1,016 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
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
Spatial distribution of introduced brook trout Salvelinus fontinalis (Salmonidae) within alpine lakes: evidences from a fish eradication campaign
Brook trout Salvelinus fontinalis have been used worldwide to stock fishless alpine lakes, negatively affecting native biota. Understanding its spatial ecology in invaded ecosystems can provide information to interpret and contrast its ecological impact. We opportunistically used capture points of brook trout gillnetted during an eradication campaign to assess the distribution patterns of four unexploited populations inhabiting high-altitude lakes. The main eradication method implies the use of many gillnets with several mesh sizes, which are selective for different fish sizes. For each lake we drew six capture maps associated with as many different mesh sizes, and we tested whether the distance from the coastline (which in alpine lakes is a reliable proxy of the most important spatial gradients, e.g. depth, temperature, prey availability, lighting conditions) influences the proportion of captured fish belonging to different size classes and the number of fish captured by the nets with different mesh sizes. To interpret the results, we also provide a cartographic description of the lakesâ bathymetry and littoral microhabitats. We found (1) a negative relationship between brook trout distribution and the distance from the coastline in all of the size classes, lakes and mesh sizes; (2) that large brook trout can thrive in the lakesâ center, while small ones are limited to the littoral areas; and (3) that the distance from the coastline alone cannot explain all the differences in the catch densities in different parts of the lakes. As in their native range, introduced brook trout populations also have littoral habits. Microhabitats, prey availability and distance from the spawning ground are other likely factors determining the distribution patterns of brook trout populations introduced in alpine lakes. The obtained results also provide useful information on how to plan new eradication campaigns
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
Heating Cooling Flows with Weak Shock Waves
The discovery of extended, approximately spherical weak shock waves in the
hot intercluster gas in Perseus and Virgo has precipitated the notion that
these waves may be the primary heating process that explains why so little gas
cools to low temperatures. This type of heating has received additional support
from recent gasdynamical models. We show here that outward propagating,
dissipating waves deposit most of their energy near the center of the cluster
atmosphere. Consequently, if the gas is heated by (intermittent) weak shocks
for several Gyrs, the gas within 30-50 kpc is heated to temperatures that far
exceed observed values. This heating can be avoided if dissipating shocks are
sufficiently infrequent or weak so as not to be the primary source of global
heating. Local PV and viscous heating associated with newly formed X-ray
cavities are likely to be small, which is consistent with the low gas
temperatures generally observed near the centers of groups and clusters where
the cavities are located.Comment: 14 pages; Accepted by Ap
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
Meiofaunal ecology in harsh environments: refugia and stepping stones, a case study in a deglaciating Alpine area
Climate change and progressive glacier loss are leading to rapid ecological shifts in alpine aquatic systems. Rock glaciers and paraglacial features such as proglacial lakes, moraines, and taluses can alter the gradients of glacial influence along alpine river networks. Particularly relevant is the effect of rock glacial streams on invertebrates, although the hydrology and ecology of such high-elevation stream types is still scarcely known. We investigated the main meiofaunal component of benthic communities of different stream types in a deglaciating area of the Italian Alps, i.e., Crustacea Copepoda. We used an index of habitat mildness based on water temperature, channel stability, turbidity, and organic detritus, to measure the difference in community metrics over a gradient of habitat amelioration, driven by the mixing of distinct stream types (glacial, rock-glacial, snowmelt, mixed) and their interactions with paraglacial features. The composition of copepod communities of rock-glacial sites differed from the one of the other stream types, particularly it was very different from the kryal sites, and more similar to the rhithral and krenal ones. Under progressive deglaciation, rock glaciers and paraglacial features will increasingly influence the meiofaunal communities of alpine river networks. As they host a higher number of taxa and individuals than non-glacial locations, rock glacial streams may act as stepping stones facilitating colonization following glacier retreat. After glacier loss, rock glacial streams may represent climate refugia for cold adapted taxa and/or kryal specialists, because the slow thawing of their ice might sustain cold water conditions for a longer period of time
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
Radiative cooling, heating and thermal conduction in M87
The crisis of the standard cooling flow model brought about by Chandra and
XMM-Newton observations of galaxy clusters, has led to the development of
several models which explore different heating processes in order to assess if
they can quench the cooling flow. Among the most appealing mechanisms are
thermal conduction and heating through buoyant gas deposited in the ICM by
AGNs. We combine Virgo/M87 observations of three satellites (Chandra,
XMM-Newton and Beppo-SAX) to inspect the dynamics of the ICM in the center of
the cluster. Using the spectral deprojection technique, we derive the physical
quantities describing the ICM and determine the extra-heating needed to balance
the cooling flow assuming that thermal conduction operates at a fixed fraction
of the Spitzer value. We assume that the extra-heating is due to buoyant gas
and we fit the data using the model developed by Ruszkowski and Begelman
(2002). We derive a scale radius for the model of kpc, which is
comparable with the M87 AGN jet extension, and a required luminosity of the AGN
of a erg s, which is comparable to the observed AGN
luminosity. We discuss a scenario where the buoyant bubbles are filled of
relativistic particles and magnetic field responsible for the radio emission in
M87. The AGN is supposed to be intermittent and to inject populations of
buoyant bubbles through a succession of outbursts. We also study the X-ray cool
component detected in the radio lobes and suggest that it is structured in
blobs which are tied to the radio buoyant bubbles.Comment: 25 pages, 10 figures and 2 tables. Accepted for publication in Ap
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