1,300 research outputs found

    AGN triggering in the infall regions of distant X-ray luminous galaxy clusters at 0.9 < z <~ 1.6

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    Observational constraints on the average radial distribution profile of AGN in distant galaxy clusters can provide important clues on the triggering mechanisms of AGN activity in dense environments and are essential for a completeness evaluation of cluster selection techniques in the X-ray and mm-wavebands. The aim of this work is a statistical study with XMM-Newton of the presence and distribution of X-ray AGN in the large-scale structure environments of 22 X-ray luminous galaxy clusters in the redshift range 0.9 < z \lesssim 1.6 compiled by the XMM-Newton Distant Cluster Project (XDCP). To this end, the X-ray point source lists from detections in the soft-band (0.35-2.4 keV) and full-band (0.3-7.5 keV) were stacked in cluster-centric coordinates and compared to average background number counts extracted from three independent control fields in the same observations. A significant full-band (soft-band) excess of \sim78 (67) X-ray point sources is found in the cluster fields within an angular distance of 8' (4Mpc) at a statistical confidence level of 4.0 sigma (4.2 sigma), corresponding to an average number of detected excess AGN per cluster environment of 3.5\pm0.9 (3.0\pm0.7). The data point towards a rising radial profile in the cluster region (r<1Mpc) of predominantly low-luminosity AGN with an average detected excess of about one point source per system, with a tentative preferred occurrence along the main cluster elongation axis. A second statistically significant overdensity of brighter soft-band detected AGN is found at cluster-centric distances of 4'-6' (2-3Mpc), corresponding to about three times the average cluster radius R200 of the systems. If confirmed, these results would support the idea of two different physical triggering mechanisms of X-ray AGN activity in dependence of the radially changing large-scale structure environment of the distant clusters.Comment: 18 pages, 7 color figures, accepted for publication in Advances in Astronomy for the special issue 'Seeking for the Leading Actor on the Cosmic Stage: Galaxies versus Supermassive Black Holes

    Electrical conductivity cell and method for fabricating the same

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    A flask having a threaded neck and a cap adapted for threaded engagement on the neck are used. A laminated disc between the cap and the neck forms a gas tight seal and the cap has a central opening that exposes a medial region of the disc. Piercing the disc through the opening are two electrodes, the inner ends of which contact the sample within the flask and the outer ends of which can be connected to test equipment. Cylindric glass tubes are fitted over the external portion of the electrodes to provide physical support and silicone rubber or a similar material serves to retain the glass cylinders in place and form a gas tight seal between the cylinders and the electrodes. Shrinkable tubing is shrunk over the glass tubes to afford further mechanical support and sealing. A final relatively large diameter shrinkable tube is shrunk over both electrodes and their associated glass cylinders. The support and sealing means for the electrodes is confined to a limited portion of the medial region of the disc so that the remainder of such region can be punctured by a hollow needle to introduce a test sample within the flask

    The detection of ultra-faint low surface brightness dwarf galaxies in the Virgo Cluster: a Probe of Dark Matter and Baryonic Physics

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    We have discovered 11 ultra-faint (r≲22.1r\lesssim 22.1) low surface brightness (LSB, central surface brightness 23≲μr≲2623\lesssim \mu_r\lesssim 26) dwarf galaxy candidates in one deep Virgo field of just 576576 arcmin2^2 obtained by the Large Binocular Camera (LBC) at the Large Binocular Telescope (LBT). Their association with the Virgo cluster is supported by their distinct position in the central surface brightness - total magnitude plane with respect to the background galaxies of similar total magnitude. They have typical absolute magnitudes and scale sizes, if at the distance of Virgo, in the range −13≲Mr≲−9-13\lesssim M_r\lesssim -9 and 250≲rs≲850250\lesssim r_s\lesssim 850 pc, respectively. Their colors are consistent with a gradually declining star formation history with a specific star formation rate of the order of 10−1110^{-11} yr−1^{-1}, i.e. 10 times lower than that of main sequence star forming galaxies. They are older than the cluster formation age and appear regular in morphology. They represent the faintest extremes of the population of low luminosity LSB dwarfs that has been recently detected in wider surveys of the Virgo cluster. Thanks to the depth of our observations we are able to extend the Virgo luminosity function down to Mr∼−9.3M_r\sim -9.3 (corresponding to total masses M∼107M\sim 10^7 M⊙_{\odot}), finding an average faint-end slope α≃−1.4\alpha\simeq -1.4. This relatively steep slope puts interesting constraints on the nature of the Dark Matter and in particular on warm Dark Matter (WDM) often invoked to solve the overprediction of the dwarf number density by the standard CDM scenario. We derive a lower limit on the WDM particle mass >1.5>1.5 keV.Comment: accepted for publication in ApJ, 13 pages, 6 figure

