4,378 research outputs found
A Regulatory Impact Analysis (RIA) approach based on evolutionary association patterns
The present paper focuses on ex post analysis to asses the impact of an adopted policy by measuring system performance. Since accurate impact assessment requires in-depth knowledge of the structure underlying the system, this contribution proposes a suitable use of multidimensional data analysis (MDA) to investigate the associations characterizing the indicators/attributes of the system. The general aim is to identify homogeneous subsets of objects that are described by subsets of attributes. This approach was planned to study students performance in Italian universities: the focus is on student careers. The example data set is a data mart selected from the University of Macerata data base and refers to the students at the Economics Faculty from 2001 to 2007
Dynamic Visualization of Changes in Association Patterns
The present proposal deals with high-dimensional binary data collected in different occasions in time or space. Studying the associations of data collected at different occasions, a primary aim is to detect changes in the association structure from one occasion to another. A suitable exploratory technique for the analysis of multiple associations in high-dimensional data is the multiple correspondence analysis (MCA; Greenacre, 2007). However, the comparison of MCA factorial displays referring to different occasions is meaningless. A possible solution to link the association structures of different data batches is to start from an MCA display of a reference and incrementally update the solution with further batches (Iodice D'Enza and Greenacre, 2010). This approach, does not take into account the presence of a cluster structure in the set of statistical units.
This contribution intend to present an approach that, through the combination of clustering and factorial techniques, aims to visualize the evolution of the association structure of binary attributes over different data batches. The proposal is to introduce a latent categorical variable which is determined and updated at each incoming batch; in other words this variable is determined according to the association structure and represents the 'link' among the solutions. The latent categorical variable is endogenously determined by the procedure; in particular, it refers to the cluster structure characterizing the data set in question. A starting solution is updated incrementally as new data sets are analysed. The factorial display will describe the patterns of change in the multiple associations when shifting the analysis from one occasion to the other.
Procedures suitably combining clustering with factorial analysis techniques have been proposed. Vichi and Kiers (2001) propose a combination of principal component analysis (PCA) with k-means clustering method. In the framework of categorical data, another interesting approach combining clustering and multiple correspondence analysis (MCA) is proposed by Hwang et al. (2006). Similarly, yet dealing with binary data, Palumbo and Iodice D'Enza (2010) propose a suitable dimension reduction and clustering. The present proposal is an enhancement of the latter approach to the comparative analysis of multiple batches
An Ultra diffuse Galaxy in the NGC 5846 group from the VEGAS survey
Many ultra diffuse galaxies (UDGs) have now been identified in clusters of
galaxies. However, the number of nearby UDGs suitable for detailed follow-up
remain rare. Our aim is to begin to identify UDGs in the environments of nearby
bright early-type galaxies from the VEGAS survey. Here we use a deep g band
image of the NGC 5846 group, taken as part of the VEGAS survey, to search for
UDGs. We found one object with properties of a UDG if it associated with the
NGC 5846 group, which seems likely. The galaxy, we name NGC 5846UDG1, has
an absolute magnitude of M = -14.2, corresponding to a stellar mass of
10 M. It also reveals a system of compact sources which are
likely globular clusters. Based on the number of globular clusters detected we
estimate a halo mass that is greater than 810 M for
UDG1.Comment: 5 pages, 4 figures, accepted for publication in A&
A panoramic VISTA of the stellar halo of NGC 253
Outskirts of large galaxies contain important information about the galaxy
formation and assembly process, and resolved star count studies can probe the
extremely low surface brightness of the outer halos. We use images obtained
with the VISTA telescope to construct spatially resolved J vs Z-J
colour-magnitude diagrams (CMDs) of NGC 253, a nearly edge-on disk galaxy in
the Sculptor group. The very deep photometry, down to J ~ 23.5, and the wide
area covered allows us to trace the red giant branch (RGB) and asymptotic giant
branch (AGB) stars that belong to the outer disk and the halo of NGC 253, out
to 50 kpc along the galaxy minor axis. We confirm the existence of an extra
planar stellar component of the disk, with a very prominent southern shelf and
a symmetrical feature on the north side. The only additional visible
sub-structure is an overdensity in the north-west part of the halo at about 28
kpc from the plane and extending over ~ 20 kpc parallel with the disk of the
galaxy. From the stellar count profile along the major axis we measure the
transition from the disk to the halo at a radial distance of about 25 kpc,
where a clear break appears in the number density profile. The isodensity
contours show that the inner halo is a flattened structure that blends with a
more extended, diffuse, rounder outer halo. Such external structure can be
traced to the very edge of our image out to 50 kpc from the disk plane. The
number density profile of the stars in the stellar halo follows a power law
with index -1.6, as function of radius. The CMD shows a very homogeneous
stellar population across the whole field; by comparison with theoretical
isochrones we conclude that the RGB stars are ~ 8 Gyr old or more, while the
AGB stars trace a population of about 2 x 10^8 Mo, formed from ~ 0.5 to a few
Gyr ago. Surprisingly, part of this latter population appears scattered over a
wide area.Comment: To appear on Astronomy and Astrophysic
The NIR structure of the barred galaxy NGC253 from VISTA
[abridged] We used J and Ks band images acquired with the VISTA telescope as
part of the science verification to quantify the structures in the stellar disk
of the barred Sc galaxy NGC253. Moving outward from the galaxy center, we find
a nuclear ring within the bright 1 kpc diameter nucleus, then a bar, a ring
with 2.9 kpc radius. From the Ks image we obtain a new measure of the
deprojected length of the bar of 2.5 kpc. The bar's strength, as derived from
the curvature of the dust lanes in the J-Ks image, is typical of weak bars.
