1,731 research outputs found
The Merger Rates and Mass Assembly Histories of Dark Matter Haloes in the Two Millennium Simulations
We construct merger trees of dark matter haloes and quantify their merger
rates and mass growth rates using the joint dataset from the Millennium and
Millennium-II simulations. The finer resolution of the Millennium-II Simulation
has allowed us to extend our earlier analysis of halo merger statistics to an
unprecedentedly wide range of descendant halo mass (10^10 < M0 < 10^15 Msun),
progenitor mass ratio (10^-5 < xi < 1), and redshift (0 < z < 15). We update
our earlier fitting form for the mean merger rate per halo as a function of
M_0, xi, and z. The overall behavior of this quantity is unchanged: the rate
per unit redshift is nearly independent of z out to z~15; the dependence on
halo mass is weak (M0^0.13); and it is nearly a power law in the progenitor
mass ratio (xi^-2). We also present a simple and accurate fitting formula for
the mean mass growth rate of haloes as a function of mass and redshift. This
mean rate is 46 Msun/yr for 10^12 Msun haloes at z=0, and it increases with
mass as M^{1.1} and with redshift as (1+z)^2.5 (for z > 1). When the fit for
the mean mass growth rate is integrated over a halo's history, we find
excellent match to the mean mass assembly histories of the simulated haloes. By
combining merger rates and mass assembly histories, we present results for the
number of mergers over a halo's history and the statistics of the redshift of
the last major merger.Comment: 12 pages, 9 figures, accepted in MNRA
Prescription Drug Manufacturer Attempts to Prevent Abuse of Controlled Substances
In the United States, prescription drug abuse is on the rise. This trend has impacted the makers of OxyContin®, as well as the manufacturers of other controlled substances,to reevaluate how they formulate their products, resulting in medications, that are more difficult to abuse. These abuse-deterrent formulations utilize physical, chemical and aversion barriers, specific delivery systems, and prodrug technology to prevent abuse. Additionally, some manufacturers have implemented the use of risk-management campaigns and education programs, to reduce the misuse of their products. Working together with prescription drug manufacturers, pharmacists play an important role in preventing abuse and educating patients on the appropriate use of their prescriptions
The imprint of dissipation on the shapes of merger remnant LOSVDs
The properties of elliptical galaxies are broadly consistent with simulated
remnants of gas-rich mergers between spirals, motivating more detailed studies
of the imprint of this formation mechanism on the remnant distribution
function. Gas has a strong impact on the non-Gaussian shapes of the
line-of-sight velocity distributions (LOSVDs) of the merger remnant, owing to
the embedded disk that forms out of the gas that retains its angular momentum
during the merger, and the strong central mass concentration from the gas that
falls to the center. The deviations from Gaussianity are parametrized by the
Gauss-Hermite moments h_3 and h_4, which are related to the skewness and
kurtosis of the LOSVDs. We quantify the dependence of the (h_3,h_4)-v/sigma
relations on the initial gas fraction of the progenitor disks in 1:1 mergers,
using Gadget-2 simulations including star formation, radiative cooling, and
feedback from supernovae and AGN. For gas fractions f_gas < ~15% the overall
correlation between h_3 and v/sigma is weak, consisting of a flat negatively
correlated component arising from edge-on viewing angles plus a steep
positively correlated part from face-on projections. The spread in v/sigma
values decreases toward high positive h_4, and there is a trend toward lower
h_4 as the gas fraction increases from 0 to 15%. For f_gas > ~20% the (h_3,4)-
v/sigma distributions look quite different - there is a tight negative h_3-
v/sigma correlation, and a wide spread in v/sigma values at all h_4, in better
agreement with observations. Re-mergers of the high-f_gas remnants (dry
mergers) produce slowly rotating systems with nearly Gaussian LOSVDs. We
explain all of these trends in terms of the underlying orbit structure of the
remnants, as molded by their dissipative formation histories.Comment: ApJ accepted - added some references and background on previous
studies. 9 pages, 4 figure
The growth of dark matter halos: evidence for significant smooth accretion
We study the growth of dark matter halos in the concordance LCDM cosmology
using several N-body simulations of large cosmological volumes. We build merger
trees from the Millennium and Millennium-II simulations, covering a range
10^9-10^15 Msun in halo mass and 1-10^5 in merger mass ratio. Our algorithm
takes special care of halo fragmentation and ensures that the mass contribution
of each merger to halo growth is only counted once. This way the integrated
merger rate converges and we can consistently determine the contribution of
mergers of different mass ratios to halo growth. We find that all resolved
mergers, up to mass ratios of 10^5, contribute only ~60% of the total halo mass
growth, while major mergers are subdominant, e.g. mergers with mass ratios
smaller than 3:1 (10:1) contribute only ~20% (~30%). This is verified with an
analysis of two additional simulation boxes, where we follow all particles
individually throughout cosmic time. Our results are also robust against using
several halo definitions. Under the assumption that the power-law behaviour of
the merger rate at large mass ratios can be extrapolated to arbitrarily large
mass ratios, it is found that, independently of halo mass, ~40% of the mass in
halos comes from genuinely smooth accretion of dark matter that was never bound
