We assess the effects of simulated active galactic nuclei (AGN) on the colour and morphology measurements of their host galaxies. To test the morphology measurements, we select a sample of galaxies ...not known to host AGN and add a series of point sources scaled to represent specified fractions of the observed V-band light detected from the resulting systems; we then compare morphology measurements of the simulated systems to measurements of the original galaxies. AGN contributions >rsim 20 per cent bias most of the morphology measurements tested, though the extent of the apparent bias depends on the morphological characteristics of the original galaxies. We test colour measurements by adding to non-AGN galaxy spectra a quasar spectrum scaled to contribute specified fractions of the rest-frame B-band light detected from the resulting systems. A quasar fraction of 5 per cent can move the NUV −r colour of an elliptical galaxy from the ultraviolet–optical red sequence to the green valley, and 20 per cent can move it into the blue cloud. Combining the colour and morphology results, we find that a galaxy/AGN system with an AGN contribution >rsim 20 per cent may appear bluer and more bulge dominated than the underlying galaxy. We conclude that (1) bulge-dominated, E/S0/Sa, and early-type morphology classifications are accurate for red AGN host galaxies and may be accurate for blue host galaxies, unless the AGN manifests itself as a well-defined point source and (2) although highly unobscured AGN, such as the quasar used for our experiments, can significantly bias the measured colours of AGN host galaxies, it is possible to identify such systems by examining optical images of the hosts for the presence of a point source and/or measuring the level of nuclear obscuration.
We estimate the galaxy density along lines of sight to hard extragalactic gamma-ray sources by correlating source positions on the sky with a void catalogue based on the Sloan Digital Sky Survey ...(SDSS). Extragalactic gamma-ray sources that are detected at very high energy (VHE; E > 100 GeV) or have been highlighted as VHE-emitting candidates in the Fermi Large Area Telescope hard source catalogue (together referred to as 'VHE-like' sources) are distributed along underdense lines of sight at the 2.4... level. There is also a less suggestive correlation for the Fermi hard source population (1.7...). A correlation between 10 and 500 GeV flux and underdense fraction along the line of sight for VHE-like and Fermi hard sources is found at 2.4... and 2.6..., calculated from the Pearson correlation coefficients of r = 0.57 and 0.47, respectively. The preference for underdense sight lines is not displayed by gamma-ray emitting galaxies within the second Fermi catalogue, containing sources detected above 100 MeV, or the SDSS Data Release 7 quasar catalogue. We investigate whether this marginal correlation might be a result of lower extragalactic background light (EBL) photon density within the underdense regions and find that, even in the most extreme case of a entirely underdense sight line, the EBL photon density is only 2 per cent less than the nominal EBL density. Translating this into gamma-ray attenuation along the line of sight for a highly attenuated source with opacity ..., we estimate that the attenuation of gamma-rays decreases no more than 10 per cent. This decrease, although non-negligible, is unable to account for the apparent hard source correlation with underdense lines of sight. (ProQuest: ... denotes formulae/symbols omitted.)
The nature of high-redshift galaxies Somerville, Rachel S.; Primack, Joel R.; Faber, S. M.
Monthly notices of the Royal Astronomical Society,
02/2001, Letnik:
320, Številka:
4
Journal Article
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Using semi‐analytic models of galaxy formation set within the cold dark matter (CDM) merging hierarchy, we investigate several scenarios for the nature of the high‐redshift ) Lyman‐break galaxies ...(LBGs). We consider a ‘collisional starburst’ model in which bursts of star formation are triggered by galaxy–galaxy mergers, and find that a significant fraction of LBGs are predicted to be starbursts. This model reproduces the observed comoving number density of bright LBGs as a function of redshift and the observed luminosity function at and with a reasonable amount of dust extinction. Model galaxies at have star formation rates, half‐light radii, colours and internal velocity dispersions that are in good agreement with the data. Global quantities such as the star formation rate density and cold gas and metal content of the Universe as a function of redshift also agree well. Two ‘quiescent’ models without starbursts are also investigated. In one, the star formation efficiency in galaxies remains constant with redshift, while in the other, it scales inversely with disc dynamical time, and thus increases rapidly with redshift. The first quiescent model is strongly ruled out, as it does not produce enough high‐redshift galaxies once realistic dust extinction is accounted for. The second quiescent model fits marginally, but underproduces cold gas and very bright galaxies at high redshift. A general conclusion is that star formation at high redshift must be more efficient than locally. The collisional starburst model appears to accomplish this naturally without violating other observational constraints.
