Origin of spatial variations of scattering polarization in the wings of the Ca {\sc i} 4227 \AA line

Polarization that is produced by coherent scattering can be modified by magnetic fields via the Hanle effect. According to standard theory the Hanle effect should only be operating in the Doppler core of spectral lines but not in the wings. In contrast, our observations of the scattering polarization in the Ca {\sc i} 4227 \AA line reveals the existence of spatial variations of the scattering polarization throughout the far line wings. This raises the question whether the observed spatial variations in wing polarization have a magnetic or non-magnetic origin. A magnetic origin may be possible if elastic collisions are able to cause sufficient frequency redistribution to make the Hanle effect effective in the wings without causing excessive collisional depolarization, as suggested by recent theories for partial frequency redistribution with coherent scattering in magnetic fields. To model the wing polarization we apply an extended version of the technique based on the "last scattering approximation". This model is highly successful in reproducing the observed Stokes $Q/I$ polarization (linear polarization parallel to the nearest solar limb), including the location of the wing polarization maxima and the minima around the Doppler core, but it fails to reproduce the observed spatial variations of the wing polarization in terms of magnetic field effects with frequency redistribution. This null result points in the direction of a non-magnetic origin in terms of local inhomogeneities (varying collisional depolarization, radiation-field anisotropies, and deviations from a plane-parallel atmospheric stratification).Comment: Accepted in May 2009 for publication in The Astrophysical Journa

Zero-Field Dichroism in the Solar Chromosphere

We explain the linear polarization of the Ca II infrared triplet observed close to the edge of the solar disk. In particular, we demonstrate that the physical origin of the enigmatic polarizations of the 866.2 nm and 854.2 nm lines lies in the existence of atomic polarization in their metastable lower levels, which produces differential absorption of polarization components (dichroism). To this end, we have solved the problem of the generation and transfer of polarized radiation by taking fully into account all the relevant optical pumping mechanisms in multilevel atomic models. We argue that `zero-field' dichroism may be of great diagnostic value in astrophysics.Comment: 10 pages, 3 figure

The stellar host in blue compact dwarf galaxies: the need for a two-dimensional fit

The structural properties of the low surface brightness stellar host in blue compact dwarf galaxies are often studied by fitting r^{1/n} models to the outer regions of their radial profiles. The limitations imposed by the presence of a large starburst emission overlapping the underlying component makes this kind of analysis a difficult task. We propose a two-dimensional fitting methodology in order to improve the extraction of the structural parameters of the LSB host. We discuss its advantages and weaknesses by using a set of simulated galaxies and compare the results for a sample of eight objects with those already obtained using a one-dimensional technique. We fit a PSF convolved Sersic model to synthetic galaxies, and to real galaxy images in the B, V, R filters. We restrict the fit to the stellar host by masking out the starburst region and take special care to minimize the sky-subtraction uncertainties. In order to test the robustness and flexibility of the method, we carry out a set of fits with synthetic galaxies. Furthermore consistency checks are performed to assess the reliability and accuracy of the derived structural parameters. The more accurate isolation of the starburst emission is the most important advantage and strength of the method. Thus, we fit the host galaxy in a range of surface brightness and in a portion of area larger than in previous published 1D fits with the same dataset. We obtain robust fits for all the sample galaxies, all of which, except one, show Sersic indices n very close to 1, with good agreement in the three bands. These findings suggest that the stellar hosts in BCDs have near-exponential profiles, a result that will help us to understand the mechanisms that form and shape BCD galaxies, and how they relate to the other dwarf galaxy classes.Comment: 22 pages, 15 figures (low resolution), accepted for publication in A&A. A higher resolution version of the figures can be provided upon reques

Stellar population and the origin of intra-cluster stars around brightest cluster galaxies: the case of NGC 3311

Context. We investigate the stellar population and the origin of diffuse light around brightest cluster galaxies. Aims. We study the stellar population of the dynamically hot stellar halo of NGC 3311, the brightest galaxy in the Hydra I cluster, and that of photometric substructures in the diffuse light to constrain the origin of these components. Methods. We analyze absorption lines in medium-resolution, long-slit spectra in the wavelength range 4800-5800 angstrom obtained with FORS2 at the Very Large Telescope. We measure the equivalent width of Lick indices out to 20 kpc from the center of NGC 3311 and fit them with stellar population models that account for the [alpha/Fe] overabundance. Results. Stars in the dynamically hot halo of NGC 3311 are old (age > 13 Gyr), metal-poor ([Z/H] ~ -0.35), and alpha-enhanced ([alpha/Fe] ~ 0.48). Together with the high velocity dispersion, these measurements indicate that the stars in the halo were accreted from the outskirts of other early-type galaxies, with a possible contribution from dwarf galaxies. We identify a region in the halo of NGC 3311 associated with a photometric substructure where the stellar population is even more metal-poor ([Z/H] ~ -0.73). In this region, our measurements are consistent with a composite stellar population superposed along the line of sight, consisting of stars from the dynamically hot halo of NGC 3311 and stars stripped from dwarf galaxies. The latter component contributes < 28% to the local surface brightness. Conclusions. The build-up of diffuse light around NGC 3311 is on-going. Based on the observed stellar population properties, the dominant part of these stars may have come from the outskirts of bright early-type galaxies, while stars from stripped dwarf galaxies are presently being added.Comment: 8 pages, 4 figures. Accepted for publication in Astronomy & Astrophysic

