179 research outputs found
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
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
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