1,509 research outputs found
Radiative cooling of swept up gas in AGN-driven galactic winds and its implications for molecular outflows
We recently used hydro-chemical simulations to demonstrate that molecular
outflows observed in luminous quasars can be explained by molecule formation
within the AGN wind. However, these simulations cover a limited parameter
space, due to their computational cost. We have therefore developed an analytic
model to follow cooling in the shocked ISM layer of an AGN wind. We explore
different ambient densities (), density profile
slopes (), AGN luminosities (), and metallicities (). The swept up gas
mostly cools within ~1 Myr. Based on our previous simulations, we predict that
this gas would produce observable molecular outflows. The instantaneous
momentum boost initially increases as the outflow decelerates. However, it
reaches a maximum of 20, due to work done against the gravitational
potential. The predicted time-averaged observational estimate of the molecular
outflow momentum boost reaches a maximum of , partly due to our
assumed molecular fraction, 0.2, but also because the instantaneous and
observational, time-averaged definitions are not equivalent. Thus recent
observational estimates of order unity momentum boosts do not necessarily rule
out energy-driven outflows. Finally, we find that dust grains are likely to
re-form by accretion of metals after the shocked ISM layer has cooled, assuming
that a small fraction of dust grains swept up after this layer has cooled are
able to mix into the cool phase, and assuming that grain growth remains
efficient in the presence of the strong AGN radiation field. This would enable
rapid molecule formation, as assumed in our models.Comment: 22 pages, 16 figures (including appendices). Accepted for publication
in MNRA
The origin of fast molecular outflows in quasars: molecule formation in AGN-driven galactic winds
We explore the origin of fast molecular outflows that have been observed in
Active Galactic Nuclei (AGN). Previous numerical studies have shown that it is
difficult to create such an outflow by accelerating existing molecular clouds
in the host galaxy, as the clouds will be destroyed before they can reach the
high velocities that are observed. In this work, we consider an alternative
scenario where molecules form in-situ within the AGN outflow. We present a
series of hydro-chemical simulations of an isotropic AGN wind interacting with
a uniform medium. We follow the time-dependent chemistry of 157 species,
including 20 molecules, to determine whether molecules can form rapidly enough
to produce the observed molecular outflows. We find H outflow rates up to
140 M yr, which is sensitive to density, AGN luminosity, and
metallicity. We compute emission and absorption lines of CO, OH and warm (a few
hundred K) H from the simulations in post-processing. The CO-derived
outflow rates and OH absorption strengths at solar metallicity agree with
observations, although the maximum line of sight velocities from the model CO
spectra are a factor 2 lower than is observed. We derive a CO (1-0) to
H conversion factor of = 0.13 M (K km
s pc), 6 times lower than is commonly assumed in observations
of such systems. We find strong emission from the mid-infrared lines of H.
The mass of H traced by this infrared emission is within a few per cent of
the total H mass. This H emission may be observable by JWST.Comment: 30 pages, 21 figures (including appendices), resubmitted to MNRAS
following referee's report. Some results have changed from the previous
version, in particular for warm H2 emission (see Figs. 5 and 13
A Direct Measurement of the IGM Opacity to HI Ionizing Photons
We present a new method to directly measure the opacity from HI Lyman limit
(LL) absorption k_LL along quasar sightlines by the intergalactic medium (IGM).
The approach analyzes the average (``stacked'') spectrum of an ensemble of
quasars at a common redshift to infer the mean free path (MFP) to ionizing
radiation. We apply this technique to 1800 quasars at z=3.50-4.34 drawn from
the Sloan Digital Sky Survey (SDSS), giving the most precise measurements on
k_LL at any redshift. From z=3.6 to 4.3, the opacity increases steadily as
expected and is well parameterized by MFP = (48.4 +/- 2.1) - (38.0 +/-
5.3)*(z-3.6) h^-1 Mpc (proper distance). The relatively high MFP values
indicate that the incidence of systems which dominate k_LL evolves less
strongly at z>3 than that of the Lya forest. We infer a mean free path three
times higher than some previous estimates, a result which has important
implications for the photo-ionization rate derived from the emissivity of star
forming galaxies and quasars. Finally, our analysis reveals a previously
unreported, systematic bias in the SDSS quasar sample related to the survey's
color targeting criteria. This bias potentially affects all z~3 IGM studies
using the SDSS database.Comment: 7 pages, 4 figures; Accepted to ApJ
Resonantly enhanced filamentation in gases
In this Letter, a low-loss Kerr-driven optical filament in Krypton gas is
experimentally reported in the ultraviolet. The experimental findings are
supported by ab initio quantum calculations describing the atomic optical
response. Higher-order Kerr effect induced by three-photon resonant transitions
is identified as the underlying physical mechanism responsible for the
intensity stabilization during the filamentation process, while ionization
plays only a minor role. This result goes beyond the commonly-admitted paradigm
of filamentation, in which ionization is a necessary condition of the filament
intensity clamping. At resonance, it is also experimentally demonstrated that
the filament length is greatly extended because of a strong decrease of the
optical losses
Magnetic and Thermodynamic Properties of the Collective Paramagnet-Spin Liquid Pyrochlore Tb2Ti2O7
In a recent letter [Phys. Rev. Lett. {\bf 82}, 1012 (1999)] it was found that
the Tb magnetic moments in the TbTiO pyrochlore lattice of
corner-sharing tetrahedra remain in a {\it collective paramagnetic} state down
to 70mK. In this paper we present results from d.c. magnetic susceptibility,
specific heat data, inelastic neutron scattering measurements, and crystal
field calculations that strongly suggest that (1) the Tb ions in
TbTiO possess a moment of approximatively 5, and (2)
the ground state tensor is extremely anisotropic below a temperature of
K, with Ising-like Tb magnetic moments confined to point along
a local cubic direction
dramatically reduces the frustration otherwise present in a Heisenberg
pyrochlore antiferromagnet. The results presented herein underpin the
conceptual difficulty in understanding the microscopic mechanism(s) responsible
for TbTiO failing to develop long-range order at a temperature of
the order of the paramagnetic Curie-Weiss temperature K. We suggest that dipolar interactions and extra perturbative exchange
coupling(s)beyond nearest-neighbors may be responsible for the lack of ordering
of TbTiO.Comment: 8 POSTSCRIPT figures included. Submitted to Physical Review B.
Contact: [email protected]
Spectral dependence of purely-Kerr driven filamentation in air and argon
Based on numerical simulations, we show that higher-order nonlinear indices
(up to and , respectively) of air and argon have a dominant
contribution to both focusing and defocusing in the self-guiding of ultrashort
laser pulses over most of the spectrum. Plasma generation and filamentation are
therefore decoupled. As a consequence, ultraviolet wavelength may not be the
optimal wavelengths for applications requiring to maximize ionization.Comment: 14 pages, 4 figures (14 panels
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