We use high-resolution, dissipationless simulations of the concordance flat CDM model to make predictions for the galaxy-mass correlations and compare them with the recent Sloan Digital Sky Survey ...(SDSS) weak-lensing measurements of Sheldon et al. The simulations resolve both isolated galaxy-sized host halos and satellite halos (subhalos) within them. We use a simple scheme based on matching the circular velocity function of halos to the galaxy luminosity function and on using the observed density-color correlation of the SDSS galaxies to assign luminosities and colors to the halos. This allows us to closely match the selection criteria used to define observational samples. The simulations reproduce the observed galaxy-mass correlation function and the observed dependence of its shape and amplitude on luminosity and color, if a reasonable amount of scatter between galaxy luminosity and circular velocity is assumed. We find that the luminosity dependence of the correlation function is primarily determined by the changing relative contribution of central and satellite galaxies at different luminosities. The color dependence of the galaxy-mass correlations reflects the difference in the typical environments of blue and red galaxies. We compare the cross-biases, b sub(x) = b/r, measured in simulations and observations and find a good agreement at all probed scales. We show that the galaxy-mass correlation coefficient r is close to unity on scales greater than 61 h super(-1) Mpc. This indicates that the cross-bias measured in weak-lensing observations should measure the actual bias b of galaxy clustering on these scales. In agreement with previous studies, we find that the aperture mass-to-light ratio is independent of galaxy color in the range of luminosities probed by observational samples. The aperture mass scales approximately linearly with luminosity at L sub(r) > 10 super(10) h super(-2) L sub( ), while at lower luminosities the scaling is shallower: M sub( capital sigma ) 8 L super(0) sub(r) super(.5). We show that most of the luminous galaxies (M sub(r) < -21) are near the centers of their halos and that their galaxy-mass correlation function at r 100 h super(-1) kpc can therefore be interpreted as the average dark matter density profile of these galaxies. Finally, we find that for galaxies in a given narrow luminosity range, there is a broad and possibly non-Gaussian distribution of halo virial masses. Therefore, the average relation between mass and luminosity derived from the weak-lensing analyses should be interpreted with caution.
We construct a physically motivated model for predicting the properties of the remnants of gaseous galaxy mergers, given the properties of the progenitors and the orbit. The model is calibrated using ...a large suite of smoothed particle hydrodynamics (SPH) merger simulations. It implements generalized energy conservation while accounting for dissipative energy losses and star formation. The dissipative effects are evaluated from the initial gas fractions and from the orbital parameters via an ‘impulse’ parameter, which characterizes the strength of the encounter. Given the progenitor properties, the model predicts the remnant stellar mass, half-mass radius and velocity dispersion to an accuracy of 25 per cent. The model is valid for both major and minor mergers. We provide an explicit recipe for semi-analytic models of galaxy formation.
Terrestrial ecosystems contribute significant amounts of dissolved organic carbon (DOC) to aquatic ecosystems. Temperate lakes vary in DOC concentration as a result of variation in the spatial ...configuration and composition of vegetation within the watershed, hydrology, and within-lake processes. We have developed and parameterized a spatially explicit model of lake DOC concentrations, using data from 428 watersheds in the Adirondack Park of New York. Our analysis estimates watershed loading to each lake as a function of the cover type of each 10 x 10 m grid cell within the watershed, and its flow-path distance to the lake. The estimated export rates for the three main forest cover types were$37.7-47.0 kg C\cdot ha^{-1}\cdot yr^{-1}$. The four main wetland cover types had much higher rates of export per unit area ($188.4-227.0 kg C\cdot ha^{-1}\cdot yr^{-1}$), but wetlands occupied only 11%, on average, of watershed area. As a result, upland forests were the source of ~70% of DOC loading. There was evidence of significant interannual variation in DOC loading, correlated with interannual variation in precipitation. Estimated net in situ DOC production within the lakes was extremely low ($<1 kg C\cdot ha^{-1}\cdot yr^{-1}$). Many of the lakes have large watersheds relative to lake volume and have correspondingly high flushing rates. As a result, losses due to lake discharge generally had a larger effect on lake DOC concentrations than in-lake decay. Our approach can be readily incorporated within a GIS framework and allows examination of scenarios such as loss of wetlands, alterations in forest management, or increases in conserved areas, as a function of the unique configuration of individual watersheds.
