764 research outputs found
Outer edges of face-on spiral galaxies
We present deep optical imaging of three face-on disk galaxies together with
a detailed description of the reduction and calibration methods used, in order
to measure the intrinsic shape of their outer stellar edges. Whereas it is now
well accepted that disks of spiral galaxies are not infinite exponential beyond
galactocentric distances of about 3-5 radial scalelengths, the genuine
structure of the truncation region is not yet well known. Our data
quantitatively establish a smooth truncation behaviour of the radial surface
brightness profiles and is best described by a two-slope model, characterised
by an inner and outer exponential scalelength separated at a relatively well
defined break radius. This result disagrees with the frequently assumed sharply
truncated nature of the radial surface brightness profiles and implies the
presence of stars and even star-formation beyond the break radius. In addition,
we do not find a strong influence of a nearby companion on the ratio of the
break radius to the radial scalelength. Our results denote new observational
constraints for the search of the physical explanation for these smooth disk
truncations.Comment: LaTeX, 10 pages, 17 figures, accepted to be published in A&A, minor
changes to the quality of figure
Social media adoption in Italian firms. Opportunities and challenges for lagging regions
Social media are an important growth opportunity for firms, especially small-sized ones operating in peripheral and lagging regions. In this paper, we investigate not only whether firms are able to take this opportunity, but also if they are able to face the challenge of adopting social media at a professional level to obtain a significant economic impact, measured in terms of exporting activities. Exploring the Italian case, our empirical study indicates that smaller firms in lagging areas are more likely to adopt social media but at the same time less likely to use them at a professional level. This reflects poor strategic targets of social media adoption and lower probabilities of entering international markets
K-Band Observations of Boxy Bulges. I. Morphology and Surface Brightness Profiles
Kn-band images, unsharp-masked images, as well as major-axis and vertically-
summed surface brightness profiles are presented for 30 edge-on spiral
galaxies, most with a boxy or peanut-shaped (B/PS) bulge. Such galaxies have
more complex morphologies than galaxies of other bulge types, more often
showing (off-)centered X structures, secondary major-axis maxima and
spiral-like structures. Those features are also observed in N-body simulations
of barred discs and may trace the main bar orbit families. The surface
brightness profiles of galaxies with a B/PS bulge are also more complex, with
typically 3 or more clearly separated regions, including a flat intermediate
region (Freeman Type II profiles). Those radial breaks offer evidence for
bar-driven transfer of angular momentum and radial redistribution of material.
The profiles also suggest a rapid variation of the scaleheight of the disc
material, contrary to conventional wisdom but again as expected from vertical
resonances and instabilities in barred discs. The steep inner region of the
surface brightness profiles is often shorter than the isophotally thick part of
the galaxies, itself always shorter than the flat region of the profiles.
Contrary to the standard `bulge + disc' model, we thus propose that galaxies
with a B/PS bulge are composed of a thin concentrated disc (a disc-like bulge)
contained within a partially thick bar (the B/PS bulge) and a thin outer disc.
The inner disc likely formed secularly through bar-driven processes and is
responsible for the steep inner region of the surface brightness profiles,
while the bar is responsible for the flat region and the thick complex
morphological structures observed. Those components are strongly coupled
dynamically and are formed mostly of the same (disc) material. [Abridged]Comment: 23 pages, 34 figures. Accepted for publication in MNRAS. A version
with full resolution figures is available at
http://www-astro.physics.ox.ac.uk/~bureau/pub_list.htm
Bar Diagnostics in Edge-On Spiral Galaxies. III. N-Body Simulations of Disks
Present in over 45% of local spirals, boxy and peanut-shaped bulges are
generally interpreted as edge-on bars and may represent a key phase in the
evolution of bulges. Aiming to test such claims, the kinematic properties of
self-consistent 3D N-body simulations of bar-unstable disks are studied. Using
Gauss-Hermite polynomials to describe the stellar kinematics, a number of
characteristic bar signatures are identified in edge-on disks: 1) a major-axis
light profile with a quasi-exponential central peak and a plateau at moderate
radii (Freeman Type II profile); 2) a ``double-hump'' rotation curve; 3) a
sometime flat central velocity dispersion peak with a plateau at moderate radii
and occasional local central minimum and secondary peak; 4) an h3-V correlation
over the projected bar length. All those kinematic features are spatially
correlated and can easily be understood from the orbital structure of barred
disks. They thus provide a reliable and easy-to-use tool to identify edge-on
bars. Interestingly, they are all produced without dissipation and are
increasingly realized to be common in spirals, lending support to bar-driven
evolution scenarios for bulge formation. So called ``figure-of-eight''
position-velocity diagrams are never observed, as expected for realistic
orbital configurations. Although not uniquely related to triaxiality,
line-of-sight velocity distributions with a high velocity tail (i.e. an h3-V
correlation) appear as particularly promising tracers of bars. The stellar
kinematic features identified grow in strength as the bar evolves and vary
little for small inclination variations. Many can be used to trace the bar
length. Comparisons with observations are encouraging and support the view that
boxy and peanut-shaped bulges are simply thick bars viewed edge-on.Comment: 32 pages, 4 figures, AASTeX preprint. Revised following referees'
comments. Now accepted for publication in The Astrophysical Journal. We
strongly suggest you download the version with full resolution figures at
http://www.astro.columbia.edu/~bureau/Publications/Nbody_ApJ04.ps.g
Utility of post mortem computed tomography in clivus fracture diagnosis. Case illustration and literature review
Clivus fractures are usually associated with head blunt trauma due to traffic accident and falls. A 23–year-old
man died immediately after a smash-up while he was stopping on his motorcycle. Post-mortem Computed tomography (PMCT), performed before autopsy, revealed a complex basilar skull base fractures associated with
brainstem and cranio-vertebral junction injuries, improving the diagnostic performance of conventional autopsy.
