3,245 research outputs found
Formation and evolution of dwarf galaxies in the CDM Universe
We first review the results of the tidal stirring model for the
transformation of gas-rich dwarf irregulars into dwarf spheroidals, which turns
rotationally supported stellar systems into pressure supported ones. We
emphasize the importance of the combined effect of ram pressure stripping and
heating from the cosmic ultraviolet background in removing the gas and
converting the object into a gas poor system as dSphs. We discuss how the
timing of infall of dwarfs into the primary halo determines the final
mass-to-light ratio and star formation history. Secondly we review the results
of recent cosmological simulations of the formation of gas-rich dwarfs. These
simulations are finally capable to produce a realistic object with no bulge, an
exponential profile and a slowly rising rotation curve. The result owes to the
inclusion of an inhomogeneous ISM and a star formation scheme based on regions
having the typical density of molecular cloud complexes. Supernovae-driven
winds become more effective in such mode, driving low angular momentum baryons
outside the virial radius at high redshift and turning the dark matter cusp
into a core. Finally we show the first tidal stirring experiments adopting
dwarfs formed in cosmological simulations as initial conditions. The latter are
gas dominated and have have turbulent thick gaseous and stellar disks disks
that cannot develop strong bars, yet they are efficiently heated into spheroids
by tidal shocks.Comment: 14 pages, 4 Figures, o appear in the proceedings of the CRAL
conference, Lyon, June 2010, "A Universe of Dwarf Galaxies", eds. Philippe
Prugniel & Mina Koleva; EDP Sciences in the European Astronomical Society
Publications Series. (invited talk
How to make an ultra-faint dwarf spheroidal galaxy: tidal stirring of disky dwarfs with shallow dark matter density profiles
In recent years the Sloan Digital Sky Survey has unraveled a new population
of ultra-faint dwarf galaxies (UFDs) in the vicinity of the Milky Way (MW)
whose origin remains a puzzle. Using a suite of collisionless N-body
simulations, we investigate the formation of UFDs in the context of the tidal
stirring model for the formation of dwarf spheroidal galaxies in the Local
Group (LG). Our simulations are designed to reproduce the tidal interactions
between MW-sized host galaxies and rotationally supported dwarfs embedded in
10^9 M_sun dark matter (DM) halos. We explore a variety of inner density slopes
\rho \propto r^{-\alpha} for the dwarf DM halos, ranging from core-like (\alpha
= 0.2) to cuspy (\alpha = 1), and different dwarf orbital configurations. Our
experiments demonstrate that UFDs can be produced via tidal stirring of disky
dwarfs on relatively tight orbits, consistent with a redshift of accretion by
the host galaxy of z \sim 1, and with intermediate values for the halo inner
density slopes (\rho \propto r^{-0.6}). The inferred slopes are in excellent
agreement with those resulting from both the modeling of the rotation curves of
dwarf galaxies and recent cosmological simulations of dwarf galaxy formation.
Comparing the properties of observed UFDs with those of their simulated
counterparts, we find remarkable similarities in terms of basic observational
parameters. We conclude that tidal stirring of rotationally supported dwarfs
represents a viable mechanism for the formation of UFDs in the LG environment.Comment: 6 pages, 4 figures, revised version accepted for publication in ApJ
Letter
Home-Based Monitoring of Pulmonary Function in Patients with Duchenne Muscular Dystroph
BACKGROUND: Loss of pulmonary function is a main cause of early morbidity and mortality in patients with Duchenne muscular dystrophy (DMD). Standard of care guidelines recommend regular assessment of pulmonary function by hospital-based spirometry to detect onset and monitor progression of pulmonary function decline. OBJECTIVE: To assess the feasibility of home-based monitoring of pulmonary function by a hand-held device (HHD) in adolescent and adult patients with DMD over a period of 12 months. METHODS: In the phase III randomized placebo-controlled DELOS trial in 10-18 year old DMD patients, peak expiratory flow (PEF) measurements were collected weekly at home by the patient (assisted by parent/caregiver) using a peak flow meter HHD. Adherence to the use of the HHD was assessed and 12-month changes in PEF as percent of predicted (PEF% p) for the idebenone (N = 31) and the placebo treatment groups (N = 33) from HHD-derived data were compared to results from hospital-based spirometry. RESULTS: A total of 2689 individual HHD assessments were analysed. Overall adherence to the use of the HHD over the course of the 12-month study duration was good (75.9%, SD 21.5%) and PEF% p data obtained at the same day by HHD and standard spirometry correlated well (Spearman's rho 0.80; p < 0.001). Several analysis methods of HHD-derived data for PEF% p consistently demonstrate that idebenone treatment slowed the decline in PEF% p compared to placebo, which supports the statistically significant difference in favour of idebenone for PEF% p measured by standard spirometry. CONCLUSIONS: This study demonstrates that home-based monitoring of pulmonary function in adolescent patients with DMD using a HHD is feasible, provides reliable data compared to hospital-based spirometry and is therefore suitable for use in clinical practice and for clinical trials
Fast rebinding increases dwell time of Src homology 2 (SH2)-containing proteins near the plasma membrane
