422 research outputs found
The 3D soft X-ray cluster-AGN cross-correlation function in the ROSAT NEP survey
X-ray surveys facilitate investigations of the environment of AGNs. Deep
Chandra observations revealed that the AGNs source surface density rises near
clusters of galaxies. The natural extension of these works is the measurement
of spatial clustering of AGNs around clusters and the investigation of relative
biasing between active galactic nuclei and galaxies near clusters.The major
aims of this work are to obtain a measurement of the correlation length of AGNs
around clusters and a measure of the averaged clustering properties of a
complete sample of AGNs in dense environments. We present the first measurement
of the soft X-ray cluster-AGN cross-correlation function in redshift space
using the data of the ROSAT-NEP survey. The survey covers 9x9 deg^2 around the
North Ecliptic Pole where 442 X-ray sources were detected and almost completely
spectroscopically identified. We detected a >3sigma significant clustering
signal on scales s<50 h70^-1 Mpc. We performed a classical maximum-likelihood
power-law fit to the data and obtained a correlation length s_0=8.7+1.2-0.3
h_70-1 Mpc and a slope gamma=1.7$^+0.2_-0.7 (1sigma errors). This is a strong
evidence that AGNs are good tracers of the large scale structure of the
Universe. Our data were compared to the results obtained by cross-correlating
X-ray clusters and galaxies. We observe, with a large uncertainty, that the
bias factor of AGN is similar to that of galaxies.Comment: 4 pages, 2 figure, proceedings of the Conference "At the edge of the
Universe", Sintra Portugal, October 2006. To be published on the Astronomical
Society of the Pacific Conference Series (ASPCS
Cosmic Structure Traced by Precision Measurements of the X-Ray Brightest Galaxy Clusters in the Sky
The current status of our efforts to trace cosmic structure with 10^6
galaxies (2MASS), 10^3 galaxy clusters (NORAS II cluster survey), and precision
measurements for 10^2 galaxy clusters (HIFLUGCS) is given. The latter is
illustrated in more detail with results on the gas temperature and metal
abundance structure for 10^0 cluster (A1644) obtained with XMM-Newton.Comment: 4 pages; to be published in the Proceedings of the Conference: The
Emergence of Cosmic Structure, College Park, MD (2002), editors: S.S. Holt
and C. Reynolds; also available at http://www.reiprich.ne
The REFLEX Cluster Survey: Probing the Mass Distribution in the Universe
We summarize some of the major results obtained so far from the REFLEX survey
of X-ray clusters of galaxies, concentrating on the latest measurements of the
cluster X-ray luminosity function and two-point correlation function. The
REFLEX luminosity function provides the most homogeneous census of the
distribution function of masses in the local Universe, representing a unique
zero-redshift reference quantity for evolutionary studies. On the other hand,
the observed clustering of REFLEX clusters is very well described both in
amplitude and shape by the correlation function computed for a low-Omega_M CDM
model. The bidimensional correlation map xi(r_p,pi) shows no stretching along
the line of sight, indicating negligible spurious effects in the sample, with
at the same time a clear compression of the contours as expected in the
presence of coherent large-scale motions.Comment: 8 pages, 9 PS figures. Uses newpasp.sty (included). To appear in
"Tracing Cosmic Evolution with Galaxy Clusters" (Sesto 2001), ASP Conference
Series, S. Borgani, M. Mezzetti & R. Valdarnini eds., in pres
Supermassive Black Holes in Elliptical Galaxies: Switching from Very Bright to Very Dim
Relativistic outflows (mainly observed in the radio) are a characteristic
feature of both Galactic stellar mass black holes and supermassive black holes
(SMBHs). Simultaneous radio and X-ray observations of Galactic sources have
shown that the outflow is strong at low accretion rates, but it weakens
dramatically or disappears completely at high accretion rates, manifesting
structural changes in the accretion flow. It is reasonable to assume that SMBHs
follow the same trend. For low luminosity SMBHs in nearby elliptical galaxies
and clusters, recent observations strongly suggest that the outflows play the
central role in keeping the gas hot (mechanical feedback). If the outflow is
quenched in SMBHs at high accretion rates similarly to the behavior of galactic
sources, then the straightforward consequence is a relatively weak feedback of
rapidly accreting SMBHs. We argue that elliptical galaxies and their central
engines should then evolve through two stages. Early on, the central SMBH
rapidly grows by accreting cooling gas at a near-Eddington rate with high
radiative efficiency but with weak feedback on the infalling gas. This stage
terminates when the black hole has grown to a sufficiently large mass that its
feedback (radiative and/or mechanical), despite the low gas heating efficiency,
is able to suppress gas cooling. After that the system switches to a stable
state corresponding to passively evolving ellipticals, when the accretion rate
and radiative efficiency are very low, but the gas heating efficiency is high
and energy input from the relativistic outflow keeps the gas hot.Comment: MNRAS Letters, accepted, 6 pages, 2 figure
