783 research outputs found
Time-dependent Circulation Flows: Iron Enrichment in Cooling Flows with Heated Return Flows
We describe a new type of dynamical model for hot gas in galaxy groups and
clusters in which gas moves simultaneously in both radial directions.
Circulation flows are consistent with (1) the failure to observe cooling gas in
X-ray spectra, (2) multiphase gas observed near the centers of these flows and
(3) the accumulation of iron in the hot gas from Type Ia supernovae in the
central galaxy. Dense inflowing gas cools, producing a positive central
temperature gradient, as in normal cooling flows. Bubbles of hot, buoyant gas
flow outward. Circulation flows eventually cool catastrophically if the outward
flowing gas transports mass but no heat; to maintain the circulation both mass
and energy must be supplied to the inflowing gas over a large volume, extending
to the cooling radius. The rapid radial recirculation of gas produces a flat
central core in the gas iron abundance, similar to many observations. We
believe the circulation flows described here are the first gasdynamic,
long-term evolutionary models that are in good agreement with all essential
features observed in the hot gas: little or no gas cools as required by XMM
spectra, the gas temperature increases outward near the center, and the gaseous
iron abundance is about solar near the center and decreases outward.Comment: 17 pages (emulateapj5) with 6 figures; accepted by The Astrophysical
Journa
O clima de São Joaquim para a viticultura de vinhos finos: classificação pelo sistema CCM Geovitícola.
bitstream/item/37166/1/BRIGHENTI-TONIETTO-2004.pdfArtigo técnico online
Relative Sizes of X-ray and Optical Images of Elliptical Galaxies; Correlation with X-ray Luminosity
Optical parameters of elliptical galaxies are tightly correlated, but their
x-ray parameters vary widely. The x-ray luminosity L_x ranges over more than an
order of magnitude for ellipticals having similar optical luminosity L_B. The
source of this scatter has been elusive. We show here that the dispersion in
L_x for fixed optical luminosity L_B correlates strongly with the dimensionless
ratio of the sizes of the x-ray and optical images, r_ex/r_e. Specifically, we
find that (L_x/L_B) is proportional to (r_{ex}/r_e)^{0.60 \pm 0.30}, a version
of the correlation that is independent of distance. This correlation may be a
natural result of mergings and tidal truncations that are expected during the
formation and early evolution of ellipticals in groups of galaxies. The radial
structure of x-ray images also varies: some are compact (e.g. NGC 4649, 7626,
5044), others diffuse (e.g. NGC 4636, 1399).Comment: 5 pages, 3 figures. Accepted for publication in Astrophysical Journal
Letter
Galactic Outflows and the pollution of the Galactic Environment by Supernovae
We here explore the effects of the SN explosions into the environment of
star-forming galaxies like the Milky Way. Successive randomly distributed and
clustered SNe explosions cause the formation of hot superbubbles that drive
either fountains or galactic winds above the galactic disk, depending on the
amount and concentration of energy that is injected by the SNe. In a galactic
fountain, the ejected gas is re-captured by the gravitational potential and
falls back onto the disk. From 3D nonequilibrium radiative cooling
hydrodynamical simulations of these fountains, we find that they may reach
altitudes up to about 5 kpc in the halo and thus allow for the formation of the
so called intermediate-velocity-clouds (IVCs) which are often observed in the
halos of disk galaxies. The high-velocity-clouds that are also observed but at
higher altitudes (of up to 12 kpc) require another mechanism to explain their
production. We argue that they could be formed either by the capture of gas
from the intergalactic medium and/or by the action of magnetic fields that are
carried to the halo with the gas in the fountains. Due to angular momentum
losses to the halo, we find that the fountain material falls back to smaller
radii and is not largely spread over the galactic disk. Instead, the SNe ejecta
fall nearby the region where the fountain was produced, a result which is
consistent with recent chemical models of the galaxy. The fall back material
leads to the formation of new generations of molecular clouds and to supersonic
turbulence feedback in the disk.Comment: 10 pages, 5 figures; paper of invited talk for the Procs. of the 2007
WISER Workshop (World Space Environment Forum), Alexandria, Egypt, October
2007, Spa. Sci. Rev
Chandra Detection of Massive Black Holes in Galactic Cooling Flows
Anticipating forthcoming observations with the Chandra X-ray telescope, we
describe the continuation of interstellar cooling flows deep into the cores of
elliptical galaxies. Interstellar gas within about r = 50 parsecs from the
massive black hole is heated to T > 1 keV and should be visible unless thermal
heating is diluted by non-thermal pressure. Since our flows are subsonic near
the massive black holes, distributed cooling continues within 300 pc from the
center. Dark, low mass stars formed in this region may be responsible for some
of the mass attributed to central black holes.Comment: 6 pages with 3 figures; accepted by Astrophysical Journal Letter
