25 research outputs found
Density of States and Critical Behavior of the Coulomb Glass
We present zero-temperature simulations for the single-particle density of
states of the Coulomb glass. Our results in three dimensions are consistent
with the Efros and Shklovskii prediction for the density of states.
Finite-temperature Monte Carlo simulations show no sign of a thermodynamic
glass transition down to low temperatures, in disagreement with mean-field
theory. Furthermore, the random-displacement formulation of the model undergoes
a transition into a distorted Wigner crystal for a surprisingly broad range of
the disorder strength.Comment: 4 pages, 2 figures, 1 tabl
The Shape of Dark Matter Halos: Dependence on Mass, Redshift, Radius, and Formation
Using six high resolution dissipationless simulations with a varying box size
in a flat LCDM universe, we study the mass and redshift dependence of dark
matter halo shapes for M_vir = 9.0e11 - 2.0e14, over the redshift range z=0-3,
and for two values of sigma_8=0.75 and 0.9. Remarkably, we find that the
redshift, mass, and sigma_8 dependence of the mean smallest-to-largest axis
ratio of halos is well described by the simple power-law relation = (0.54
+- 0.02)(M_vir/M_*)^(-0.050 +- 0.003), where s is measured at 0.3 R_vir and the
z and sigma_8 dependences are governed by the characteristic nonlinear mass,
M_*=M_*(z,sigma_8). We find that the scatter about the mean s is well described
by a Gaussian with sigma ~ 0.1, for all masses and redshifts. We compare our
results to a variety of previous works on halo shapes and find that reported
differences between studies are primarily explained by differences in their
methodologies. We address the evolutionary aspects of individual halo shapes by
following the shapes of the halos through ~100 snapshots in time. We determine
the formation scalefactor a_c as defined by Wechsler et al. (2002) and find
that it can be related to the halo shape at z = 0 and its evolution over time.Comment: 18 pages, 21 figures, submitted to MNRA
The Effect of Gas Cooling on the Shapes of Dark Matter Halos
We analyze the effect of dissipation on the shapes of dark matter (DM) halos
using high-resolution cosmological gasdynamics simulations of clusters and
galaxies in the LCDM cosmology. We find that halos formed in simulations with
gas cooling are significantly more spherical than corresponding halos formed in
adiabatic simulations. Gas cooling results in an average increase of the
principle axis ratios of halos by ~ 0.2-0.4 in the inner regions. The
systematic difference decreases slowly with radius but persists almost to the
virial radius. We argue that the differences in simulations with and without
cooling arise both during periods of quiescent evolution, when gas cools and
condenses toward the center, and during major mergers. We perform a series of
high-resolution N-body simulations to study the shapes of remnants in major
mergers of DM halos and halos with embedded stellar disks. In the DM halo-only
mergers, the shape of the remnants depends only on the orbital angular momentum
of the encounter and not on the internal structure of the halos. However,
significant shape changes in the DM distribution may result if stellar disks
are included. In this case the shape of the DM halos is correlated with the
morphology of the stellar remnants.Comment: Accepted for publication in ApJL, 5 pages, 3 figures, LaTeX (uses
emulateapj5.sty
The Shape of Galaxy Cluster Dark Matter Haloes: Systematics of Its Imprint on Cluster Gas, and Comparison to Observations
(Abridged) We study predictions for galaxy cluster observables that can test
the statistics of dark matter halo shapes expected in a flat LCDM universe. We
present a simple analytical model for the prediction of cluster-scale X-ray
observations, approximating clusters as isothermal systems in hydrostatic
equilibrium, and dark matter haloes as ellipsoids with uniform axial ratios. We
test the model against high-resolution, hydrodynamic cluster simulations to
gauge its reliability. We find that this simple prescription does a good job of
predicting the distribution of cluster X-ray ellipticities compared to the
simulations as long as one focuses on cluster regions that are less sensitive
to recent mergers. Based on this simple model, the distribution of cluster-size
halo shapes expected in the concordance LCDM cosmology implies an X-ray
ellipticity distribution with a mean of 0.32 +- 0.01 and a scatter of 0.14 +-
0.01 for the mass range (1-4)x10^{14} Msun/h. We find it important to include
the mass dependence of halo shape to make comparisons to observational samples
that contain many, very massive clusters. We analyse the systematics of four
observational samples of cluster ellipticities and find that our results are
statistically compatible with observations. In particular, we find remarkably
good agreement between two recent ROSAT samples and LCDM predictions that DO
NOT include gas cooling. We also test how well our analytical model can predict
Sunyaev-Zel'dovich decrement maps and find that it is less successful although
still useful; the model does not perform as well as a function of flux level in
this case because of the changing triaxiality of dark matter haloes as a
function of radial distance. Both this effect and the changing alignment of
isodensity shells of dark matter haloes leave an imprint on cluster gas...Comment: 16 pages, 9 figures; corrected typo (no result affected) submitted to
MNRA
The Shape of galaxy cluster dark matter haloes: Systematics of its imprint on cluster gas, and comparison to observations