Imaging data were re-assessable and PMCT offers the possibility to perform multiplanar and volume rendered
reconstructions, increasing forensic medicine knowledge related to traumatic injuries
Why Buckling Stellar Bars Weaken in Disk Galaxies
Young stellar bars in disk galaxies experience a vertical buckling
instability which terminates their growth and thickens them, resulting in a
characteristic peanut/boxy shape when viewed edge on. Using N-body simulations
of galactic disks embedded in live halos, we have analyzed the bar structure
throughout this instability and found that the outer third of the bar dissolves
completely while the inner part (within the vertical inner Lindblad resonance)
becomes less oval. The bar acquires the frequently observed peanut/boxy-shaped
isophotes. We also find that the bar buckling is responsible for a mass
injection above the plane, which is subsequently trapped by specific 3-D
families of periodic orbits of particular shapes explaining the observed
isophotes, in line with previous work. Using a 3-D orbit analysis and surfaces
of sections, we infer that the outer part of the bar is dissolved by a rapidly
widening stochastic region around its corotation radius -- a process related to
the bar growth. This leads to a dramatic decrease in the bar size, decrease in
the overall bar strength and a mild increase in its pattern speed, but is not
expected to lead to a complete bar dissolution. The buckling instability
appears primarily responsible for shortening the secular diffusion timescale to
a dynamical one when building the boxy isophotes. The sufficiently long
timescale of described evolution, ~1 Gyr, can affect the observed bar fraction
in local universe and at higher redshifts, both through reduced bar strength
and the absence of dust offset lanes in the bar.Comment: 7 pages, 4 figures, ApJ Letters, in pres
Calcineurin controls expression of EAAT1/GLAST in mouse and human cultured astrocytes through dynamic regulation of protein synthesis and degradation
Alterations in the expression of glutamate/aspartate transporter (GLAST) have been associated with several neuropathological conditions including Alzheimer\u2019s disease and epilepsy. However, the mechanisms by which GLAST expression is altered are poorly understood. Here we used a combination of pharmacological and genetic approaches coupled with quantitative PCR and Western blot to investigate the mechanism of the regulation of GLAST expression by a Ca2+ /calmodulin-activated phosphatase calcineurin (CaN). We show that treatment of cultured hippocampal mouse and fetal human astrocytes with a CaN inhibitor FK506 resulted in a dynamic modulation of GLAST protein expression, being downregulated after 24\u201348 h, but upregulated after 7 days of continuous FK506 (200 nM) treatment. Protein synthesis, as assessed by puromycin incorporation in neo-synthesized polypeptides, was inhibited already after 1 h of FK506 treatment, while the use of a proteasome inhibitor MG132 (1 \ub5M) shows that GLAST protein degradation was only suppressed after 7 days of FK506 treatment. In astrocytes with constitutive genetic ablation of CaN both protein synthesis and degradation were significantly inhibited. Taken together, our data suggest that, in cultured astrocytes, CaN controls GLAST expression at a posttranscriptional level through regulation of GLAST protein synthesis and degradation
Comparing peanut-shaped `bulges' to N-body simulations and orbital calculations
We present a near-infrared K_n-band photometric study of edge-on galaxies
with a box/peanut-shaped `bulge'. The morphology of the galaxies is analysed
using unsharp masking and fits to the vertical surface brightness profiles, and
the results are compared to N-body simulations and orbital calculations of
barred galaxies. Both theoretical approaches reproduce the main structures
observed.Comment: 4 pages, 5 figures, to appear in the proceedings of "The Evoution of
Galaxies. III. From simple approaches to self-consistent models" (Kiel, July
2002), G. Hensler et al. (eds.
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