LambdaCDM has become the standard cosmological model because its predictions agree so well with observations of the cosmic microwave background and the large-scale structure of the universe. However ...LambdaCDM has faced challenges on smaller scales. Some of these challenges, including the "angular momentum catastrophe" and the absence of density cusps in the centers of small galaxies, may be overcome with improvements in simulation resolution and feedback. Recent simulations appear to form realistic galaxies in agreement with observed scaling relations. Although dark matter halos start small and grow by accretion, the existence of a star-forming band of halo masses naturally explains why the most massive galaxies have the oldest stars, a phenomenon known as known as galactic "downsizing." The discovery of many faint galaxies in the Local Group is consistent with LambdaCDM predictions, as is the increasing evidence for substructure in galaxy dark matter halos from gravitational lensing flux anomalies and gaps in cold stellar streams. However, the "too big to fail" (TBTF) problem challenges LambdaCDM. It arose from analysis of the Acquarius and Via Lactea very high-resolution LambdaCDM simulations of dark matter halos like that of the Milky Way. Each simulated halo has 10 subhalos that were so massive and dense that they would be too big to fail to form lots of stars. The TBTF problem is that none of the observed satellite galaxies of the Milky Way or Andromeda have stars moving as fast as would be expected in these densest subhalos. This may indicate the need for a more complex theory of dark matter - or perhaps just better understanding of dark matter simulations and/or baryonic physics. PUBLICATION ABSTRACT
After a four-century rupture between science and the questions of value and meaning, this groundbreaking book presents an explosive and potentially life-altering idea: if the world could agree on a ...shared creation story based on modern cosmology and biology-a story that has just become available-it would redefine our relationship with Planet Earth and benefit all of humanity, now and into the distant future.
Written in eloquent, accessible prose and illustrated in magnificent color throughout, including images from innovative simulations of the evolving universe, this book brings the new scientific picture of the universe to life. It interprets what our human place in the cosmos may mean for us and our descendants. It offers unique insights into the potential use of this newfound knowledge to find solutions to seemingly intractable global problems such as climate change and unsustainable growth. And it explains why we need to "think cosmically, act globally" if we're going to have a long-term, prosperous future on Earth.
A new Monte Carlo radiative transfer code, SUNRISE, is used in conjunction with hydrodynamic simulations of major galaxy mergers to calculate the effects of dust in such systems. Dust has a profound ...effect on the emerging radiation, consistent with observations of dust absorption in starburst galaxies. The dust attenuation increases with luminosity such that at peak luminosities 690% of the bolometric luminosity is absorbed by dust. We find that our predictions agree with observed relationships between the UV spectral slope and the fraction of light absorbed by dust (IRX-b) and observational estimates of the optical depth as a function of intrinsic B-band or UV luminosity. In general, the detailed appearance of the merging event depends on the stage of the merger and the geometry of the encounter. The fraction of bolometric energy absorbed by the dust, however, is a robust quantity that can be predicted from the intrinsic properties bolometric luminosity, baryonic mass, star formation rate, and metallicity of the simulated system. This paper presents fitting formulae, valid over a wide range of masses and metallicities, from which the absorbed fraction of luminosity (and consequently also the infrared dust luminosity) can be predicted. The attenuation of the luminosity at specific wavelengths can also be predicted, albeit with a larger scatter due to the variation with viewing angle. These formulae for dust attenuation are consistent with earlier studies and would be suitable for inclusion in theoretical models, e.g., semianalytic models, of galaxy formation and evolution.
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