5,195 research outputs found
An Assessment of Cross-National Regulatory Burden Comparisons
The Article compares several rankings systems for national regulatory compliance costs. It finds the ranking systems are limited to differentiating between those countries least burdened by regulation from those most burdened by regulation. It concludes the rankings could be an important tool for deciding which countries would be the most promising for regulatory burden reduction initiatives
Chart Accompanying: An Assessment of Cross-National Regulatory Burden Comparisons
The Article compares several rankings systems for national regulatory compliance costs. It finds the ranking systems are limited to differentiating between those countries least burdened by regulation from those most burdened by regulation. It concludes the rankings could be an important tool for deciding which countries would be the most promising for regulatory burden reduction initiatives
An Assessment of Cross-National Regulatory Burden Comparisons
The Article compares several rankings systems for national regulatory compliance costs. It finds the ranking systems are limited to differentiating between those countries least burdened by regulation from those most burdened by regulation. It concludes the rankings could be an important tool for deciding which countries would be the most promising for regulatory burden reduction initiatives
Chart Accompanying: An Assessment of Cross-National Regulatory Burden Comparisons
The Article compares several rankings systems for national regulatory compliance costs. It finds the ranking systems are limited to differentiating between those countries least burdened by regulation from those most burdened by regulation. It concludes the rankings could be an important tool for deciding which countries would be the most promising for regulatory burden reduction initiatives
Lee Porter (1947-1989)
Lee Porter died unexpectedly in Seattle, Washington, on 18 January 1989. ... [After initially training in journalism] Lee became a specialist in the Pleistocene biostratigraphy of arctic regions, particularly Alaska and the U.S.S.R. ... Lee's subsequent career took her to the British Museum of Natural History on a research fellowship, then to teaching and administration at Northern Arizona University. She also worked as a consultant for several engineering firms, applying her knowledge of Quaternary deposits to environmental problems, such as geologic hazards, site stability, and waste disposal. At the time of her death, she was working as an engineering geologist for the California State Department of Transportation. ..
The mass-metallicity relation of local active galaxies
We systematically measure the gas-phase metallicities and the
mass-metallicity relation of a large sample of local active galaxies for the
first time. Observed emission-line fluxes from the Sloan Digital Sky Survey
(SDSS) are compared to a four-dimensional grid of photoionization models using
the Bayesian parameter estimation code NebulaBayes. For the first time we take
into account arbitrary mixing between HII region and narrow-line region (NLR)
emission, and the models are also varied with metallicity, ionization parameter
in the NLR, and the gas pressure. The active galactic nucleus (AGN) oxygen
abundance is found to increase by dex as a function
of host galaxy stellar mass over the range .
We also measure the metallicity and ionization parameter of 231000 star-forming
galaxies for comparison with the sample of 7670 Seyfert 2 galaxies. A
systematic offset in oxygen abundance of 0.09 dex is observed between the
mass-metallicity relations of the star-forming and active galaxies. We
investigate potential causes of the offset, including sample selection and the
treatment in the models of diffuse ionized gas, pressure, and ionization
parameter. We cannot identify the major cause(s), but suspect contributions due
to deficiencies in modeling the ionizing spectra and the treatment of dust
physics. Optical diagnostic diagrams are presented with the star-forming and
Seyfert data colored by the inferred oxygen abundance, ionization parameter and
gas pressure, clearly illustrating the trends in these quantities.Comment: 12 pages, 4 figures and 1 table; accepted for publication in Ap
How Do Disks Survive Mergers?