Receptor tyrosine kinases (RTKs) control a host of biological functions by phosphorylating tyrosine residues of intracellular proteins upon extracellular ligand binding. The phosphotyrosines (p-Tyr) then recruit a subset of ∼100 Src homology 2 (SH2) domain-containing proteins to the cell membrane. The in vivo kinetics of this process are not well understood. Here we use total internal reflection (TIR) microscopy and single-molecule imaging to monitor interactions between SH2 modules and p-Tyr sites near the cell membrane. We found that the dwell time of SH2 modules within the TIR illumination field is significantly longer than predictions based on chemical dissociation rate constants, suggesting that SH2 modules quickly rebind to nearby p-Tyr sites after dissociation. We also found that, consistent with the rebinding model, the effective diffusion constant is negatively correlated with the respective dwell time for different SH2 domains and the dwell time is positively correlated with the local density of RTK phosphorylation. These results suggest a mechanism whereby signal output can be regulated through the spatial organization of multiple binding sites, which will prompt reevaluation of many aspects of RTK signaling, such as signaling specificity, mechanisms of spatial control, and noise suppression
Electronic stress tensor analysis of hydrogenated palladium clusters
We study the chemical bonds of small palladium clusters Pd_n (n=2-9)
saturated by hydrogen atoms using electronic stress tensor. Our calculation
includes bond orders which are recently proposed based on the stress tensor. It
is shown that our bond orders can classify the different types of chemical
bonds in those clusters. In particular, we discuss Pd-H bonds associated with
the H atoms with high coordination numbers and the difference of H-H bonds in
the different Pd clusters from viewpoint of the electronic stress tensor. The
notion of "pseudo-spindle structure" is proposed as the region between two
atoms where the largest eigenvalue of the electronic stress tensor is negative
and corresponding eigenvectors forming a pattern which connects them.Comment: 22 pages, 13 figures, published online, Theoretical Chemistry
Account
The central slope of dark matter cores in dwarf galaxies: Simulations vs. THINGS
We make a direct comparison of the derived dark matter (DM) distributions
between hydrodynamical simulations of dwarf galaxies assuming a LCDM cosmology
and the observed dwarf galaxies sample from the THINGS survey in terms of (1)
the rotation curve shape and (2) the logarithmic inner density slope alpha of
mass density profiles. The simulations, which include the effect of baryonic
feedback processes, such as gas cooling, star formation, cosmic UV background
heating and most importantly physically motivated gas outflows driven by
supernovae (SNe), form bulgeless galaxies with DM cores. We show that the
stellar and baryonic mass is similar to that inferred from photometric and
kinematic methods for galaxies of similar circular velocity. Analyzing the
simulations in exactly the same way as the observational sample allows us to
address directly the so-called "cusp/core" problem in the LCDM model. We show
that the rotation curves of the simulated dwarf galaxies rise less steeply than
CDM rotation curves and are consistent with those of the THINGS dwarf galaxies.
The mean value of the logarithmic inner density slopes alpha of the simulated
galaxies' dark matter density profiles is ~ -0.4 +- 0.1, which shows good
agreement with \alpha = -0.29 +- 0.07 of the THINGS dwarf galaxies. The effect
of non-circular motions is not significant enough to affect the results. This
confirms that the baryonic feedback processes included in the simulations are
efficiently able to make the initial cusps with \alpha ~ -1.0 to -1.5 predicted
by dark-matter-only simulations shallower, and induce DM halos with a central
mass distribution similar to that observed in nearby dwarf galaxies.Comment: 13 pages, 7 figures; Accepted for publication in AJ; minor
correction
The impact of dark matter cusps and cores on the satellite galaxy population around spiral galaxies
(Abridged) We use N-body simulations to study the effects that a divergent
(i.e. "cuspy") dark matter (DM) profile introduces on the tidal evolution of
dwarf spheroidal galaxies (dSphs). Our models assume cosmologically-motivated
initial conditions where dSphs are DM-dominated systems on eccentric orbits
about a host galaxy composed of a dark halo and a baryonic disc. We find that
the resilience of dSphs to tidal stripping is extremely sensitive to the halo
cuspiness; whereas dwarfs with a cored profile can be easily destroyed by the
host disc, those with cusps always retain a bound remnant. For a given halo
profile the evolution of the structural parameters as driven by tides is
controlled solely by the total amount of mass lost. This information is used to
construct a semi-analytic code that simulates the hierarchical build-up of
spiral galaxies assuming different halo profiles and disc masses. We find that
tidal encounters with discs tend to decrease the average mass of satellites at
all galactocentric radii. Interestingly, satellites accreted before
re-ionization (z>6), which may be singled out by anomalous metallicity
patterns, survive only if haloes are cuspy. We show that the size-mass relation
established from Milky Way (MW) dwarfs strongly supports the presence of cusps
in the majority of these systems, as cored models systematically underestimate
the masses of the known Ultra-Faint dSphs. Our models also indicate that a