Turbulent Mixing in Clusters of Galaxies
We present a spherically-symmetric, steady-state model of galaxy clusters in
which radiative cooling from the hot gas is balanced by heat transport through
turbulent mixing. We assume that the gas is in hydrostatic equilibrium, and
describe the turbulent heat diffusion by means of a mixing length prescription
with a dimensionless parameter alpha_mix. Models with alpha_mix ~ 0.01-0.03
yield reasonably good fits to the observed density and temperature profiles of
cooling core clusters. Making the strong simplification that alpha_mix is
time-independent and that it is roughly the same in all clusters, the model
reproduces remarkably well the observed scalings of X-ray luminosity, gas mass
fraction and entropy with temperature. The break in the scaling relations at kT
\~ 1-2 keV is explained by the break in the cooling function at around this
temperature, and the entropy floor observed in galaxy groups is reproduced
naturally.Comment: Accepted for publication in ApJ
Growth of boundary-layer streaks due to free-stream turbulence
The growth of laminar boundary-layer streaks caused by free-stream turbulence
encountering a flat plate in zero-pressure-gradient conditions is investigated experimentally
in a wind tunnel and numerically by solving the unsteady boundary-region
equations. A comparative discussion amongst the most relevant theoretical frameworks,
such as the Goldstein theory, the Taylor-Stewartson theory, the optimalgrowth
theory and the Orr-Sommerfeld theory, is first presented and parallels and
complimentary aspects of the theories are pointed out to justify the use of the
Goldstein theory in our study. The statistical properties of the positive and negative
fluctuations of the laminar streaks are discussed, showing how the total time average
of the boundary-layer fluctuations masks the true character of the disturbance flow
and revealing that the maximum values and the root-mean-square of positive and
negative fluctuations grow downstream at the same rate. The downstream growth
rate of the low-frequency disturbances and the decay rate of the high-frequency
disturbances are also computed for the first time. The numerical solutions of the unsteady
boundary-region equations are compared successfully with the streak profiles
measured in the wind tunnel and with direct numerical simulation results available
in the literatur
Thermal Evolution of Supernova Iron in Elliptical Galaxies
In explaining the relative metal abundances observed in galaxy groups and
clusters, it is generally assumed that all metals created by supernovae are
present either in visible stars or the hot gas. We discuss here the possibility
that some of the iron expelled into the hot gas by Type Ia supernovae may have
radiatively cooled, avoiding detection by X-ray and optical observers.
Hydrodynamic models of Type Ia explosions in the hot gas inside elliptical
galaxies result in a gas of nearly pure iron that is several times hotter than
the local interstellar gas. We describe the subsequent thermal evolution of the
iron-rich gas as it radiates and thermally mixes with the surrounding gas.
There is a critical time by which the iron ions must mix into the ambient gas
to avoid rapid radiative cooling. We find that successful mixing is possible if
the iron ions diffuse with large mean free paths, as in an unmagnetized plasma.
However, in microgauss fields the Larmor radii of the iron ions are
exceptionally small, so the field geometry must be highly tangled or radial to
allow the iron to mix by diffusion faster than it cools by radiative losses.
The possibility that some of the supernova iron cools cannot be easily
discounted.Comment: 27 pages (aastex) including 7 figures; accepted by The Astrophysical
Journa
Evolution of Hot Gas and Dark Halos in Group-Dominant Elliptical Galaxies: Influence of Cosmic Inflow
We study the complete dynamical evolution of hot interstellar gas in massive
elliptical galaxies born into a simple flat universe beginning with an
overdense perturbation. Within the turn-around radius dark matter flows in a
self-similar fashion into a stationary Navarro-Frenk-White halo and the
baryonic gas shocks. After a few gigayears, when enough gas accumulates within
the accretion shock, the de Vaucouleurs stellar system is constructed and the
energy from Type II supernovae is released. The stars and dark halo are matched
to NGC 4472. Gas continues to enter the galaxy by secondary infall and by
stellar mass loss based on a Salpeter IMF. After about 13 Gyrs the temperature
and density distribution in the hot gas agree quite well with the hot
interstellar gas observed in NGC 4472. As a result of supernova-driven outflow,
the present day baryonic fraction has a deep minimum in the outer galactic
halo. When relatively gas-rich, X-ray luminous models are spatially truncated
at early times, simulating tidal events that may have occurred during galaxy
group dynamics, the current locus of truncated models lies just along the
, X-ray size correlation among well-observed ellipticals, providing
another striking confirmation of our simple model of elliptical evolution.Comment: 16 pages in AASTEX LaTeX with 14 figures; accepted by Astrophysical
Journa
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