ABSTRACT We study predictions for galaxy cluster observables that can test the statistics of dark matter halo shapes expected in a flat cold dark matter (CDM) universe. We present a simple analytical model for the prediction of cluster-scale X-ray observations, approximating clusters as isothermal systems in hydrostatic equilibrium, and dark matter haloes as ellipsoids with uniform axial ratios (homeoidal ellipsoids). We test the model against high-resolution, hydrodynamic cluster simulations to gauge its reliability. We find that this simple prescription does a good job of predicting cluster X-ray ellipticities compared to the simulations as long as one focuses on cluster regions that are less sensitive to recent mergers. Based on this simple model, the distribution of cluster-size halo shapes expected in the concordance CDM cosmology implies an X-ray ellipticity distribution with a mean X = 0.32 ± 0.01, and a scatter σ = 0.14 ± 0.01 for the mass range (1-4) × 10 14 h −1 M . We find it important to include the mass dependence of halo shape when making comparisons to observational samples. We analyse the systematics of four observational samples of cluster ellipticities and find that our results are statistically compatible with these observations. In particular, we find remarkably good agreement between two recent ROSAT samples and CDM predictions that do not include gas cooling. We also test how well our analytical model can predict Sunyaev-Zel'dovich decrement maps and find that it is less successful although still useful; the model does not perform as well as a function of flux level in this case because of the changing triaxiality of dark matter haloes as a function of radial distance. Both this effect and the changing alignment of isodensity shells of dark matter haloes leave an imprint on cluster gas that appears to be seen in observational data. Thus, dark matter haloes cannot be accurately characterized as homeoidal ellipsoids for all comparisons. Key words: cosmology: theory -dark matter -X-rays: galaxies: clusters. I N T RO D U C T I O N Clusters of galaxies are the largest bound structures in the Universe and the most recently formed ones according to the very successful cold dark matter (CDM) cosmology. As such, their dark matter (DM) haloes are expected to be less evolved and more aspherical than, say, galaxy-size haloes. Most gas in cluster DM haloes has E-mail: [email protected] (RAF); [email protected] (BA); [email protected] (AVK); [email protected] (JRP); [email protected] (DAB); [email protected] (JSB) not had time to cool, and since it is gravitationally subdominant, we can expect it to reflect the underlying 3D shape of their dark matter haloes. Indeed, large samples of X-ray clusters have been known to show a broad distribution of ellipticities in their surface brightness (SB) maps since the work of McMillan, The general expectation that in CDM-based theories DM haloes are flattened, are approximately ellipsoidal and have short-to-long axial ratios as small as s ≡ c/a ∼ 0.5 has been known for mor
The Radius of Baryonic Collapse in Disc Galaxy Formation
In the standard picture of disc galaxy formation, baryons and dark matter
receive the same tidal torques, and therefore approximately the same initial
specific angular momentum. However, observations indicate that disc galaxies
typically have only about half as much specific angular momentum as their dark
matter haloes. We argue this does not necessarily imply that baryons lose this
much specific angular momentum as they form galaxies. It may instead indicate
that galaxies are most directly related to the inner regions of their host
haloes, as may be expected in a scenario where baryons in the inner parts of
haloes collapse first. A limiting case is examined under the idealised
assumption of perfect angular momentum conservation. Namely, we determine the
density contrast Delta, with respect to the critical density of the Universe,
by which dark matter haloes need to be defined in order to have the same
average specific angular momentum as the galaxies they host. Under the
assumption that galaxies are related to haloes via their characteristic
rotation velocities, the necessary Delta is ~600. This Delta corresponds to an
average halo radius and mass which are ~60% and ~75%, respectively, of the
virial values (i.e., for Delta = 200). We refer to this radius as the radius of
baryonic collapse R_BC, since if specific angular momentum is conserved
perfectly, baryons would come from within it. It is not likely a simple step
function due to the complex gastrophysics involved, therefore we regard it as
an effective radius. In summary, the difference between the predicted initial
and the observed final specific angular momentum of galaxies, which is
conventionally attributed solely to angular momentum loss, can more naturally
be explained by a preference for collapse of baryons within R_BC, with possibly
some later angular momentum transfer.Comment: MNRAS accepted, 7 page
Best Practices for Artificial Intelligence in Life Sciences Research
We describe 11 best practices for the successful use of Artificial Intelligence and Machine Learning in the pharmaceutical and biotechnology research, on the data, technology, and organizational management levels
The Shape of Dark Matter Haloes: Dependence on Mass, Redshift, Radius and Formation
Using six high-resolution dissipationless simulations with a varying box size in a flat Lambda cold dark matter (ΛCDM) universe, we study the mass and redshift dependence of dark matter halo shapes for Mvir= 9.0 × 1011− 2.0 × 1014h−1 M⊙, over the redshift range z= 0–3, and for two values of σ8= 0.75 and 0.9. Remarkably, we find that the redshift, mass and σ8 dependence of the mean smallest-to-largest axis ratio of haloes is well described by the simple power-law relation 〈s〉= (0.54 ± 0.02)(Mvir/M*)−0.050±0.003, where s is measured at 0.3Rvir, and the z and σ8 dependences are governed by the characteristic non-linear mass, M*=M*(z, σ8). We find that the scatter about the mean s is well described by a Gaussian with σ∼ 0.1, for all masses and redshifts. We compare our results to a variety of previous works on halo shapes and find that reported differences between studies are primarily explained by differences in their methodologies. We address the evolutionary aspects of individual halo shapes by following the shapes of the haloes through ∼100 snapshots in time. We determine the formation scalefactor ac as defined by Wechsler et al. and find that it can be related to the halo shape at z= 0 and its evolution over time