1,570 research outputs found
On DDO154 and Cold Dark Matter halo profiles
We investigate the claim by Burkert and Silk (1997) that the observed
rotation curve of the dwarf irregular galaxy DDO154 cannot be reconciled with
the universal CDM halo profile of Navarro, Frenk & White (1996,1997) even when
allowance is made for the effect of violent gas outflow events on the structure
of the galaxy. By means of N-body simulations we show that under certain
conditions it is possible to obtain a reasonable fit to the observed rotation
curve without invoking Burkert & Silk's proposed spheroidal MACHO component. We
are able to best reproduce the observed decline in the rotation curve by
postulating additional hidden disc mass, in an amount that is compatible with
disc stability requirements. In the process we improve upon the results of
Navarro, Eke & Frenk (1996) on the formation of halo cores by mass loss by
using actual haloes from Cold Dark Matter simulations instead of Hernquist
(1990) distributions.Comment: LaTeX (mn.sty), 8 pages, 6 figures included; updated to match final
version to appear in MNRA
Disk Evolution and Bar Triggering Driven by Interactions with Dark Matter Substructure
We study formation and evolution of bar-disk systems in fully self-consistent
cosmological simulations of galaxy formation in the LCDM WMAP3 Universe. In a
representative model we find that the first generation of bars form in response
to the asymmetric dark matter (DM) distribution (i.e., DM filament) and quickly
decay. Subsequent bar generations form and are destroyed during the major
merger epoch permeated by interactions with a DM substructure (subhalos). A
long-lived bar is triggered by a tide from a subhalo and survives for ~10 Gyr.
The evolution of this bar is followed during the subsequent numerous minor
mergers and interactions with the substructure. Together with intrinsic
factors, these interactions largely determine the stellar bar evolution. The
bar strength and its pattern speed anticorrelate, except during interactions
and when the secondary (nuclear) bar is present. For about 5 Gyr bar pattern
speed increases substantially despite the loss of angular momentum to stars and
cuspy DM halo. We analyze the evolution of stellar populations in the bar-disk
and relate them to the underlying dynamics. While the bar is made mainly of an
intermediate age, ~5-6 Gyr, disk stars at z=0, a secondary nuclear bar which
surfaces at z~0.1 is made of younger, ~1-3 Gyr stars.Comment: 5 pages, 5 figures, accepted for publication in ApJ Letter
Dissecting Galaxy Formation: II. Comparing Substructure in Pure Dark Matter and Baryonic Models
We compare the substructure evolution in pure dark matter (DM) halos with
those in the presence of baryons (PDM and BDM). The prime halos have been
analyzed by Romano-Diaz et al (2009). Models have been evolved from identical
initial conditions using Constrained Realizations, including star formation and
feedback. A comprehensive catalog of subhalos has been compiled and properties
of subhalos analyzed in the mass range of 10^8 Mo - 10^11 Mo. We find that
subhalo mass functions are consistent with a single power law, M_sbh^{alpha},
but detect a nonnegligible shift between these functions, alpha -0.86 for the
PDM, and -0.98 for the BDM. Overall, alpha const. in time with variations of
+-15%. Second, we find that the radial mass distribution of subhalos can be
approximated by a power law, R^{gamma} with a steepening around the radius of a
maximal circular velocity, Rvmax, in the prime halos. Gamma ~-1.5 for the PDM
and -1 for the BDM, inside Rvmax, and is steeper outside. We detect little
spatial bias between the subhalo populations and the DM of the main halos. The
subhalo population exhibits much less triaxiality with baryons, in tandem with
the prime halo. Finally, we find that, counter-intuitively, the BDM population
is depleted at a faster rate than the PDM one within the central 30kpc of the
prime. Although the baryons provide a substantial glue to the subhalos, the
main halos exhibit the same trend. This assures a more efficient tidal
disruption of the BDM subhalos. This effect can be reversed for a more
efficient feedback from stellar evolution and supermassive black holes, which
will expel baryons from the center and decrease the concentration of the prime
halo. We compare our results with via Lactea and Aquarius simulations and other
published results.Comment: 12 pages, 9 figures, to be published by the Astrophysical Journa
Constrained simulations of the Local Group: on the radial distribution of substructures
We examine the properties of satellites found in high resolution simulations
of the local group. We use constrained simulations designed to reproduce the
main dynamical features that characterize the local neighborhood, i.e. within
tens of Mpc around the Local Group (LG). Specifically, a LG-like object is
found located within the 'correct' dynamical environment and consisting of
three main objects which are associated with the Milky Way, M31 and M33. By
running two simulations of this LG from identical initial conditions - one with
and one without baryons modeled hydrodynamically - we can quantify the effect
of gas physics on the population of subhaloes in an environment similar
to our own. We find that above a certain mass cut, subhaloes in hydrodynamic simulations are more
radially concentrated than those in simulations with out gas. This is caused by
the collapse of baryons into stars that typically sit in the central regions of
subhaloes, making them denser. The increased central density of such a subhalo,
results in less mass loss due to tidal stripping than the same subhalo
simulated with only dark matter. The increased mass in hydrodynamic subhaloes
with respect to dark matter ones, causes dynamical friction to be more