We develop a physical model for how galactic disks survive and/or are
destroyed in interactions. Based on dynamical arguments, we show gas primarily
loses angular momentum to internal torques in a merger. Gas within some
characteristic radius (a function of the orbital parameters, mass ratio, and
gas fraction of the merging galaxies), will quickly lose angular momentum to
the stars sharing the perturbed disk, fall to the center and be consumed in a
starburst. A similar analysis predicts where violent relaxation of the stellar
disks is efficient. Our model allows us to predict the stellar and gas content
that will survive to re-form a disk in the remnant, versus being violently
relaxed or contributing to a starburst. We test this in hydrodynamic
simulations and find good agreement as a function of mass ratio, orbital
parameters, and gas fraction, in simulations spanning a wide range in these
properties and others, including different prescriptions for gas physics and
feedback. In an immediate sense, the amount of disk that re-forms can be
understood in terms of well-understood gravitational physics, independent of
details of ISM gas physics or feedback. This allows us to explicitly quantify
the requirements for such feedback to (indirectly) enable disk survival, by
changing the pre-merger gas content and distribution. The efficiency of disk
destruction is a strong function of gas content: we show how and why
sufficiently gas-rich major mergers can, under general conditions, yield
systems with small bulges (B/T<0.2). We provide prescriptions for inclusion of
our results in semi-analytic models.Comment: 32 pages, 16 figures, accepted to ApJ (minor revisions to match
accepted version
The Formation of High Redshift Submillimeter Galaxies
We describe a model for the formation of \zsim 2 Submillimeter Galaxies
(SMGs) which simultaneously accounts for both average and bright SMGs while
providing a reasonable match to their mean observed spectral energy
distributions (SEDs). By coupling hydrodynamic simulations of galaxy mergers
with the high resolution 3D polychromatic radiative transfer code Sunrise, we
find that a mass sequence of merger models which use observational constraints
as physical input naturally yield objects which exhibit black hole, bulge, and
H2 gas masses similar to those observed in SMGs. The dominant drivers behind
the 850 micron flux are the masses of the merging galaxies and the stellar
birthcloud covering fraction. The most luminous (S850 ~ 15 mJy) sources are
recovered by ~10^13 Msun 1:1 major mergers with a birthcloud covering fraction
close to unity, whereas more average SMGs ~5-7 mJy) may be formed in lower mass
halos ~5x10^12 Msun. These models demonstrate the need for high spatial
resolution hydrodynamic and radiative transfer simulations in matching both the
most luminous sources as well as the full SEDs of SMGs. While these models
suggest a natural formation mechanism for SMGs, they do not attempt to match
cosmological statistics of galaxy populations; future efforts along this line
will help ascertain the robustness of these models.Comment: MNRAS Accepted; Revised version includes expanded discussion of
simulated radio properties of SMG
Interrogating Seyferts with NebulaBayes: Spatially probing the narrow-line region radiation fields and chemical abundances
NebulaBayes is a new Bayesian code that implements a general method of
comparing observed emission-line fluxes to photoionization model grids. The
code enables us to extract robust, spatially resolved measurements of
abundances in the extended narrow line regions (ENLRs) produced by Active
Galactic Nuclei (AGN). We observe near-constant ionization parameters but
steeply radially-declining pressures, which together imply that radiation
pressure regulates the ENLR density structure on large scales. Our sample
includes four `pure Seyfert' galaxies from the S7 survey that have extensive
ENLRs. NGC2992 shows steep metallicity gradients from the nucleus into the
ionization cones. An {\it inverse} metallicity gradient is observed in
ESO138-G01, which we attribute to a recent gas inflow or minor merger. A
uniformly high metallicity and hard ionizing continuum are inferred across the
ENLR of Mrk573. Our analysis of IC5063 is likely affected by contamination from
shock excitation, which appears to soften the inferred ionizing spectrum. The
peak of the ionizing continuum E_peak is determined by the nuclear spectrum and
the absorbing column between the nucleus and the ionized nebula. We cannot
separate variation in this intrinsic E_peak from the effects of shock or HII
region contamination, but E_peak measurements nevertheless give insights into
ENLR excitation. We demonstrate the general applicability of NebulaBayes by
analyzing a nuclear spectrum from the non-active galaxy NGC4691 using a HII
region grid. The NLR and HII region model grids are provided with NebulaBayes
for use by the astronomical community.Comment: Accepted for publication in ApJ; 29 pages with 10 figures and 3
table
A Theoretical Interpretation of the Black Hole Fundamental Plane
We examine the origin and evolution of correlations between properties of
supermassive black holes (BHs) and their host galaxies using simulations of
major galaxy mergers, including the effects of gas dissipation, cooling, star
formation, and BH accretion and feedback. We demonstrate that the simulations
predict the existence of a BH 'fundamental plane' (BHFP), of the form M_BH
sigma^(3.0+-0.3)*R_e^(0.43+-0.19) or M_BH
M_bulge^(0.54+-0.17)*sigma^(2.2+-0.5), similar to relations found
observationally. The simulations indicate that the BHFP can be understood
roughly as a tilted intrinsic correlation between BH mass and spheroid binding
energy, or the condition for feedback coupling to power a pressure-driven
outflow. While changes in halo circular velocity, merger orbital parameters,
progenitor disk redshifts and gas fractions, ISM gas pressurization, and other
parameters can drive changes in e.g. sigma at fixed M_bulge, and therefore
changes in the M_BH-sigma or M_BH-M_bulge relations, the BHFP is robust. Given
the empirical trend of decreasing R_e for a given M_bulge at high redshift, the
BHFP predicts that BHs will be more massive at fixed M_bulge, in good agreement
with recent observations. This evolution in the structural properties of merger
remnants, to smaller R_e and larger sigma (and therefore larger M_BH,
conserving the BHFP) at a given M_bulge, is driven by the fact that bulge
progenitors have characteristically larger gas fractions at high redshifts.
Adopting the observed evolution of disk gas fractions with redshift, our
simulations predict the observed trends in both R_e(M_bulge) and M_BH(M_bulge).Comment: 22 pages, 19 figures, replaced with version accepted to ApJ.
Companion paper to arXiv:0707.400
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