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

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_☉