392 research outputs found
The catalytically inactive tyrosine phosphatase HD-PTP/PTPN23 is a novel regulator of SMN complex localization
The survival motor neuron (SMN) complex fulfils essential functions in the assembly of snRNPs, which are key components in the splicing of pre-mRNAs. Little is known about the regulation of SMN complex activity by posttranslational modification despite its complicated phosphorylation pattern. Several phosphatases had been implicated in the regulation of SMN, including the nuclear phosphatases PPM1G and PP1γ. Here we systematically screened all human phosphatase gene products for a regulatory role in the SMN complex. We used the accumulation of SMN in Cajal bodies of intact proliferating cells, which actively assemble snRNPs, as a readout for unperturbed SMN complex function. Knockdown of 29 protein phosphatases interfered with SMN accumulation in Cajal bodies, suggesting impaired SMN complex function, among those the catalytically inactive, non–receptor-type tyrosine phosphatase PTPN23/HD-PTP. Knockdown of PTPN23 also led to changes in the phosphorylation pattern of SMN without affecting the assembly of the SMN complex. We further show interaction between SMN and PTPN23 and document that PTPN23, like SMN, shuttles between nucleus and cytoplasm. Our data provide the first comprehensive screen for SMN complex regulators and establish a novel regulatory function of PTPN23 in maintaining a highly phosphorylated state of SMN, which is important for its proper function in snRNP assembly
Dust properties in M31.I.Basic properties and a discussion on age-dependent dust heating
Context. Spitzer Space Telescope observations and dust emission models are
used to discuss the distribution of dust and its characteristics in M31.
Together with GALEX FUV, NUV, and SDSS images we studied the age dependence of
the dust heating process. Methods.Spitzer IRAC/MIPS maps of M31 were matched
together and compared to dust emission models allowing to constrain the dust
mass, the intensity of the mean radiation field, the abundance of Polycyclic
Aromatic Hydrocarbons (PAH) particles. The total infrared emission (TIR) was
analyzed in function of UV and Optical colors and compared to predictions of
models which consider the age-dependent dust heating. Results. We demonstrate
that cold-dust component emission dominates the infrared spectral energy
distribution of M31. The mean intensity of the radiation field heating the dust
is low (typically U<2, where U=1 is the value in the solar surrounding). Due to
the lack of submillimetric measurements the dust mass (M_{dust}) is only weakly
constrained by the infrared spectrum. We show that across the spiral-ring
structure of M31 a fraction >3% of the total dust mass is in PAHs. UV and
optical colors are correlated to (TIR/FUV) ratios in \sim 670 pc-sized regions
overall the disk of M31, although deviating from the IRX-beta relationship for
starburst galaxies. We derived that in 83% of the regions analyzed across the
10kpc ring more than 50% of the energy absorbed by the dust is rediated at
\lambda > 4000 \AA and that dust in M31 appears mainly heated by populations a
few Gyr old even across the star-forming ring. The attenuation is varying
radially peaking near 10kpc and decreasing faster in the inner regions of M31
than in the outer regions. We finally derived the attenuation map of M31 at
6"/px resolution (\sim 100 pc/px along the plane of M31).[abridged]Comment: 21 pages, 18 figures, accepted for publication in A&A. Only low
resolution images included, full resolution images will be avaiable in the
journal electronic version. Fig.14 and Fig.17 will be avaiable via CD
Coulombically Interacting Electrons in a One-dimensional Quantum Dot
The spectral properties of up to four interacting electrons confined within a
quasi one--dimensional system of finite length are determined by numerical
diagonalization including the spin degree of freedom. The ground state energy
is investigated as a function of the electron number and of the system length.
The limitations of a description in terms of a capacitance are demonstrated.
The energetically lowest lying excitations are physically explained as
vibrational and tunneling modes. The limits of a dilute, Wigner-type
arrangement of the electrons, and a dense, more homogeneous charge distribution
are discussed.Comment: 10 pages (excl. Figures), Figures added in POSTSCRIPT, LaTe
The young star cluster system of the Antennae galaxies
“The original publication is available at www.springerlink.com”. Copyright Springer. DOI: 10.1007/s10509-009-0103-xThe study of young star cluster (YSC) systems, preferentially in starburst and merging galaxies, has seen great interest in the recent past, as it provides important input to models of star formation. However, even some basic properties (such as the luminosity function; LF) of YSC systems are still being debated. Here, we study the photometric properties of the YSC system in the nearest major merger system, the Antennae galaxies. We find evidence for the existence of a statistically significant turnover in the LF.Peer reviewe
Multiwavelength star formation indicators: Observations
We present a compilation of multiwavelength data on different star formation
indicators for a sample of nearby star forming galaxies. Here we discuss the
observations, reductions and measurements of ultraviolet images obtained with
STIS, on board the Hubble Space Telescope, ground-based Halpha, and VLA 8.46
GHz radio images. These observations are complemented with infrared fluxes, as
well as large aperture optical radio and ultraviolet data from the literature.