A machine vision system using circular autoregressive models for rapid recognition of salmonella typhimurium

The objective of this research was to develop a machine vision system using image processing and statistical modeling techniques to identify and enumerate bacteria on slides containing Salmonella typhimurium. Pictures of bacterial cells were acquired with a CCD camera attached to a motorized fluorescence microscope. A shape boundary modeling technique, based on the use of circular autoregressive model parameters, was used. A feature weighting classifier was trained with ten images belonging to each shape class (rod shape and circle shape). In order to enhance the discrimination of circular shapes, a size range was added to the recognition algorithm. Experimental results showed that the model parameters could be used as descriptors of shape boundaries detected in digitized binary images of bacterial cells. The introduction of the rotated coordinate method and the circular size restriction, reduced the differences between automated and manual recognition/enumeration from 7% to less than 1%. The computer analyzed each image in approximately 5 s (a total of 2 h including sample preparation), while the bacteriologist spent an average of 1 min for each image

Improved timed-mating, non-invasive method using fewer unproven female rats with pregnancy validation via early body mass increases

For studies requiring accurate conception-timing, reliable, efficient methods of detecting oestrus reduce time and costs, whilst improving welfare. Standard methods use vaginal cytology to stage cycle, and breeders are paired–up using approximately five proven females with proven males to achieve at least one conception on a specific day. We describe an alternative, fast, consistent, non-invasive method of timed-mating using detection of lordosis behaviour in Wistar and Lister-Hooded rats that used unproven females with high success rates. Rats under reverse-lighting had body masses recorded pre-mating, day (d) 3-4, d8, d10 and d18 of pregnancy. Using only the presence of the oestrus dance to time-mate females for 24-hrs, 89% Wistar and 88% Lister-Hooded rats successfully conceived. We did not observe behavioural oestrus in Sprague-Dawleys without males present. Significant body mass increases following mating distinguished pregnant from non-pregnant rats, as early as d4 of pregnancy (10% ± 1.0 increase cf 3% ± 1.2). The pattern of increases throughout gestation was similar for all pregnant rats until late pregnancy, when there were smaller increases for primi- and multiparous rats (32% ± 2.5; 25% ± 2.4), whereas nulliparous rats had highest gains (38% ± 1.5). This method demonstrated a distinct refinement of the previous timed-mating common practice used, as disturbance of females was minimised. Only the number required of nulli-, primi- or multiparous rats were mated, and body mass increases validated pregnancy status. This new breeding-management method is now established practice for two strains of rat and resulted in a reduction in animal use

L1 transfer in the acquisition of manner and path in Spanish by native speakers of English

In this article the authors argue that L1 transfer from English is not only important in the early stages of L2 acquisition of Spanish, but remains influential in later stages if there is not enough positive evidence for the learners to progress in their development (Lefebvre, White, & Jourdan, 2006). The findings are based on analyses of path and manner of movement in stories told by British students of Spanish (N = 68) of three different proficiency levels. Verbs that conflate motion and path, on the one hand, are mastered early, possibly because the existence of Latinate path verbs, such as enter and ascend in English, facilitate their early acquisition by British learners of Spanish. Contrary to the findings of Cadierno (2004) and Cadierno and Ruiz (2006), the encoding of manner, in particular in boundary crossing contexts, seems to pose enormous difficulties, even among students who had been abroad on a placement in a Spanish-speaking country prior to the data collection. An analysis of the frequency of manner verbs in Spanish corpora shows that one of the key reasons why students struggle with manner is that manner verbs are so infrequent in Spanish. The authors claim that scarce positive evidence in the language exposed to and little or no negative evidence are responsible for the long-lasting effect of transfer on the expression of manner

Alignment of galaxy spins in the vicinity of voids

We provide limits on the alignment of galaxy orientations with the direction to the void center for galaxies lying near the edges of voids. We locate spherical voids in volume limited samples of galaxies from the Sloan Digital Sky Survey using the HB inspired void finder and investigate the orientation of (color selected) spiral galaxies that are nearly edge-on or face-on. In contrast with previous literature, we find no statistical evidence for departure from random orientations. Expressed in terms of the parameter c, introduced by Lee & Pen to describe the strength of such an alignment, we find that c<0.11(0.13) at 95% (99.7%) confidence limit within a context of a toy model that assumes a perfectly spherical voids with sharp boundaries.Comment: 8 pages, 4 figures; v2 discussion expanded, references fixed, matches version accepted by JCA