From the deprojected length of the bar, we establish the corotation radius
(R_CR=3 kpc) and bar pattern speed (Omega_b = 61.3 km /s kpc), which provides
the connection between the high-frequency structures in the disk and the
orbital resonances induced by the bar. The nuclear ring is located at the inner
Lindblad resonance. The second ring does not have a resonant origin, but it
could be a merger remnant or a transient structure formed during an
intermediate stage of the bar formation. The inferred bar pattern speed places
the outer Lindblad resonance within the optical disk at 4.9 kpc, in the same
radial range as the peak in the HI surface density. The disk of NGC253 has a
down-bending profile with a break at R~9.3 kpc, which corresponds to about 3
times the scale length of the inner disk. We discuss the evidence for a
threshold in star formation efficiency as a possible explanation of the steep
gradient in the surface brightness profile at large radii. The NIR photometry
unveils the dynamical response of the NGC253 stellar disk to its central bar.
The formation of the bar may be related to the merger event that determined the
truncation of stars and gas at large radii and the perturbation of the disk's
outer edge.Comment: Accepted for publication in Astronomy & Astrphysics. High resolution
pdf file is available at the following link:
https://www.dropbox.com/s/4o4cofs1lyjrtpv/NGC253.pd
Phenotypic effects of expanded ataxin-1 polyglutamines with interruptions in vitro
Spinocerebellar ataxia type 1 is a neurodegenerative disease caused by expansion of an uninterrupted glutamine repeat in ataxin-1 protein. Protein aggregation and immunoreactivity to 1C2 monoclonal antibody are two distinct pathognomonic features of expanded ataxin-1, as well as of other polyglutamine disorders. Rare cases of non-affected elderly subjects carrying expanded ataxin-1 alleles were found in random population. However, in these alleles the glutamine stretch was interrupted by histidines. Due to lack of phenotype, these alleles should be considered "normal". Most importantly, occurrence of these unusual alleles provides a unique opportunity to investigate which molecular properties of expanded ataxin-1 are not coupled to polyglutamine pathogenesis. Towards this goal, we compared in vitro the immunoreactivity to 1C2 antibody and the ability to form aggregates of interrupted and uninterrupted alleles. Immunoblotting showed that expanded-interrupted ataxin-1 had an affinity to 1C2 resembling that of normal ataxin-1. On the contrary, filter assay showed that aggregation rate of expanded-interrupted ataxin-1 resembles that of expanded-uninterrupted ataxin-1. These observations indicate that affinity for 1C2 does not directly correlate with self-aggregation of ataxin-1. Moreover, self-aggregation is not directly affected by histidine interruptions. In conclusion, these results support the hypothesis that mechanisms underlying neuronal degeneration are triggered by protein misfolding rather than by protein aggregation
VEGAS: a VST Early-type GAlaxy Survey. IV. NGC 1533, IC 2038 and IC 2039: an interacting triplet in the Dorado group
This paper focuses on NGC 1533 and the pair IC 2038 and IC 2039 in Dorado a
nearby, clumpy, still un-virialized group. We obtained their surface photometry
from deep OmegaCAM@ESO-VST images in g and r bands. For NGC 1533, we map the
surface brightness down to mag/arcsec and mag/arcsec and out to about . At such faint levels
the structure of NGC 1533 appear amazingly disturbed with clear structural
asymmetry between inner and outer isophotes in the North-East direction. We
detect new spiral arm-like tails in the outskirts, which might likely be the
signature of a past interaction/merging event. Similarly, IC 2038 and IC 2039
show tails and distortions indicative of their ongoing interaction. Taking
advantages of deep images, we are able to detect the optical counterpart to the
HI gas. The analysis of the new deep data suggests that NGC 1533 had a complex
history made of several interactions with low-mass satellites that generated
the star-forming spiral-like structure in the inner regions and are shaping the
stellar envelope. In addition, the VST observations show that also the two less
luminous galaxies, IC 2038 and IC 2039, are probably interacting each-other
and, in the past, IC 2038 could have also interacted with NGC 1533, which
stripped away gas and stars from its outskirts. The new picture emerging from
this study is of an interacting triplet, where the brightest galaxy NGC 1533
has ongoing mass assembly in the outskirts.Comment: Accepted for publication in The Astronomical Journal. High-resolution
version of paper is available at the following link:
https://www.dropbox.com/preview/VEGAS_IV.pdf?role=persona
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