effective, dragging the subhalo towards the centre of the host. This results in
these subhaloes being effectively more radially concentrated then their dark
matter counterparts.Comment: 12 pages, 9 figure
On the physical origin of dark matter density profiles
The radial mass distribution of dark matter haloes is investigated within the
framework of the spherical infall model. We present a new formulation of
spherical collapse including non-radial motions, and compare the analytical
profiles with a set of high-resolution N-body simulations ranging from galactic
to cluster scales. We argue that the dark matter density profile is entirely
determined by the initial conditions, which are described by only two
parameters: the height of the primordial peak and the smoothing scale. These
are physically meaningful quantities in our model, related to the mass and
formation time of the halo. Angular momentum is dominated by velocity
dispersion, and it is responsible for the shape of the density profile near the
centre. The phase-space density of our simulated haloes is well described by a
power-law profile, rho/sigma^3 = 10^{1.46\pm0.04} (rho_c/Vvir^3)
(r/Rvir)^{-1.90\pm0.05}. Setting the eccentricity of particle orbits according
to the numerical results, our model is able to reproduce the mass distribution
of individual haloes.Comment: 12 pages, 13 figures, submitted to MNRA
Dynamical difference between the cD galaxy and the stellar diffuse component in simulated galaxy clusters
Member galaxies within galaxy clusters nowadays can be routinely identified
in cosmological, hydrodynamical simulations using methods based on identifying
self bound, locally over dense substructures. However, distinguishing the
central galaxy from the stellar diffuse component within clusters is
notoriously difficult, and in the center it is not even clear if two distinct
stellar populations exist. Here, after subtracting all member galaxies, we use
the velocity distribution of the remaining stars and detect two dynamically,
well-distinct stellar components within simulated galaxy clusters. These
differences in the dynamics can be used to apply an un-binding procedure which
leads to a spatial separation of the two components into a cD and a diffuse
stellar component (DSC). Applying our new algorithm to a cosmological,
hydrodynamical simulation we find that -- in line with previous studies --
these two components have clearly distinguished spatial and velocity
distributions as well as different star formation histories. We show that the
DSC fraction -- which can broadly be associated with the observed intra cluster
light -- does not depend on the virial mass of the galaxy cluster and is much
more sensitive to the formation history of the cluster. We conclude that the
separation of the cD and the DSC in simulations, based on our dynamical
criteria, is more physically motivated than current methods which depend on
implicit assumptions on a length scale associated with the cD galaxy and
therefore represent a step forward in understanding the different stellar
components within galaxy clusters. Our results also show the importance of
analyzing the dynamics of the DSC to characterize its properties and understand
its origin.Comment: 15 pages, 18 figures, MNRAS in pres
ccz-1 mediates the digestion of apoptotic corpses in C. elegans
During development, the processes of cell division, differentiation and apoptosis must be precisely coordinated in order to maintain tissue homeostasis. The nematode C. elegans is a powerful model system in which to study cell death and its control. C. elegans apoptotic cells condense and form refractile corpses under differential interference contrast (DIC) microscopy. Activation of the GTPase CED-10 (Rac) in a neighbouring cell mediates the recognition and engulfment of the cell corpse. After inclusion of the engulfed corpse in a phagosome, different proteins are sequentially recruited onto this organelle to promote its acidification and fusion with lysosomes, leading to the enzymatic degradation of the cell corpse. We show that CCZ-1, a protein conserved from yeasts to humans, mediates the digestion of these apoptotic corpses. CCZ-1 seems to act in lysosome biogenesis and phagosome maturation by recruiting the GTPase RAB-7 over the phagosome
Density profiles of dark matter haloes on Galactic and Cluster scales
In the present paper, we improve the "Extended Secondary Infall Model" (ESIM)
of Williams et al. (2004) to obtain further insights on the cusp/core problem.
The model takes into account the effect of ordered and random angular momentum,
dynamical friction and baryon adiabatic contraction in order to obtain a
secondary infall model more close to the collapse reality. The model is applied
to structures on galactic scales (normal and dwarf spiral galaxies) and on
cluster of galaxies scales. The results obtained suggest that angular momentum
and dynamical friction are able, on galactic scales, to overcome the competing
effect of adiabatic contraction eliminating the cusp. The NFW profile can be
reobtained, in our model only if the system is constituted just by dark matter
and the magnitude of angular momentum and dynamical friction are reduced with
respect to the values predicted by the model itself. The rotation curves of
four LSB galaxies from de Blok & Bosma (2002) are compared to the rotation
curves obtained by the model in the present paper obtaining a good fit to the
observational data. On scales smaller than
the slope and on cluster scales we observe a similar
evolution of the dark matter density profile but in this case the density
profile slope flattens to for a cluster of . The total mass profile, differently from that of dark
matter, shows a central cusp well fitted by a NFW model.Comment: 26 pages; 4 figures A&A Accepte
- âŠ