37,120 research outputs found
Galaxies in LCDM with Halo Abundance Matching: luminosity-velocity relation, baryonic mass-velocity relation, velocity function and clustering
It has long been regarded as difficult for a cosmological model to account
simultaneously for the galaxy luminosity, mass, and velocity distributions. We
revisit this issue using a modern compilation of observational data along with
the best available large-scale cosmological simulation of dark matter. We find
that the standard cosmological model, used in conjunction with halo abundance
matching (HAM) and simple dynamical corrections, fits all basic statistics of
galaxies with circular velocities Vcirc > 80 km/s. Our observational constraint
is the luminosity-velocity relation which allows all types of galaxies to be
included. We have compiled data for a variety of galaxies ranging from dwarf
irregulars to giant ellipticals. The data present a clear monotonic
luminosity-velocity relation from 50 km/s to 500 km/s, with a bend below 80
km/s and a systematic offset between late- and early-type galaxies. For
comparison to theory, we employ our LCDM "Bolshoi" simulation of dark matter,
which has unprecedented mass and force resolution. We use halo abundance
matching to assign rank-ordered galaxy luminosities to the dark matter halos.
The resulting predictions for the luminosity-velocity relation are in excellent
agreement with the available data on both early-type and late-type galaxies for
the luminosity range from Mr = -14-22. We also compare our predictions for the
"cold" baryon mass (i.e., stars and cold gas) of galaxies as a function of
circular velocity with the available observations, again finding a very good
agreement. The predicted circular velocity function is in agreement with the
galaxy velocity function for 80-400 km/s. However, we find that the dark matter
halos with Vcirc < 80 km/s are much more abundant than observed galaxies with
the same Vcirc . We find that the two-point correlation function of galaxies in
our model matches very well the results from the SDSS.Comment: 40 pages, 18 figures, published in Ap
Heavy particle concentration in turbulence at dissipative and inertial scales
Spatial distributions of heavy particles suspended in an incompressible
isotropic and homogeneous turbulent flow are investigated by means of high
resolution direct numerical simulations. In the dissipative range, it is shown
that particles form fractal clusters with properties independent of the
Reynolds number. Clustering is there optimal when the particle response time is
of the order of the Kolmogorov time scale . In the inertial range,
the particle distribution is no longer scale-invariant. It is however shown
that deviations from uniformity depend on a rescaled contraction rate, which is
different from the local Stokes number given by dimensional analysis. Particle
distribution is characterized by voids spanning all scales of the turbulent
flow; their signature in the coarse-grained mass probability distribution is an
algebraic behavior at small densities.Comment: 4 RevTeX pgs + 4 color Figures included, 1 figure eliminated second
part of the paper completely revise
Simultaneous non-negative matrix factorization for multiple large scale gene expression datasets in toxicology
Non-negative matrix factorization is a useful tool for reducing the dimension of large datasets. This work considers simultaneous non-negative matrix factorization of multiple sources of data. In particular, we perform the first study that involves more than two datasets. We discuss the algorithmic issues required to convert the approach into a practical computational tool and apply the technique to new gene expression data quantifying the molecular changes in four tissue types due to different dosages of an experimental panPPAR agonist in mouse. This study is of interest in toxicology because, whilst PPARs form potential therapeutic targets for diabetes, it is known that they can induce serious side-effects. Our results show that the practical simultaneous non-negative matrix factorization developed here can add value to the data analysis. In particular, we find that factorizing the data as a single object allows us to distinguish between the four tissue types, but does not correctly reproduce the known dosage level groups. Applying our new approach, which treats the four tissue types as providing distinct, but related, datasets, we find that the dosage level groups are respected. The new algorithm then provides separate gene list orderings that can be studied for each tissue type, and compared with the ordering arising from the single factorization. We find that many of our conclusions can be corroborated with known biological behaviour, and others offer new insights into the toxicological effects. Overall, the algorithm shows promise for early detection of toxicity in the drug discovery process
Interlinkages and structural changes in cross-border liabilities: a network approach
We study the international interbank market through a geometrical and a
topological analysis of empirical data. The geometrical analysis of the time
series of cross-country liabilities shows that the systematic information of
the interbank international market is contained in a space of small dimension,
from which a topological characterization could be conveniently carried out.