in smaller halos. We discuss possible implications of our findings for galaxy
formation. One implication, assuming as is standard that the pristine
intergalactic medium is heated and photoionized by UV photons, is that all
halos accrete >40% of their baryons in smooth "cold" T>~10^4K gas, rather than
as warm, enriched or clumpy gas or as stars.Comment: 11 pages, 9 figures. Accepted for publication in the Astrophysical
Journa
The Origin of the Hubble Sequence in Lambda-CDM Cosmology
The Galform semi-analytic model of galaxy formation is used to explore the
mechanisms primarily responsible for the three types of galaxies seen in the
local universe: bulge, bulge+disk and disk, identified with the visual
morphological types E, S0/a-Sbc, and Sc-Scd, respectively. With a suitable
choice of parameters the Galform model can accurately reproduce the observed
local K_s-band luminosity function (LF) for galaxies split by visual
morphological type. The successful set of model parameters is used to populate
the Millennium Simulation with 9.4 million galaxies and their dark matter
halos. The resulting catalogue is then used to explore the evolution of
galaxies through cosmic history. The model predictions concur with recent
observational results including the galaxy merger rate, the star formation rate
and the seemingly anti-hierarchical evolution of ellipticals. However, the
model also predicts significant evolution of the elliptical galaxy LF that is
not observed. The discrepancy raises the possibility that samples of z~1
galaxies which have been selected using colour and morphological criteria may
be contaminated with galaxies that are not actually ellipticals.Comment: Accepted for publication in MNRAS. Missing reference adde
Anisotropic flow in sqrt(s)=2.76 TeV Pb+Pb collisions at the LHC
The results on elliptic flow in sqrt(s)=2.76 TeV Pb+Pb collisions at the
Large Hadron Collider (LHC) reported by the ALICE collaboration are remarkably
similar to those for sqrt(s)=200 GeV gold-gold collisions at the Relativistic
Heavy Ion Collider (RHIC). This result is surprising, given the expected longer
lifetime of the system at the higher collision energies. We show that it is
nevertheless consistent with 3+1 dimensional viscous event-by-event
hydrodynamic calculations, and demonstrate that elliptic flow at both RHIC and
LHC is built up mostly within the first 5 fm/c of the evolution. We conclude
that an "almost perfect liquid" is produced in heavy-ion collisions at the LHC.
Furthermore, we present predictions for triangular flow as a function of
transverse momentum for different centralities.Comment: 5 pages, 9 figures, one figure replaced; includes event-by-event
computations with increased statistics and resolution and more accurate
matching to experimental spectra; comments on different v_2 measurement
methods added; calculation of the integrated v_2 adde
Close Galaxy Pairs at z = 3: A Challenge to UV Luminosity Abundance Matching
We use a sample of z~3 Lyman Break Galaxies (LBGs) to examine close pair
clustering statistics in comparison to LCDM-based models of structure
formation. Samples are selected by matching the LBG number density and by
matching the observed LBG 3-D correlation function of LBGs over the two-halo
term region. We show that UV-luminosity abundance matching cannot reproduce the
observed data, but if subhalos are chosen to reproduce the observed clustering
of LBGs we are able to reproduce the observed LBG pair fraction, (Nc), defined
as the average number of companions per galaxy. This model suggests an over
abundance of LBGs by a factor of ~5 over those observed, suggesting that only 1
in 5 halos above a fixed mass hosts a galaxy with LBG-like UV luminosity
detectable via LBG selection techniques. We find a total observable close pair
fraction of 23 \pm 0.6% (17.7 \pm 0.5%) using a prototypical cylinder radius in
our overdense fiducial model and 8.3 \pm 0.5% (5.6 \pm 0.2%) in an abundance
matched model (impurity corrected). For the matched spectroscopic slit
analysis, we find Ncs = 5.1\pm0.2% (1.68\pm0.02%), the average number of
companions observed serendipitously in our for fiducial slits (abundance
matched), whereas the observed fraction of serendipitous spectroscopic close
pairs is 4.7\pm1.5 per cent using the full LBG sample and 7.1\pm2.3% for a
subsample with higher signal-to-noise ratio. We show that the standard method
of halo assignment fails to reproduce the break in the LBG close pair behavior
at small scale. To reconcile these discrepancies we suggest that a plausible
fraction of LBGs in close pairs with lower mass than our sample experience
interaction-induced enhanced star formation that boosts their luminosity
sufficiently to be detected in observational sample but are not included in the
abundance matched simulation sample.Comment: 18 pages, 12 figures, 1 table, published in MNRA
Resonant relaxation and the warp of the stellar disc in the Galactic centre
Observations of the spatial distribution and kinematics of young stars in the
Galactic centre can be interpreted as showing that the stars occupy one, or
possibly two, discs of radii ~0.05-0.5 pc. The most prominent (`clockwise')
disc exhibits a strong warp: the normals to the mean orbital planes in the
inner and outer third of the disc differ by ~60 deg. Using an analytical model
based on Laplace-Lagrange theory, we show that such warps arise naturally and
inevitably through vector resonant relaxation between the disc and the
surrounding old stellar cluster.Comment: 24 pages, 8 figures, accepted by MNRA
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Toward an integrative understanding of social behavior: new models and new opportunities.
