925 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
Heating, conduction and minimum temperatures in cooling flows
There is mounting observational evidence from Chandra for strong interaction
between keV gas and AGN in cooling flows. It is now widely accepted that the
temperatures of cluster cores are maintained at a level of 1 keV and that the
mass deposition rates are lower than earlier ROSAT/Einstein values. Recent
theoretical results suggest that thermal conduction can be very efficient even
in magnetized plasmas. Motivated by these discoveries, we consider a ``double
heating model'' which incorporates the effects of simultaneous heating by both
the central AGN and thermal conduction from the hot outer layers of clusters.
Using hydrodynamical simulations, we demonstrate that there exists a family of
solutions that does not suffer from the cooling catastrophe. In these cases,
clusters relax to a stable final state, which is characterized by minimum
temperatures of order 1 keV and density and temperature profiles consistent
with observations. Moreover, the accretion rates are much reduced, thereby
reducing the need for excessive mass deposition rates required by the standard
cooling flow models.Comment: 7 pages, 2 figures, minor changes, accepted for The Astrophysical
Journa
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
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
Mistura em tanque de boro e herbicidas em semeadura convencional de girassol.
Os objetivos deste experimento foi avaliar a resposta do girassol às aplicações de boro (B), isoladas ou em mistura com herbicidas, e o controle de plantas daninhas por meio de experimento conduzido na Embrapa Soja, Londrina-PR. Os tratamentos foram acetochlor (1,92 kg i.a. ha-1), oxyfluorfen (0,36 kg i.a. ha-1), sulfentrazone (0,35 kg i.a.ha-1), trifluralin (1,80 kg i.a. ha-1) e as testemunhas capinada e sem capina. Todos os tratamentos foram aplicados, isoladamente ou em mistura, com 2 kg ha-1 de B (Na2B4O7.10H20 ? bórax e H3BO3 ? ácido bórico). O tratamento mais eficiente foi acetochlor mais ácido bórico; essa combinação resultou em solução mais homogênea da calda de pulverização, quando comparada com os herbicidas mais bórax. O herbicida acetochlor aplicado isoladamente ou em mistura com as duas fontes de B foi eficiente no controle da trapoeraba (Commelina benghalensis), do picão-preto (Bidens pilosa) e da corda-de-viola (Ipomoea grandifolia). Os herbicidas oxyfluorfen e sulfentrazone, aplicados isoladamente ou em misturas com as duas fontes de B, foram eficientes no controle do amendoim-bravo (Euphorbia heterophylla) e da corda-de-viola, respectivamente. É viável a aplicação de boro juntamente com os herbicidas testados nesta pesquisa em mistura em tanque, evitando a deficiência desse micronutriente e controlando as plantas daninhas na cultura do girassol
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
Spitzer Observations of Transient, Extended Dust in Two Elliptical Galaxies: New Evidence of Recent Feedback Energy Release in Galactic Cores
Spitzer observations of extended dust in two optically normal elliptical
galaxies provide a new confirmation of buoyant feedback outflow in the hot gas
atmospheres around these galaxies. AGN feedback energy is required to prevent
wholesale cooling and star formation in these group-centered galaxies. In NGC
5044 we observe interstellar (presumably PAH) emission at 8 microns out to
about 5 kpc. Both NGC 5044 and 4636 have extended 70 microns emission from cold
dust exceeding that expected from stellar mass loss. The sputtering lifetime of
this extended dust in the ~1keV interstellar gas, ~10^7 yrs, establishes the
time when the dust first entered the hot gas. Evidently the extended dust
originated in dusty disks or clouds, commonly observed in elliptical galaxy
cores, that were disrupted, heated and buoyantly transported outward. The
surviving central dust in NGC 5044 and 4636 has been disrupted into many small
filaments. It is remarkable that the asymmetrically extended 8 micron emission
in NGC 5044 is spatially coincident with Halpha+[NII] emission from warm gas. A
calculation shows that dust-assisted cooling in buoyant hot gas moving out from
the galactic core can cool within a few kpc in about ~10^7 yrs, explaining the
optical line emission observed. The X-ray images of both galaxies are
disturbed. All timescales for transient activity - restoration of equilibrium
and buoyant transport in the hot gas, dynamics of surviving dust fragments, and
dust sputtering - are consistent with a central release of feedback energy in
both galaxies about 10^7 yrs ago.Comment: 13 pages. Accepted by ApJ; minor typos correcte
AGN Feedback in Galaxy Groups: the Delicate Touch of Self-Regulated Outflows
AGN heating, through massive subrelativistic outflows, might be the key to
solve the long-lasting `cooling flow problem' in cosmological systems. In a
previous paper, we showed that cold accretion feedback and, to a lesser degree,
Bondi self-regulated models are in fact able to quench cooling rates for
several Gyr, at the same time preserving the mainc ool core features, like
observed density and temperature profiles. Is it true also for lighter systems,
such as galaxy groups? The answer is globally yes, although with remarkable
differences. Adopting a modified version of the AMR code FLASH 3.2, we found
that successful 3D simulations with cold and Bondi models are almost convergent
in the galaxy group environment, with mechanical efficiencies in the range
5.e-4 - 1.e-3 and 5.e-2 - 1.e-1, respectively. The evolutionary storyline of
galaxy groups is dominated by a quasi-continuous gentle injection with
sub-Eddington outflows (with mechanical power and velocity around 1.e44 erg/s
and 1.e4 km/s). The cold and hybrid accretion models present, in addition, very
short quiescence periods, followed by moderate outbursts (10 times the previous
phase), which generate a series of 10-20 kpc size cavities with high density
contrast, temperatures similar to the ambient medium and cold rims. After shock
heating, a phase of turbulence promotes gas mixing and diffusion of metals,
which peak along jet-axis (up to 40 kpc) during active phases. At this stage
the tunnel, produced by the enduring outflow (hard to detect in the mock SBx
maps), is easily fragmented, producing tiny buoyant bubbles, typically a few
kpc in size. In contrast to galaxy clusters, the AGN self-regulated feedback
has to be persistent, with a `delicate touch', rather than rare and explosive
strokes. This evolutionary difference dictates in the end that galaxy groups
are not scaled-down versions of clusters.Comment: Accepted by MNRAS; 22 pages, 7 figure
- …