804 research outputs found
Galactic fountains and outflows in star forming dwarf galaxies: ISM expulsion and chemical enrichment
We investigated the impact of supernova feedback in gas-rich dwarf galaxies
experiencing a low-to-moderate star formation rate, typical of relatively
quiescent phases between starbursts. We calculated the long term evolution of
the ISM and the metal-rich SN ejecta using 3D hydrodynamic simulations, in
which the feedback energy is deposited by SNeII exploding in distinct OB
associations. We found that a circulation flow similar to galactic fountains is
generally established, with some ISM lifted at heights of one to few kpc above
the galactic plane. This gas forms an extra-planar layer, which falls back to
the plane in about yr, once the star formation stops. Very little or no
ISM is expelled outside the galaxy system for the considered SFRs, even though
in the most powerful model the SN energy is comparable to the gas binding
energy. The metal-rich SN ejecta is instead more vulnerable to the feedback and
we found that a significant fraction (25-80\%) is vented in the intergalactic
medium, even for low SN rate ( - yr).
About half of the metals retained by the galaxy are located far ( 500 pc)
from the galactic plane. Moreover, our models indicate that the circulation of
the metal-rich gas out from and back to the galactic disk is not able to erase
the chemical gradients imprinted by the (centrally concentrated) SN explosions.Comment: 19 pages, MNRAS accepte
Star formation feedback and metal enrichment by SN Ia and SN II in dwarf spheroidal galaxies: the case of Draco
We present 3D hydrodynamical simulations aimed to study the dynamical and
chemical evolution of the interstellar medium in dwarf spheroidal galaxies.
This evolution is driven by the explosions of Type II and Type Ia supernovae,
whose different contribution is explicity taken into account in our models. We
compare our results with detailed observations of the Draco galaxy. We assume
star formation histories consisting of a number of instantaneous burst
separated by quiescent periods. Because of the large effectiveness of the
radiative losses and the extended dark matter halo, no galactic wind develops,
despite the total energy released by the supernovae is much larger than the
binding energy of the gas. This explains why the galaxy is able to form stars
for a long period (> 3 Gyr), consistently with observations. In this picture,
the end of the star formation and gas removal must result from external
mechanisms, such as ram pressure and/or tidal interaction with the Galaxy. The
metallicity distributions of the stars found in our models agree very well with
the observed one. We find a mean value =-1.65 with a spread of ~1.5
dex. The chemical properties of the stars derive by the different temporal
evolution between Type Ia and Type II supernova rate, and by the different
mixing of the metals produced by the two types of SNe. We reproduce
successfully the observed [O/Fe]-[Fe/H] diagram. However, our interpretation of
this diagram differs from that generally adopted by previous chemical models.
In fact, we find that the chemical properties of the stars derive, besides the
different temporal evolution of the SNe II and SNe Ia rates, from the spatial
inhomogeneous chemical enrichment due to the different dynamical behaviour
between the remnants of the two types of supernovae.Comment: 20 pages, 14 figures (1 added), MNRAS accepted, Minor changes
following referee repor
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
On the evolution of cooling cores in X-ray galaxy clusters
(Abridged) To define a framework for the formation and evolution of the
cooling cores in X-ray galaxy clusters, we study how the physical properties
change as function of the cosmic time in the inner regions of a 4 keV and 8 keV
galaxy cluster under the action of radiative cooling and gravity only. The
cooling radius, R_cool, defined as the radius at which the cooling time equals
the Universe age at given redshift, evolves from ~0.01 R200 at z>2, where the
structures begin their evolution, to ~0.05 R200 at z=0. The values measured at
0.01 R200 show an increase of about 15-20 per cent per Gyr in the gas density
and surface brightness and a decrease with a mean rate of 10 per cent per Gyr
in the gas temperature. The emission-weighted temperature diminishes by about
25 per cent and the bolometric X-ray luminosity rises by a factor ~2 after 10
Gyrs when all the cluster emission is considered in the computation. On the
contrary, when the core region within 0.15 R500 is excluded, the gas
temperature value does not change and the X-ray luminosity varies by 10-20 per
cent only. The cooling time and gas entropy radial profiles are well
represented by power-law functions. The behaviour of the inner slopes of the
gas temperature and density profiles are the most sensitive and unambiguous
tracers of an evolving cooling core. Their values after 10 Gyrs of radiative
losses, T_gas ~ r^0.4 and n_gas ~ r^(-1.2) for the hot (cool) object, are
remarkably in agreement with the observational constraints available for nearby
X-ray luminous cooling core clusters. Because our simulations do not consider
any AGN heating, they imply that the feedback process does not greatly alter
the gas density and temperature profiles as generated by radiative cooling
alone.Comment: 8 pages. MNRAS in pres
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
Circulation Flows: Cooling Flows with Bubble Return
The failure of the XMM-Newton and Chandra X-ray telescopes to detect cooling
gas in elliptical galaxies and clusters of galaxies has led many to adopt the
position that the gas is not cooling at all and that heating by an active
nucleus in the central E or cD galaxy is sufficient to offset radiative
cooling. In this paper we explore an idealized limiting example of this point
of view in which hot, buoyant bubbles formed near the center return the
inflowing, radiatively cooling gas to distant regions in the flow. We show that
idealized steady state, centrally heated non-cooling flows can indeed be
constructed. In addition, the emission-weighted temperature profiles in these
circulating flows resemble those of normal cooling flows. However, these
solutions are valid only (1) for a range of bubble parameters for which there
is no independent justification, (2) for a limited spatial region in the
cooling flow and (3) for a limited period of time after which cooling seems
inevitable. Our exploration of non-cooling flows is set in the context of
galaxy/group flows.Comment: 10 pages (emulateapj5) with 4 figures; accepted by The Astrophysical
Journa
About the evolution of Dwarf Spheroidal Galaxies
We present 3D hydrodynamic simulations aimed at studying the dynamical and
chemical evolution of the interstellar medium in dwarf spheroidal galaxies.