Social interactions among conspecifics are a fundamental and adaptively significant component of the biology of numerous species. Such interactions give rise to group living as well as many of the complex forms of cooperation and conflict that occur within animal groups. Although previous conceptual models have focused on the ecological causes and fitness consequences of variation in social interactions, recent developments in endocrinology, neuroscience, and molecular genetics offer exciting opportunities to develop more integrated research programs that will facilitate new insights into the physiological causes and consequences of social variation. Here, we propose an integrative framework of social behavior that emphasizes relationships between ultimate-level function and proximate-level mechanism, thereby providing a foundation for exploring the full diversity of factors that underlie variation in social interactions, and ultimately sociality. In addition to identifying new model systems for the study of human psychopathologies, this framework provides a mechanistic basis for predicting how social behavior will change in response to environmental variation. We argue that the study of non-model organisms is essential for implementing this integrative model of social behavior because such species can be studied simultaneously in the lab and field, thereby allowing integration of rigorously controlled experimental manipulations with detailed observations of the ecological contexts in which interactions among conspecifics occur
Chandra and Very Large Array Observations of the Nearby Sd Galaxy NGC 45
We present an analysis of high angular resolution observations made in the X-ray and the radio with the Chandra X-ray Observatory and the Karl Jansky Very Large Array (VLA), respectively, of the nearby spiral galaxy NGC 45. This galaxy is the third that we have considered in a study of the supernova remnant (SNR) populations of nearby spiral galaxies and the present work represents the first detailed analysis of the discrete X-ray and radio source populations of this galaxy. We analyzed data sets from the three pointed observations made of this galaxy with Chandra along with a merged data set obtained from combining these data sets: the total effective exposure time of the merged data set is 63515 s. A total of 25 discrete X-ray sources are found in the entire field of view of the ACIS-S3 chip, with 16 sources found within the visual extent of the galaxy. We estimate that as many as half of the sources detected in the entire field of view of the ACIS-S3 chip and seven of the sources detected in the optical extent of NGC 45 may be background sources. We analyzed the spectral properties of the discrete X-ray sources within the galaxy and conclude that the majority of these sources are X-ray binaries. We have searched for counterparts at different wavelengths to the discrete X-ray sources and we find two associations: one with a star cluster and the other with a background galaxy. We have found one source that is clearly variable within one observation and seven that are seen to vary from one observation to another. We also conduct a photometric analysis to determine the near-infrared fluxes of the discrete X-ray sources in Spitzer Infrared Array Camera channels. We constructed a cumulative luminosity function of the discrete X-ray sources seen toward NGC 45: taking into account simultaneously the luminosity function of background sources, the fitted slope of the cumulative luminosity function Γ = –1.3_(-1.6)^(+0.7) (all error bounds correspond to 90% confidence intervals). The VLA observations reveal seven discrete radio sources: we find no overlaps between these sources and the X-ray detected sources. Based on their measured spectral indices and their locations with respect to the visible extent of NGC 45, we classify one source as a candidate radio SNR associated with the galaxy and the others as likely background galaxies seen in projection toward NGC 45. Finally, we discuss the properties of a background cluster of galaxies (denoted as CXOU J001354.2–231254.7) seen in projection toward NGC 45 and detected by the Chandra observations. The fit parameters to the extracted Chandra spectra of this cluster are a column density N_H = 0.07(<0.14) × 10^(22) cm^(−2), a temperature kT = 4.22_(-1.42)^(+2.08) keV, an abundance Z = 0.30(<0.75) relative to solar and a redshift z = 0.28 ± 0.14. From the fit parameters we derive an electron number density n_e = 4(±1) × 10^(−3) cm^(−3), an unabsorbed X-ray luminosity L_(0.5-7.0keV) ~ 8.77(±0.96) × 10^(43) erg s^(−1) for the cluster and an X-ray emitting mass M = 2.32(±1.75) × 10^(12)M_☉
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