462 research outputs found
Efficiency of gas cooling and accretion at the disc-corona interface
In star-forming galaxies, stellar feedback can have a dual effect on the
circumgalactic medium both suppressing and stimulating gas accretion. The
trigger of gas accretion can be caused by disc material ejected into the halo
in the form of fountain clouds and by its interaction with the surrounding hot
corona. Indeed, at the disc-corona interface, the mixing between the
cold/metal-rich disc gas (T ~ 10^6 K)
can dramatically reduce the cooling time of a portion of the corona and produce
its condensation and accretion. We studied the interaction between fountain
clouds and corona in different galactic environments through parsec-scale
hydrodynamical simulations, including the presence of thermal conduction, a key
mechanism that influences gas condensation. Our simulations showed that the
coronal gas condensation strongly depends on the galactic environment, in
particular it is less efficient for increasing virial temperature/mass of the
haloes where galaxies reside and it is fully ineffective for objects with
virial masses larger than 10^13 Msun. This result implies that the coronal gas
cools down quickly in haloes with low-intermediate virial mass (Mvir <~ 3 x
10^12 Msun) but the ability to cool the corona decreases going from late-type
to early-type disc galaxies, potentially leading to the switching off of
accretion and the quenching of star formation in massive systems.Comment: 14 pages, 8 figures, accepted for publication in MNRA
Magnetic fields in cosmological simulations of disk galaxies
Observationally, magnetic fields reach equipartition with thermal energy and
cosmic rays in the interstellar medium of disk galaxies such as the Milky Way.
However, thus far cosmological simulations of the formation and evolution of
galaxies have usually neglected magnetic fields. We employ the moving-mesh code
\textsc{Arepo} to follow for the first time the formation and evolution of a
Milky Way-like disk galaxy in its full cosmological context while taking into
account magnetic fields. We find that a prescribed tiny magnetic seed field
grows exponentially by a small-scale dynamo until it saturates around
with a magnetic energy of about of the kinetic energy in the center of
the galaxy's main progenitor halo. By , a well-defined gaseous disk forms
in which the magnetic field is further amplified by differential rotation,
until it saturates at an average field strength of \sim 6 \mug in the disk
plane. In this phase, the magnetic field is transformed from a chaotic
small-scale field to an ordered large-scale field coherent on scales comparable
to the disk radius. The final magnetic field strength, its radial profile and
the stellar structure of the disk compare well with observational data. A minor
merger temporarily increases the magnetic field strength by about a factor of
two, before it quickly decays back to its saturation value. Our results are
highly insensitive to the initial seed field strength and suggest that the
large-scale magnetic field in spiral galaxies can be explained as a result of
the cosmic structure formation process.Comment: 5 pages, 4 figures, accepted to ApJ
The survival of gas clouds in the Circumgalactic Medium of Milky Way-like galaxies
Observational evidence shows that low-redshift galaxies are surrounded by
extended haloes of multiphase gas, the so-called 'circumgalactic medium' (CGM).
To study the survival of relatively cool gas (T < 10^5 K) in the CGM, we
performed a set of hydrodynamical simulations of cold (T = 10^4 K) neutral gas
clouds travelling through a hot (T = 2x10^6 K) and low-density (n = 10^-4
cm^-3) coronal medium, typical of Milky Way-like galaxies at large
galactocentric distances (~ 50-150 kpc). We explored the effects of different
initial values of relative velocity and radius of the clouds. Our simulations
were performed on a two-dimensional grid with constant mesh size (2 pc) and
they include radiative cooling, photoionization heating and thermal conduction.
We found that for large clouds (radii larger than 250 pc) the cool gas survives
for very long time (larger than 250 Myr): despite that they are partially
destroyed and fragmented into smaller cloudlets during their trajectory, the
total mass of cool gas decreases at very low rates. We found that thermal
conduction plays a significant role: its effect is to hinder formation of
hydrodynamical instabilities at the cloud-corona interface, keeping the cloud
compact and therefore more difficult to destroy. The distribution of column
densities extracted from our simulations are compatible with those observed for
low-temperature ions (e.g. SiII and SiIII) and for high-temperature ions (OVI)
once we take into account that OVI covers much more extended regions than the
cool gas and, therefore, it is more likely to be detected along a generic line
of sight.Comment: 12 pages, 10 figures. Accepted for publication in MNRA
Galactic fountains and gas accretion
Star-forming disc galaxies such as the Milky Way need to accrete \gsim 1
of gas each year to sustain their star formation. This gas
accretion is likely to come from the cooling of the hot corona, however it is
still not clear how this process can take place. We present simulations
supporting the idea that this cooling and the subsequent accretion are caused
by the passage of cold galactic-fountain clouds through the hot corona. The
Kelvin-Helmholtz instability strips gas from these clouds and the stripped gas
causes coronal gas to condense in the cloud's wake. For likely parameters of
the Galactic corona and of typical fountain clouds we obtain a global accretion
rate of the order of that required to feed the star formation.Comment: 2 pages, 1 figure, to appear in "Hunting for the Dark: The Hidden
Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds. V.P. Debattista &
C.C. Popescu, AIP Conf. Se
Fountain-driven gas accretion by the Milky Way
Accretion of fresh gas at a rate of ~ 1 M_{sun} yr^{-1} is necessary in
star-forming disc galaxies, such as the Milky Way, in order to sustain their
star-formation rates. In this work we present the results of a new hydrodynamic
simulation supporting the scenario in which the gas required for star formation
is drawn from the hot corona that surrounds the star-forming disc. In
particular, the cooling of this hot gas and its accretion on to the disc are
caused by the passage of cold galactic fountain clouds through the corona.Comment: 2 pages, 1 figure. To appear in the proceedings of the conference
"Assembling the Puzzle of the Milky Way", Le Grand-Bornand 17-22 April 2011,
European Physical Journal, editors C. Reyl\'e, A. Robin and M. Schulthei
The origin of the high-velocity cloud complex C
High-velocity clouds consist of cold gas that appears to be raining down from
the halo to the disc of the Milky Way. Over the past fifty years, two competing
scenarios have attributed their origin either to gas accretion from outside the
Galaxy or to circulation of gas from the Galactic disc powered by supernova
feedback (galactic fountain). Here we show that both mechanisms are
simultaneously at work. We use a new galactic fountain model combined with
high-resolution hydrodynamical simulations. We focus on the prototypical cloud
complex C and show that it was produced by an explosion that occurred in the
Cygnus-Outer spiral arm about 150 million years ago. The ejected material has
triggered the condensation of a large portion of the circumgalactic medium and
caused its subsequent accretion onto the disc. This fountain-driven cooling of
the lower Galactic corona provides the low-metallicity gas required by chemical
evolution models of the Milky Way's disc.Comment: 6 pages, 4 figures, 1 table; accepted by MNRA
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