Levaduras autóctonas como factores de crecimiento en Rhamdia quelen: primeras aproximaciones
Rhamdia quelen is an autochthonous fish used for aquaculture in the Northeast of Argentina. Antibiotics as growing factors in fish production had been criticized in the last years and the use of microorganisms emerged as a putative replacement. The aim of this study was to isolate autochthonous yeasts from R. quelen and to evaluate the effect of the administration over biometrical parameters of larvae under intensive culture system. Fungi were isolated from the digestive tract of wild specimens, phenotypically identified and evaluated on their ability to exert beneficial properties. One selected fungal isolate was administered, four times a day, dead or alive at 1 or 2% together with balanced feed to R. quelen larvae for 15 days. aftertreatment larvae were counted and weighted in order to obtain the values of survival, mean weight and total biomass. Only four Candida tropicalis isolates were obtained. None of them expressed beneficial properties. One fungal isolate was randomly selected for in vivo assays. Results indicated that only dead yeast at 1% induced a significant increment of biomass when compared with the control group (p<0.05). Survival rate was boosted significantly (p<0.05) with dead yeast and not significantly (p>0.05) with the fungal isolate. None of the treatments induced significant increments in mean weight (p<0.05). These results allowed us to propose the treatment with dead yeast at 1% as a growing factor for R. quelen, considering this microorganism as an effective prebiotic in the aquaculture of this specie.Rhamdia quelen es un pez autóctono cultivado en el nordeste argentino. El uso de antibióticos como factores de crecimiento en acuicultura fue criticado en los últimos años y la administración de microorganismos surgió como una opción posible. El objetivo de este trabajo fue aislar levaduras autóctonas de R. quelen y evaluar el efecto de su administración sobre parámetros biométricos de larvas en cultivo intensivo. Los hongos se aislaron del tracto digestivo de especímenes de vida silvestre, se identificaron fenotípicamente y se evaluaron en cuanto a la expresión de propiedades benéficas. Se seleccionó una de las cepas y se administró cuatro veces por día, muerta o viva al 1 o 2% junto con el alimento balanceado durante 15 días. Luego del tratamiento las larvas se contaron y pesaron para obtener los valores de sobrevida, peso medio y biomasa. Sólo se obtuvieron cuatro aislamientos de Candida tropicalis. Ninguno expresó propiedades benéficas y uno fue elegido al azar para los ensayos in vivo. Solo la levadura muerta al 1% incrementó significativamente la biomasa en comparación con el control (p<0,05). La sobrevida mejoró significativamente (p<0,05) con la levadura muerta y sin significancia (p>0,05) con el hongo vivo. Ninguno de los tratamientos induce incrementos significativos del peso medio (p<0,05). Estos resultados nos permiten proponer al tratamiento con levadura muerta al 1% como un factor de crecimiento para R. quelen, considerando al microorganismo como un prebiótico efectivo para su uso en esta especie
Feedback under the microscope II: heating, gas uplift, and mixing in the nearest cluster core
Using a combination of deep 574ks Chandra data, XMM-Newton high-resolution
spectra, and optical Halpha+NII images, we study the nature and spatial
distribution of the multiphase plasma in M87. Our results provide direct
observational evidence of `radio mode' AGN feedback in action, stripping the
central galaxy of its lowest entropy gas and preventing star-formation. This
low entropy gas was entrained with and uplifted by the buoyantly rising
relativistic plasma, forming long "arms". These arms are likely oriented within
15-30 degrees of our line-of-sight. The mass of the uplifted gas in the arms is
comparable to the gas mass in the approximately spherically symmetric 3.8 kpc
core, demonstrating that the AGN has a profound effect on its immediate
surroundings. The coolest X-ray emitting gas in M87 has a temperature of ~0.5
keV and is spatially coincident with Halpha+NII nebulae, forming a multiphase
medium where the cooler gas phases are arranged in magnetized filaments. We
place strong upper limits of 0.06 Msun/yr on the amount of plasma cooling
radiatively from 0.5 keV and show that a uniform, volume-averaged heating
mechanism could not be preventing the cool gas from further cooling. All of the
bright Halpha filaments appear in the downstream region of the <3 Myr old shock
front, at smaller radii than ~0.6'. We suggest that shocks induce shearing
around the filaments, thereby promoting mixing of the cold gas with the ambient
hot ICM via instabilities. By bringing hot thermal particles into contact with
the cool, line-emitting gas, mixing can supply the power and ionizing particles
needed to explain the observed optical spectra. Mixing of the coolest X-ray
emitting plasma with the cold optical line emitting filamentary gas promotes
efficient conduction between the two phases, allowing non-radiative cooling
which could explain the lack of X-ray gas with temperatures under 0.5 keV.Comment: to appear in MNRA
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