This evolution is driven by the explosions of Type II and Type Ia supernovae,
whose different contribution is explicitly taken into account in our models. We
compare our results with avaiable properties of the Draco galaxy. Despite the
huge amount of energy released by SNe explosions, in our model the galaxy is
able to retain most of the gas allowing a long period ( Gyr) of star
formation, consistent with the star formation history derived by observations.
The stellar [Fe/H] distribution found in our model matches very well the
observed one. The chemical properties of the stars derive from the different
temporal evolution between Type Ia and Type II supernova rate, and from the
different mixing of the metals produced by the two types of supernovae. We
reproduce successfully the observed [O/Fe]-[Fe/H] diagram.Comment: 6 pages, 2 figures, to appear in the Proceedings of the CRAL
conference "Chemodynamics: from first stars to local galaxies", Lyon, France,
10-14 July 200
3D Numerical Simulations of AGN Outflows in Clusters and Groups
We compute 3D gasdynamical models of jet outflows from the central AGN, that
carry mass as well as energy to the hot gas in galaxy clusters and groups.
These flows have many attractive attributes for solving the cooling flow
problem: why the hot gas temperature and density profiles resemble cooling
flows but show no spectral evidence of cooling to low temperatures.
Subrelativistic jets, described by a few parameters, are assumed to be
activated when gas flows toward or cools near a central SMBH. Using approximate
models for a rich cluster (A1795), a poor cluster (2A 0336+096) and a group
(NGC 5044), we show that mass-carrying jets with intermediate mechanical
efficiencies () can reduce for many Gyr the global cooling rate to
or below the low values implied by X-spectra, while maintaining and
profiles similar to those observed, at least in clusters. Groups are much more
sensitive to AGN heating and present extreme time variability in both profiles.
Finally, the intermittency of the feedback generates multiple generations of
X-ray cavities similar to those observed in Perseus cluster and elsewhere. Thus
we also study the formation of buoyant bubbles and weak shocks in the ICM,
along with the injection of metals by SNIa and stellar winds.Comment: 4 pages, 2 figures, to appear in proceedings of the conference "The
Monster's Fiery Breath: Feedback in Galaxies, Groups, and Clusters", June
2009, Madison Wisconsi
Apparent high metallicity in 3-4 keV galaxy clusters: the inverse iron-bias in action in the case of the merging cluster Abell 2028
Recent work based on a global measurement of the ICM properties find evidence
for an increase of the iron abundance in galaxy clusters with temperature
around 2-4 keV up to a value about 3 times larger than that typical of very hot
clusters. We have started a study of the metal distribution in these objects
from the sample of Baumgartner et al. (2005), aiming at resolving spatially the
metal content of the ICM. We report here on a 42ks XMM observation of the first
object of the sample, the cluster Abell 2028. The XMM observation reveals a
complex structure of the cluster over scale of 300 kpc, showing an interaction
between two sub-clusters in cometary-like configurations. At the leading edges
of the two substructures cold fronts have been detected. The core of the main
subcluster is likely hosting a cool corona. We show that a one-component fit
for this region returns a biased high metallicity. This inverse iron bias is
due to the behavior of the fitting code in shaping the Fe-L complex. In
presence of a multi-temperature structure of the ICM, the best-fit metallicity
is artificially higher when the projected spectrum is modeled with a single
temperature component and it is not related to the presence of both Fe-L and
Fe-K emission lines in the spectrum. After accounting for the bias, the overall
abundance of the cluster is consistent with the one typical of hotter, more
massive clusters. We caution the interpretation of high abundances inferred
when fitting a single thermal component to spectra derived from relatively
large apertures in 3-4 keV clusters, because the inverse iron bias can be
present. Most of the inferences trying to relate high abundances in 3-4 keV
clusters to fundamental physical processes will likely have to be revised.Comment: 13 pages, 8 figures.Accepted for publication in Astronomy and
Astrophysycs. Minor changes to match published versio
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