50,702 research outputs found
The quenching of star formation in accretion-driven clumpy turbulent tori of active galactic nuclei
Galactic gas-gas collisions involving a turbulent multiphase ISM share common
ISM properties: dense extraplanar gas visible in CO, large linewidths (>= 50
km/s), strong mid-infrared H_2 line emission, low star formation activity, and
strong radio continuum emission. Gas-gas collisions can occur in the form of
ICM ram pressure stripping, galaxy head-on collisions, compression of the
intragroup gas and/or galaxy ISM by an intruder galaxy which flies through the
galaxy group at a high velocity, or external gas accretion on an existing gas
torus in a galactic center. We suggest that the common theme of all these
gas-gas interactions is adiabatic compression of the ISM leading to an increase
of the turbulent velocity dispersion of the gas. The turbulent gas clouds are
then overpressured and star formation is quenched. Within this scenario we
developed a model for turbulent clumpy gas disks where the energy to drive
turbulence is supplied by external infall or the gain of potential energy by
radial gas accretion within the disk. The cloud size is determined by the size
of a C-type shock propagating in dense molecular clouds with a low ionization
fraction at a given velocity dispersion. We give expressions for the expected
volume and area filling factors, mass, density, column density, and velocity
dispersion of the clouds. The latter is based on scaling relations of
intermittent turbulence whose open parameters are estimated for the CND in the
Galactic Center. The properties of the model gas clouds and the external mass
accretion rate necessary for the quenching of the star formation rate due to
adiabatic compression are consistent with those derived from high-resolution
H_2 line observations. Based on these findings, a scenario for the evolution of
gas tori in galactic centers is proposed and the implications for star
formation in the Galactic Center are discussed.Comment: 13 pages, 1 figure, accepted for publication by A&
The Effect of Fluctuations on the Helium-Ionizing Background
Interpretation of He II Ly{\alpha} absorption spectra after the epoch of He
II reionization requires knowledge of the He II ionizing background. While past
work has modelled the evolution of the average background, the standard
cosmological radiative transfer technique assumes a uniform radiation field
despite the discrete nature of the (rare) bright quasars that dominate the
background. We implement a cosmological radiative transfer model that includes
the most recent constraints on the ionizing spectra and luminosity function of
quasars and the distribution of IGM absorbers. We also estimate, for the first
time, the effects of fluctuations on the evolving continuum opacity in two
ways: by incorporating the complete distribution of ionizing background
amplitudes into the standard approach, and by explicitly treating the quasars
as discrete -- but isolated -- sources. Our model results in a He II ionization
rate that evolves steeply with redshift, increasing by a factor ~2 from z=3.0
to z=2.5. This causes rapid evolution in the mean He II Ly{\alpha} optical
depth -- as recently observed -- without appealing to the reionization of He
II. The observed behaviour could instead result from rapid evolution in the
mean free path of ionizing photons as the helium in higher H I column density
absorbers becomes fully ionized.Comment: 14 pages, 9 figures. Accepted by MNRAS; significantly modified from
previous versio
SINFONI's take on Star Formation, Molecular Gas, and Black Hole Masses in AGN
We present some preliminary (half-way) results on our adaptive optics
spectroscopic survey of AGN at spatial scales down to 0.085arcsec. Most of the
data were obtained with SINFONI which provides integral field capability at a
spectral resolution of R~4000. The themes on which we focus in this
contribution are: star formation around the AGN, the properties of the
molecular gas and its relation to the torus, and the mass of the black hole.Comment: 5 pages, 2 figures. To appear in Science Perspectives for 3D
Spectroscopy. ESO Astrophysics Symposia. Ed by M. Kissler-Patig, M. Roth and
J. Wals
Knowledge development for organic systems: An example of weed management
Despite the large amount information on weed biology and specific weed control measures produced by researchers, organic farmers still prioritise weeds as an important area for further research. A recent project investigating weed management in organic farming systems has established that knowledge and learning are key requirements for this to be effective. Development of relevant, practically useful knowledge depends on access to information generated ‘scientifically’ by researchers and also to knowledge generated as a result of farmer experience with weeds. This requires that farmers, advisors and researchers take a participatory approach to collecting and processing information on weed management, using it to develop new and relevant knowledge. The appropriate framework for knowledge development is thus a collegiate one in which all stakeholders’ value and learn from the observations and experience of others. These findings have implications for the way in which research is conducted and funded
The ultimate outcome of black hole - neutron star mergers
We present a simple, semi--analytical description for the final stages of
mergers of black hole (BH) -- neutron star (NS) systems. Such systems are of
much interest as gravitational wave sources and gamma--ray burst progenitors.
Numerical studies show that in general the neutron star is not disrupted at the
first phase of mass transfer. Instead, what remains of the neutron star is left
on a wider, eccentric, orbit. We consider the evolution of such systems as they
lose angular momentum via gravitational radiation and come into contact for
further phases of mass transfer. During each mass transfer event the neutron
star mass is reduced until a critical value where mass loss leads to a rapid
increase in the stellar radius. At this point Roche lobe overflow shreds what
remains of the neutron star, most of the mass forming a disc around the black
hole. Such a disc may be massive enough to power a gamma--ray burst. The mass
of the neutron star at the time of disruption (and therefore the disc mass) is
largely independent of the initial masses of the black hole and neutron star,
indicating that BH--NS star mergers may be standard candles.Comment: MNRAS, in pres
Neutron star binaries and long duration gamma-ray bursts
Cosmological long-duration gamma-ray bursts (LGRBs) are thought to originate
from the core collapse to black holes of stripped massive stars. Those with
sufficient rotation form a centrifugally-supported torus whose collapse powers
the GRB. We investigate the role of tidal locking within a tight binary as a
source of the necessary angular momentum. We find that the binary orbit must be
no wider than a few solar radii for a torus to form upon core collapse.
Comparing this criterion to the observed population of binaries containing two
compact objects suggests that rotation may have been important in the formation
of up to 50% of the observed systems. As these systems created a neutron star
and not a black hole they presumably did not produce highly luminous GRBs. We
suggest instead that they make the subset of GRBs in the relatively local
universe which have much lower luminosity.Comment: 7 pages, accepted for publication in MNRA
Determining the Nature of Late Gunn-Peterson Troughs with Galaxy Surveys
Recent observations have discovered long (up to ~110 Mpc/h), opaque
Gunn-Peterson troughs in the z ~ 5.5 Lyman-alpha forest, which are challenging
to explain with conventional models of the post-reionization intergalactic
medium. Here we demonstrate that observations of the galaxy populations in the
vicinity of the deepest troughs can distinguish two competing models for these
features: deep voids where the ionizing background is weak due to fluctuations
in the mean free path of ionizing photons would show a deficit of galaxies,
while residual temperature variations from extended, inhomogeneous reionization
would show an overdensity of galaxies. We use large (~550 Mpc/h) semi-numerical
simulations of these competing explanations to predict the galaxy populations
in the largest of the known troughs at z ~ 5.7. We quantify the strong
correlation of Lyman-alpha effective optical depth and galaxy surface density
in both models and estimate the degree to which realistic surveys can measure
such a correlation. While a spectroscopic galaxy survey is ideal, we also show
that a relatively inexpensive narrowband survey of Lyman-alpha-emitting
galaxies is ~90% likely to distinguish between the competing models.Comment: 12 pages, 16 figures. Submitted to Ap
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