320 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
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
HIV-associated Cutaneous Dissemination of Visceral Leishmaniasis, Despite Negligible Immunodeficiency. Failure of Liposomal Amphotericin B Administration, Followed by Successful Pentamidine-Paromomycin Administration
and post hoc Tukey test. Statistical analysis of survival time was carried out with Kaplan-Meier (Log-Rank) test. Results: The immunized mice with this DNA vaccine presented an important reduction in diameter of lesion and increasing of weight compared to the control mice and was indicated a significant difference between the immunized group and the control groups (p < 0.05). The survival time of the immunized mice was significantly higher than the control groups (p < 0.05) after challenge with Leishmania major. The immunized mice had significantly lower parasite load compared to the control mice (p < 0.05). Conclusion: The findings of this study, indicated that the TSA encoded DNA vaccine induced protection against infection with Leishmania major in mice. In this study, we demonstrated that, the TSA -encoded DNA vaccine may be an excellent candidate for futher vaccine development
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
Commutativity, comonotonicity, and Choquet integration of self-adjoint operators
In this work, we propose a definition of comonotonicity for elements of [Formula: see text], i.e. bounded self-adjoint operators defined over a complex Hilbert space [Formula: see text]. We show that this notion of comonotonicity coincides with a form of commutativity. Intuitively, comonotonicity is to commutativity as monotonicity is to bounded variation. We also define a notion of Choquet expectation for elements of [Formula: see text] that generalizes quantum expectations. We characterize Choquet expectations as the real-valued functionals over [Formula: see text] which are comonotonic additive, [Formula: see text]-monotone, and normalized
Baryonic impact on the dark matter distribution in Milky Way-sized galaxies and their satellites
We study the impact of baryons on the distribution of dark matter in a Milky Way-sized halo by comparing a high-resolution, moving mesh cosmological simulation with its dark matter-only counterpart. We identify three main processes related to baryons \u2013 adiabatic contraction, tidal disruption, and reionization \u2013 which jointly shape the dark matter distribution in both the main halo and its subhaloes. The relative effect of each baryonic process depends strongly on the subhalo mass. For massive subhaloes with maximum circular velocity vmax > 35\u2009km\u2009s 121, adiabatic contraction increases the dark matter concentration, making these haloes less susceptible to tidal disruption. For low-mass subhaloes with vmax < 20\u2009km\u2009s 121, reionization effectively reduces their mass on average by 4830 per cent and vmax by 4820 per cent. For intermediate subhaloes with 20\u2009km\u2009s 121 < vmax < 35\u2009km\u2009s 121, which share a similar mass range as the classical dwarf spheroidals, strong tidal truncation induced by the main galaxy reduces their vmax. As a combined result of reionization and increased tidal disruption, the total number of low-mass subhaloes in the hydrodynamic simulation is nearly halved compared to that of the N-body simulation. We do not find dark matter cores in dwarf galaxies, unlike previous studies that employed bursty feedback-driven outflows. The substantial impact of baryons on the abundance and internal structure of subhaloes suggests that galaxy formation and evolution models based on N-body simulations should include these physical processes as major components
A Deep Learning Approach to Galaxy Cluster X-ray Masses
We present a machine-learning approach for estimating galaxy cluster masses
from Chandra mock images. We utilize a Convolutional Neural Network (CNN), a
deep machine learning tool commonly used in image recognition tasks. The CNN is
trained and tested on our sample of 7,896 Chandra X-ray mock observations,
which are based on 329 massive clusters from the IllustrisTNG simulation. Our
CNN learns from a low resolution spatial distribution of photon counts and does
not use spectral information. Despite our simplifying assumption to neglect
spectral information, the resulting mass values estimated by the CNN exhibit
small bias in comparison to the true masses of the simulated clusters (-0.02
dex) and reproduce the cluster masses with low intrinsic scatter, 8% in our
best fold and 12% averaging over all. In contrast, a more standard core-excised
luminosity method achieves 15-18% scatter. We interpret the results with an
approach inspired by Google DeepDream and find that the CNN ignores the central
regions of clusters, which are known to have high scatter with mass.Comment: 10 pages, 6 figures, accepted for publication in The Astrophysical
Journa
Quenched fractions in the IllustrisTNG simulations: Comparison with observations and other theoretical models
We make an in-depth comparison of the IllustrisTNG cosmological simulations with observed quenched fractions of central and satellite galaxies, for Mstars = 109-12 M⊙ at 0 ≤ z ≤ 3. We show how measurement choices [aperture, quenched definition, and star formation rate (SFR) indicator time-scale], as well as sample selection issues (projection effects, satellite/central misclassification, and host mass distribution sampling), impact this comparison. The quenched definition produces differences of up to 70 (30) percentage points for centrals (satellites) above ∼1010.5 M⊙. At z Z 2, a larger aperture within which SFR is measured suppresses the quenched fractions by up to ∼50 percentage points. Proper consideration of the stellar and host mass distributions is crucial: Naive comparisons to volume-limited samples from simulations lead to misinterpretation of the quenched fractions as a function of redshift by up to 20 percentage points. Including observational uncertainties to theoretical values of Mstars and SFR changes the quenched fraction values and their trend and/or slope with mass. Taking projected rather than three-dimensional distances for satellites decreases the quenched fractions by up to 10 per cent. TNG produces quenched fractions for both centrals and satellites broadly consistent with observations and predicts up to ∼80 (90) per cent of quenched centrals at z = 0 (z = 2), in line with recent observations, and higher than other theoretical models. The quantitative agreement of TNG and Sloan Digital Sky Survey for satellite quenched fractions in groups and clusters depends strongly on the galaxy and host mass range. Our mock comparison highlights the importance of properly accounting for observational effects and biases
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