to tidal interaction with the
Milky Way. Although none of these is entirely satisfactory, the last one
appears the least problematic, but warrants much deeper observations to track
evidence of such tidal interaction.Comment: 20 pages, 11 figures, ApJ in press; some typos corrected, magnitude
of BooII corrected (thanks go to Shane Walsh for spotting the erroneous
original value
Mapping low-latitude stellar substructure with SEGUE photometry
Encircling the Milky Way at low latitudes, the Low Latitude Stream is a large
stellar structure, the origin of which is as yet unknown. As part of the SEGUE
survey, several photometric scans have been obtained that cross the Galactic
plane, spread over a longitude range of 50 to 203 degrees. These data allow a
systematic study of the structure of the Galaxy at low latitudes, where the Low
Latitude Stream resides. We apply colour-magnitude diagram fitting techniques
to map the stellar (sub)structure in these regions, enabling the detection of
overdensities with respect to smooth models. These detections can be used to
distinguish between different models of the Low Latitude Stream, and help to
shed light on the nature of the system.Comment: To appear in the proceedings of IAU Symposium 254 "The Galaxy disk in
a cosmological context", Copenhagen, June 200
Understanding farmer needs and unlocking local genetic resources for potato improvement: a case study in Ethiopia
Understanding farmers’ needs and local genetic resources are crucial steps to improve and conserve the potato crop. A study was conducted to understand what potato traits Ethiopian farmers consider most important, and to characterize the diversity and distribution of local varieties. Growers from six districts were surveyed in 2012 and 2014. Based on the survey results, participatory variety selection (PVS) activities were conducted in two districts during two production seasons. Simultaneously, local varieties were collected from northwest and southern Ethiopia and characterized using molecular and morphological markers. Farmers identified 23 traits they considered important for variety selection, with the degree of importance for each trait varying across gender groups, agro-ecological zones, and growing seasons, as well as with extent of market access. The distribution of local varieties varied by agro-ecological zone, cropping system and proximity to markets. Our genetic fingerprinting and morphological characterization further revealed that, 34% of 44 local potato varieties collected are truly unique, the rest were duplicates, known by different names. These unique Ethiopian local varieties harbour considerable genetic variation, compared to the variation found in European and North American clones. Although local varieties may have lower yield than commercial varieties, they have other desirable attributes that make them well suited for alternative uses and different agro-ecologies.Keywords: Participatory variety selection, Gender, Local potato varieties, agroecological zones, growing seasons, genetic fingerprintin
The enigmatic pair of dwarf galaxies Leo IV and Leo V: coincidence or common origin?
We have obtained deep photometry in two 1x1 degree fields covering the close
pair of dwarf spheroidal galaxies (dSph) Leo IV and Leo V and part of the area
in between. We find that both systems are significantly larger than indicated
by previous measurements based on shallower data and also significantly
elongated. With half-light radii of r_h=4'.6 +- 0'.8 (206 +- 36 pc) and
r_h=2'.6 +- 0'.6 (133 +- 31 pc), respectively, they are now well within the
physical size bracket of typical Milky Way dSph satellites. Their ellipticities
of epsilon ~0.5 are shared by many faint (M_V>-8) Milky Way dSphs. The large
spatial extent of our survey allows us to search for extra-tidal features with
unprecedented sensitivity. The spatial distribution of candidate red giant
branch and horizontal branch stars is found to be non-uniform at the ~3 sigma
level. This substructure is aligned along the direction connecting the two
systems, indicative of a possible `bridge' of extra-tidal material. Fitting the
stellar distribution with a linear Gaussian model yields a significance of 4
sigma for this overdensity, a most likely FWHM of ~16 arcmin and a central
surface brightness of ~32 mag arcsec^{-2}. We investigate different scenarios
to explain the close proximity of Leo IV and Leo V and the possible tidal
bridge between them. Orbit calculations demonstrate that they are unlikely to
be remnants of a single disrupted progenitor, while a comparison with
cosmological simulations shows that a chance collision between unrelated
subhalos is negligibly small. Leo IV and Leo V could, however, be a bound
`tumbling pair' if their combined mass exceeds 8 +- 4 x 10^9 M_sun. The
scenario of an internally interacting pair appears to be the most viable
explanation for this close celestial companionship. (abridged)Comment: 9 pages, 8 figures, small number of minor textual changes, accepted
for publication in Astrophysical Journa
Mismatch between the 1997/1998 influenza vaccine and the major epidemic A (H3N2) virus strain as the cause of an inadequate vaccine-induced antibody response to this strain in the elderly.