    MnSi1.7_{1.7} nanoparticles embedded in Si: Superparamagnetism with a collective behavior

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    The doping of Mn in Si is attracting research attentions due to the possibility to fabricate Si-based diluted magnetic semiconductors. However, the low solubility of Mn in Si favors the precipitation of Mn ions even at non-equilibrium growth conditions. MnSi1.7_{1.7} nanoparticles are the common precipitates, which show exotic magnetic properties in comparison with the MnSi1.7_{1.7} bulk phase. In this paper we present the static and dynamic magnetic properties of MnSi1.7_{1.7} nanoparticles. Using the Preisach model, we derive the magnetic parameters, such as the magnetization of individual particles, the distribution of coercive fields and the inter-particle interaction field. Time-dependent magnetization measurements reveal a spin-glass behavior of the system.Comment: 11 pages, 6 figures, submitted to PR

    Lorentz TEM imaging of stripe structures embedded in a soft magnetic matrix

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    N\'eel walls in soft magnetic NiFe/NiFeGa hybrid stripe structures surrounded by a NiFe film are investigated by high resolution Lorentz transmission electron microscopic imaging. An anti-parallel orientation of magnetization in 1000 nm wide neighboring unirradiated-irradiated stripes is observed by forming high angle domain walls during magnetization reversal. Upon downscaling the stripe structure size from 1000 nm to 200 nm a transition from a discrete domain pattern to an effective magnetic medium is observed for external magnetic field reversal. This transition is associated with vanishing ability of hosting high angle domain walls between adjacent stripes. The investigation also demonstrated the potentiality of Lorentz microscopy to image periodic stripe structures well under micron length-scale.Comment: 7 pages, 6 figure

    A study on the multicolour evolution of Red Sequence galaxy populations: insights from hydrodynamical simulations and semi-analytical models

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    By means of our own cosmological-hydrodynamical simulation and semi-analytical model we studied galaxy population properties in clusters and groups, spanning over 10 different bands from UV to NIR, and their evolution since redshift z=2. We compare our results in terms of galaxy red/blue fractions and luminous-to-faint ratio (LFR) on the Red Sequence (RS) with recent observational data reaching beyond z=1.5. Different selection criteria were tested in order to retrieve galaxies belonging to the RS: either by their quiescence degree measured from their specific SFR ("Dead Sequence"), or by their position in a colour-colour plane which is also a function of sSFR. In both cases, the colour cut and the limiting magnitude threshold were let evolving with redshift, in order to follow the natural shift of the characteristic luminosity in the LF. We find that the Butcher-Oemler effect is wavelength-dependent, with the fraction of blue galaxies increasing steeper in optical colours than in NIR. Besides, only when applying a lower limit in terms of fixed absolute magnitude, a steep BO effect can be reproduced, while the blue fraction results less evolving when selecting samples by stellar mass or an evolving magnitude limit. We then find that also the RS-LFR behaviour, highly debated in the literature, is strongly dependent on the galaxy selection function: in particular its very mild evolution recovered when measured in terms of stellar mass, is in agreement with values reported for some of the highest redshift confirmed (proto)clusters. As to differences through environments, we find that normal groups and (to a lesser extent) cluster outskirts present the highest values of both star forming fraction and LFR at low z, while fossil groups and cluster cores the lowest: this separation among groups begins after z~0.5, while earlier all group star forming properties are undistinguishable.Comment: revised version, A&A accepted (11 pages, 6 figures

    Origin of magnetic moments in defective TiO2 single crystals

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    In this paper we show that ferromagnetism can be induced in pure TiO2 single crystals by oxygen ion irradiation. By combining x-ray diffraction, Raman-scattering, and electron spin resonance spectroscopy, a defect complex, \emph{i.e.} Ti3+^{3+} ions on the substitutional sites accompanied by oxygen vacancies, has been identified in irradiated TiO2. This kind of defect complex results in a local (TiO6−x_{6-x}) stretching Raman mode. We elucidate that Ti3+^{3+} ions with one unpaired 3d electron provide the local magnetic moments.Comment: 4 pages, 4 figures, to be published at Phys. Rev.
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