massive M31 disc may explain why many of its dSphs fall below the size-mass
relationship derived from MW dSphs. We use our models to constrain the mass
threshold below which star formation is suppressed in DM haloes, finding that
luminous satellites must be accreted with masses above 10^8--10^9 M_sol in
order to explain the size-mass relation observed in MW dwarfs.Comment: 17 pages, 14 figures, MNRAS accepted after minor revisio
On the Efficiency of the Tidal Stirring Mechanism for the Origin of Dwarf Spheroidals: Dependence on the Orbital and Structural Parameters of the Progenitor Disky Dwarfs
(Abridged) The tidal stirring model posits the formation of dSph galaxies via
the tidal interactions between rotationally-supported dwarfs and MW-sized host
galaxies. Using a set of collisionless N-body simulations, we investigate the
efficiency of the tidal stirring mechanism. We explore a wide variety of dwarf
orbital configurations and initial structures and demonstrate that in most
cases the disky dwarfs experience significant mass loss and their stellar
components undergo a dramatic morphological and dynamical transformation: from
disks to bars and finally to pressure-supported spheroidal systems with
kinematic and structural properties akin to those of the classic dSphs in the
Local Group (LG). Our results suggest that such tidal transformations should be
common occurrences within the currently favored cosmological paradigm and
highlight the key factor responsible for an effective metamorphosis to be the
strength of the tidal shocks at the pericenters of the orbit. We demonstrate
that the combination of short orbital times and small pericenters,
characteristic of dwarfs being accreted at high redshift, induces the strongest
transformations. Our models also indicate that the transformation efficiency is
affected significantly by the structure of the progenitor disky dwarfs. Lastly,
we find that the dwarf remnants satisfy the relation Vmax = \sqrt{3} * sigma,
where sigma is the 1D, central stellar velocity dispersion and Vmax is the
maximum halo circular velocity, with intriguing implications for the missing
satellites problem. Overall, we conclude that the action of tidal forces from
the hosts constitutes a crucial evolutionary mechanism for shaping the nature
of dwarf galaxies in environments such as that of the LG. Environmental
processes of this type should thus be included as ingredients in models of
dwarf galaxy formation and evolution.Comment: submitted to ApJ, 34 pages, 15 figures, LaTeX (uses emulateapj.cls
Cuspy No More: How Outflows Affect the Central Dark Matter and Baryon Distribution in Lambda CDM Galaxies
We examine the evolution of the inner dark matter (DM) and baryonic density
profile of a new sample of simulated field galaxies using fully cosmological,
Lambda CDM, high resolution SPH + N-Body simulations. These simulations include
explicit H2 and metal cooling, star formation (SF) and supernovae (SNe) driven
gas outflows. Starting at high redshift, rapid, repeated gas outflows following
bursty SF transfer energy to the DM component and significantly flatten the
originally `cuspy' central DM mass profile of galaxies with present day stellar
masses in the 10^4.5 -- 10^9.8 Msolar range. At z=0, the central slope of the
DM density profile of our galaxies (measured between 0.3 and 0.7 kpc from their
centre) is well fitted by rhoDM propto r^alpha with alpha \simeq -0.5 + 0.35
log_10(Mstar/10^8Msolar) where Mstar is the stellar mass of the galaxy and 4 <
log_10 Mstar < 9.4. These values imply DM profiles flatter than those obtained
in DM--only simulations and in close agreement with those inferred in galaxies
from the THINGS and LITTLE THINGS survey. Only in very small halos, where by
z=0 star formation has converted less than ~ 0.03% of the original baryon
abundance into stars, outflows do not flatten the original cuspy DM profile out
to radii resolved by our simulations. The mass (DM and baryonic) measured
within the inner 500 pc of each simulated galaxy remains nearly constant over
four orders of magnitudes in stellar mass for Mstar 10^9 Msolar. This finding
is consistent with estimates for faint Local Group dwarfs and field galaxies.
These results address one of the outstanding problems faced by the CDM model,
namely the strong discrepancy between the original predictions of cuspy DM
profiles and the shallower central DM distribution observed in galaxies.Comment: MNRAS in press. Accepted version, a few references added. 12 pages.
Animation at http://youtu.be/FbcgEovabDI?hd=
Period changes in six contact binaries: WZ And, V803 Aql, DF Hya, PY Lyr, FZ Ori, and AH Tau
Six contact binaries lacking a period analysis have been chosen to search for
the presence of a third body. The O-C diagrams of these binaries were analyzed
with the least-squares method by using all available times of minima. Ten new
minima times, obtained from our observations, were included in the present
research. The Light-Time Effect was adopted for the first time as the main
cause for the detailed description of the long-term period changes. Third
bodies were found with orbital periods from 49 up to 100 years, and
eccentricities from 0.0 to 0.56 for the selected binaries. In one case (WZ
And), a fourth-body LITE variation was also applied. The mass functions and the
minimal masses of such bodies were also calculated and a possible angular
separation and magnitude differences were discussed for a prospective
interferometric discovery of these bodies.Comment: 7 pages, 8 figures, 2009 New Astronomy 14, 12
- …