The success of influenza vaccination depends largely on the antigenic match between the influenza vaccine strains and the virus strains actually circulating during the season. In the past, this match has proved to be satisfactory in most seasons. In the 1997/1998 season, however, hemagglutination inhibition (HI) assays with ferret antisera indicated a considerable mismatch between the H3N2 vaccine component and the most prevalent epidemic influenza A(H3N2) virus. The results from antigenic analyses using pre- and postvaccination serum samples from volunteers of various ages, including residents of nursing homes who were more than 60 years of age, were in good agreement with the results obtained with ferret antisera. Homologous serum antibody responses to the H3N2 vaccine component as well as the cross-reactivity of the induced antibodies to the epidemic H3N2 strain, declined with increasing age of the vaccinees. As a consequence of these two effects, 84% of the vaccinees over 75 years of age did not develop HI antibody titers >/= 40 against the major H3N2 virus variant of 1997/1998, suggesting that they were not protected against infection with this virus variant. These findings support the current policy of the World Health Organization (WHO), which is to base worldwide influenza virus surveillance on results predominantly obtained by antigenic analyses of influenza virus isolates with ferret antisera in HI tests. If an antigenic mismatch is observed, the protective efficacy of the vaccine, especially for the elderly, may be insufficient. The observations also support the current policy to include the elderly in serologic efficacy trials
Mapping the stellar structure of the Milky Way thick disk and halo using SEGUE photometry
We map the stellar structure of the Galactic thick disk and halo by applying
color-magnitude diagram (CMD) fitting to photometric data from the SEGUE
survey, allowing, for the first time, a comprehensive analysis of their
structure at both high and low latitudes using uniform SDSS photometry.
Incorporating photometry of all relevant stars simultaneously, CMD fitting
bypasses the need to choose single tracer populations. Using old stellar
populations of differing metallicities as templates we obtain a sparse 3D map
of the stellar mass distribution at |Z|>1 kpc. Fitting a smooth Milky Way model
comprising exponential thin and thick disks and an axisymmetric power-law halo
allows us to constrain the structural parameters of the thick disk and halo.
The thick-disk scale height and length are well constrained at 0.75+-0.07 kpc
and 4.1+-0.4 kpc, respectively. We find a stellar halo flattening within ~25
kpc of c/a=0.88+-0.03 and a power-law index of 2.75+-0.07 (for 7<R_{GC}<~30
kpc). The model fits yield thick-disk and stellar halo densities at the solar
location of rho_{thick,sun}=10^{-2.3+-0.1} M_sun pc^{-3} and
rho_{halo,sun}=10^{-4.20+-0.05} M_sun pc^{-3}, averaging over any
substructures. Our analysis provides the first clear in situ evidence for a
radial metallicity gradient in the Milky Way's stellar halo: within R<~15 kpc
the stellar halo has a mean metallicity of [Fe/H]=-1.6, which shifts to
[Fe/H]=-2.2 at larger radii. Subtraction of the best-fit smooth and symmetric
model from the overall density maps reveals a wealth of substructures at all
latitudes, some attributable to known streams and overdensities, and some new.
A simple warp cannot account for the low latitude substructure, as
overdensities occur simultaneously above and below the Galactic plane.
(abridged)Comment: 13 pages, 10 figures, accepted for publication in Astrophysical
Journa
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