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 ($1-10^{4} \, \rm{cm}^{-3}$), density profile slopes ($0-1.5$), AGN luminosities ($10^{44}-10^{47} \, \rm{erg} \, \rm{s}^{-1}$), and metallicities ($0.1-3 \rm{Z}_{\odot}$). 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 $\approx$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 $\approx1-2$, 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$_2$ outflow rates up to 140 M$_\odot$ yr$^{-1}$, 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$_2$ 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 $\approx$2 lower than is observed. We derive a CO (1-0) to H$_2$ conversion factor of $\alpha_{\rm{CO} (1-0)}$ = 0.13 M$_\odot$ (K km s$^{-1}$ pc$^2$)$^{-1}$, 6 times lower than is commonly assumed in observations of such systems. We find strong emission from the mid-infrared lines of H$_2$. The mass of H$_2$ traced by this infrared emission is within a few per cent of the total H$_2$ mass. This H$_2$ 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$^{3+}$ magnetic moments in the Tb$_2$Ti$_2$O$_7$ 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$^{3+}$ ions in Tb$_2$Ti$_2$O$_7$ possess a moment of approximatively 5$\mu_{\rm B}$, and (2) the ground state $g-$tensor is extremely anisotropic below a temperature of $O(10^0)$K, with Ising-like Tb$^{3+}$ magnetic moments confined to point along a local cubic $$diagonal (e.g. towards the middle of the tetrahedron). Such a very large easy-axis Ising like anisotropy along a$$ 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 Tb$_2$Ti$_2$O$_7$ failing to develop long-range order at a temperature of the order of the paramagnetic Curie-Weiss temperature $\theta_{\rm CW} \approx -10^1$K. We suggest that dipolar interactions and extra perturbative exchange coupling(s)beyond nearest-neighbors may be responsible for the lack of ordering of Tb$_2$Ti$_2$O$_7$.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 $n_8$ and $n_{10}$, 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