Weighted and complete networks of financial linkages across countries are
developed, for which continuous clustering, degree centrality and closeness
centrality are computed. The behavior of these topological coefficients reveals
an important modification acting in the financial linkages in the period
1997-2011. Here we show that, besides the generalized clustering increase,
there is a persistent increment in the degree of connectivity and in the
closeness centrality of some countries. These countries seem to correspond to
critical locations where tax policies might provide opportunities to shift
debts. Such critical locations highlight the role that specific countries play
in the network structure and helps to situates the turbulent period that has
been characterizing the global financial system since the Summer 2007 as the
counterpart of a larger structural change going on for a more than one decade.Comment: 24 pages, 11 figure
The Tully-Fisher and mass-size relations from halo abundance matching
The Tully-Fisher relation (TFR) expresses the connection between rotating
galaxies and the dark matter haloes they inhabit, and therefore contains a
wealth of information about galaxy formation. We construct a general framework
to investigate whether models based on halo abundance matching are able to
reproduce the observed stellar mass TFR and mass-size relation (MSR), and use
the data to constrain galaxy formation parameters. Our model tests a range of
plausible scenarios, differing in the response of haloes to disc formation, the
relative angular momentum of baryons and dark matter, the impact of selection
effects, and the abundance matching parameters. We show that agreement with the
observed TFR puts an upper limit on the scatter between galaxy and halo
properties, requires weak or reversed halo contraction, and favours selection
effects that preferentially eliminate fast-rotating galaxies. The MSR
constrains the ratio of the disc to halo specific angular momentum to be
approximately in the range 0.6-1.2. We identify and quantify two problems that
models of this nature face. (1) They predict too large an intrinsic scatter for
the MSR, and (2) they predict too strong an anticorrelation between the TFR and
MSR residuals. We argue that resolving these problems requires introducing a
correlation between stellar surface density and enclosed dark matter mass.
Finally, we explore the expected difference between the TFRs of central and
satellite galaxies, finding that in the favoured models this difference should
be detectable in a sample of ~700 galaxies.Comment: 27 pages, 10 figures; revised to match published MNRAS versio
Semi-empirical catalog of early-type galaxy-halo systems: dark matter density profiles, halo contraction and dark matter annihilation strength
With SDSS galaxy data and halo data from up-to-date N-body simulations we
construct a semi-empirical catalog (SEC) of early-type systems by making a
self-consistent bivariate statistical match of stellar mass (M_star) and
velocity dispersion (sigma) with halo virial mass (M_vir). We then assign
stellar mass profile and velocity dispersion profile parameters to each system
in the SEC using their observed correlations with M_star and sigma.
Simultaneously, we solve for dark matter density profile of each halo using the
spherical Jeans equation. The resulting dark matter density profiles deviate in
general from the dissipationless profile of NFW or Einasto and their mean inner
density slope and concentration vary systematically with M_vir. Statistical
tests of the distribution of profiles at fixed M_vir rule out the null
hypothesis that it follows the distribution predicted by N-body simulations for
M_vir ~< 10^{13.5-14.5} M_solar. These dark matter profiles imply that dark
matter density is, on average, enhanced significantly in the inner region of
halos with M_vir ~< 10^{13.5-14.5} M_solar supporting halo contraction. The
main characteristics of halo contraction are: (1) the mean dark matter density
within the effective radius has increased by a factor varying systematically up
to ~ 3-4 at M_vir = 10^{12} M_solar, and (2) the inner density slope has a mean
of ~ 1.3 with rho(r) ~ r^{-alpha} and a halo-to-halo rms scatter of
rms(alpha) ~ 0.4-0.5 for 10^{12} M_solar ~< M_vir ~< 10^{13-14} M_solar steeper
than the NFW profile (alpha=1). Based on our results we predict that halos of
nearby elliptical and lenticular galaxies can, in principle, be promising
targets for gamma-ray emission from dark matter annihilation.Comment: 43 pages, 20 figures, JCAP, revised and accepted versio
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