115 research outputs found
Simulations of High-Velocity Clouds. II. Ablation from High-Velocity Clouds as a Source of Low-Velocity High Ions
In order to determine if the material ablated from high-velocity clouds
(HVCs) is a significant source of low-velocity high ions (C IV, N V, and O VI)
such as those found in the Galactic halo, we simulate the hydrodynamics of the
gas and the time-dependent ionization evolution of its carbon, nitrogen, and
oxygen ions. Our suite of simulations examines the ablation of warm material
from clouds of various sizes, densities, and velocities as they pass through
the hot Galactic halo. The ablated material mixes with the environmental gas,
producing an intermediate-temperature mixture that is rich in high ions and
that slows to the speed of the surrounding gas. We find that the slow mixed
material is a significant source of the low-velocity O VI that is observed in
the halo, as it can account for at least ~1/3 of the observed O VI column
density. Hence, any complete model of the high ions in the halo should include
the contribution to the O VI from ablated HVC material. However, such material
is unlikely to be a major source of the observed C IV, presumably because the
observed C IV is affected by photoionization, which our models do not include.
We discuss a composite model that includes contributions from HVCs, supernova
remnants, a cooling Galactic fountain, and photoionization by an external
radiation field. By design, this model matches the observed O VI column
density. This model can also account for most or all of the observed C IV, but
only half of the observed N V.Comment: 17 pages, 8 figures. Accepted for publication in the Astrophysical
Journa
Simulations of High-Velocity Clouds. I. Hydrodynamics and High-Velocity High Ions
We present hydrodynamic simulations of high-velocity clouds (HVCs) traveling
through the hot, tenuous medium in the Galactic halo. A suite of models was
created using the FLASH hydrodynamics code, sampling various cloud sizes,
densities, and velocities. In all cases, the cloud-halo interaction ablates
material from the clouds. The ablated material falls behind the clouds, where
it mixes with the ambient medium to produce intermediate-temperature gas, some
of which radiatively cools to less than 10,000 K. Using a non-equilibrium
ionization (NEI) algorithm, we track the ionization levels of carbon, nitrogen,
and oxygen in the gas throughout the simulation period. We present
observation-related predictions, including the expected H I and high ion (C IV,
N V, and O VI) column densities on sight lines through the clouds as functions
of evolutionary time and off-center distance. The predicted column densities
overlap those observed for Complex C. The observations are best matched by
clouds that have interacted with the Galactic environment for tens to hundreds
of megayears. Given the large distances across which the clouds would travel
during such time, our results are consistent with Complex C having an
extragalactic origin. The destruction of HVCs is also of interest; the smallest
cloud (initial mass \approx 120 Msun) lost most of its mass during the
simulation period (60 Myr), while the largest cloud (initial mass \approx 4e5
Msun) remained largely intact, although deformed, during its simulation period
(240 Myr).Comment: 20 pages, 13 figures. Accepted for publication in the Astrophysical
Journa
The Evolution of Gas Clouds Falling in the Magnetized Galactic Halo: High Velocity Clouds (HVCs) Originated in the Galactic Fountain
In the Galactic fountain scenario, supernovae and/or stellar winds propel
material into the Galactic halo. As the material cools, it condenses into
clouds. By using FLASH three-dimensional magnetohydrodynamic simulations, we
model and study the dynamical evolution of these gas clouds after they form and
begin to fall toward the Galactic plane. In our simulations, we assume that the
gas clouds form at a height of z=5 kpc above the Galactic midplane, then begin
to fall from rest. We investigate how the cloud's evolution, dynamics, and
interaction with the interstellar medium (ISM) are affected by the initial mass
of the cloud. We find that clouds with sufficiently large initial densities (>
0.1 hydrogen atoms per cc) accelerate sufficiently and maintain sufficiently
large column densities as to be observed and identified as high-velocity clouds
(HVCs) even if the ISM is weakly magnetized (1.3 micro Gauss). We also
investigate the effects of various possible magnetic field configurations. As
expected, the ISM's resistance is greatest when the magnetic field is strong
and perpendicular to the motion of the cloud. The trajectory of the cloud is
guided by the magnetic field lines in cases where the magnetic field is
oriented diagonal to the Galactic plane. The model cloud simulations show that
the interactions between the cloud and the ISM can be understood via analogy to
the shock tube problem which involves shock and rarefaction waves. We also
discuss accelerated ambient gas, streamers of material ablated from the clouds,
and the cloud's evolution from a sphere-shaped to a disk- or cigar-shaped
object.Comment: 46 pages, 16 figures, 3 tables. Accepted for publication in Ap
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