This database will be used in a forthcoming paper to compare star formation
rates at different wavebands. We also present spectral energy distributions
(SEDs) for those galaxies with at least one far-infrared measurements from ISO,
longward of 100 um. These SEDs are divided in two groups, those which are
dominated by the far-infrared emission, and those where the contribution from
the far-infrared and optical emission is comparable. These SEDs are useful
tools to study the properties of high redshift galaxies.Comment: 39 pages, 17 jpeg figures, 1 eps figure, To appear in ApJS May 200
On the dependence between UV luminosity and Lyman-alpha equivalent width in high redshift galaxies
We show that with the simple assumption of no correlation between the
Ly-alpha equivalent width and the UV luminosity of a galaxy, the observed
distribution of high redshift galaxies in an equivalent width - absolute UV
magnitude plane can be reproduced. We further show that there is no dependence
between Ly-alpha equivalent width and Ly-alpha luminosity in a sample of
Ly-alpha emitters. The test was expanded to Lyman-break galaxies and again no
dependence was found. Simultaneously, we show that a recently proposed lack of
large equivalent width, UV bright galaxies (Ando et al. 2006) can be explained
by a simple observational effect, based on too small survey volumes.Comment: 7 pages, 3 figures, 2 tables, accepted in MNRA
Star cluster formation and star formation: the role of environment and star-formation efficiencies
“The original publication is available at www.springerlink.com”. Copyright Springer. DOI: 10.1007/s10509-009-0088-5By analyzing global starburst properties in various kinds of starburst and post-starburst galaxies and relating them to the properties of the star cluster populations they form, I explore the conditions for the formation of massive, compact, long-lived star clusters. The aim is to determine whether the relative amount of star formation that goes into star cluster formation as opposed to field star formation, and into the formation of massive long-lived clusters in particular, is universal or scales with star-formation rate, burst strength, star-formation efficiency, galaxy or gas mass, and whether or not there are special conditions or some threshold for the formation of star clusters that merit to be called globular clusters a few billion years later.Peer reviewe
Effective charge-spin models for quantum dots
It is shown that at low densities, quantum dots with few electrons may be
mapped onto effective charge-spin models for the low-energy eigenstates. This
is justified by defining a lattice model based on a many-electron pocket-state
basis in which electrons are localised near their classical ground-state
positions. The equivalence to a single-band Hubbard model is then established
leading to a charge-spin () model which for most geometries reduces to a
spin (Heisenberg) model. The method is refined to include processes which
involve cyclic rotations of a ``ring'' of neighboring electrons. This is
achieved by introducing intermediate lattice points and the importance of ring
processes relative to pair-exchange processes is investigated using high-order
degenerate perturbation theory and the WKB approximation. The energy spectra
are computed from the effective models for specific cases and compared with
exact results and other approximation methods.Comment: RevTex, 24 pages, 7 figures submitted as compressed and PostScript
file
Vertically coupled double quantum dots in magnetic fields
Ground-state and excited-state properties of vertically coupled double
quantum dots are studied by exact diagonalization. Magic-number total angular
momenta that minimize the total energy are found to reflect a crossover between
electron configurations dominated by intra-layer correlation and ones dominated
by inter-layer correlation. The position of the crossover is governed by the
strength of the inter-layer electron tunneling and magnetic field. The magic
numbers should have an observable effect on the far infra-red optical
absorption spectrum, since Kohn's theorem does not hold when the confinement
potential is different for two dots. This is indeed confirmed here from a
numerical calculation that includes Landau level mixing. Our results take full
account of the effect of spin degrees of freedom. A key feature is that the
total spin, , of the system and the magic-number angular momentum are
intimately linked because of strong electron correlation. Thus jumps hand
in hand with the total angular momentum as the magnetic field is varied. One
important consequence of this is that the spin blockade (an inhibition of
single-electron tunneling) should occur in some magnetic field regions because
of a spin selection rule. Owing to the flexibility arising from the presence of
both intra-layer and inter-layer correlations, the spin blockade is easier to
realize in double dots than in single dots.Comment: to be published in Phys. Rev. B1
Comparison of large-angle production of charged pions with incident protons on cylindrical long and short targets
The HARP collaboration has presented measurements of the double-differential
pi+/pi- production cross-section in the range of momentum 100 MeV/c <= p 800
MeV/c and angle 0.35 rad <= theta <= 2.15 rad with proton beams hitting thin
nuclear targets. In many applications the extrapolation to long targets is
necessary. In this paper the analysis of data taken with long (one interaction
length) solid cylindrical targets made of carbon, tantalum and lead is
presented. The data were taken with the large acceptance HARP detector in the
T9 beam line of the CERN PS. The secondary pions were produced by beams of
protons with momenta 5 GeV/c, 8 GeV/c and 12 GeV/c. The tracking and
identification of the produced particles were performed using a small-radius
cylindrical time projection chamber (TPC) placed inside a solenoidal magnet.
Incident protons were identified by an elaborate system of beam detectors.
Results are obtained for the double-differential yields per target nucleon d2
sigma / dp dtheta. The measurements are compared with predictions of the MARS
and GEANT4 Monte Carlo simulations.Comment: 43 pages, 20 figure
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