1,041 research outputs found
AGN triggering in the infall regions of distant X-ray luminous galaxy clusters at 0.9 < z <~ 1.6
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
The detection of ultra-faint low surface brightness dwarf galaxies in the Virgo Cluster: a Probe of Dark Matter and Baryonic Physics
We have discovered 11 ultra-faint () low surface brightness
(LSB, central surface brightness ) dwarf galaxy
candidates in one deep Virgo field of just arcmin 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
and pc,
respectively. Their colors are consistent with a gradually declining star
formation history with a specific star formation rate of the order of
yr, 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 (corresponding to
total masses M), finding an average faint-end slope
. 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
keV.Comment: accepted for publication in ApJ, 13 pages, 6 figure
A study on the multicolour evolution of Red Sequence galaxy populations: insights from hydrodynamical simulations and semi-analytical models
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
Phase Coherent Precessional Magnetization Reversal in Micro-scopic Spin Valve Elements
We study the precessional switching of the magnetization in microscopic spin
valve cells induced by ultra short in-plane hard axis magnetic field pulses.
Stable and highly efficient switching is monitored following pulses as short as
140 ps with energies down to 15 pJ. Multiple application of identical pulses
reversibly toggles the cell's magnetization be-tween the two easy directions.
Variations of pulse duration and amplitude reveal alter-nating regimes of
switching and non-switching corresponding to transitions from in-phase to
out-of-phase excitations of the magnetic precession by the field pulse. In the
low field limit damping becomes predominant and a relaxational reversal is
found allowing switching by hard axis fields below the in-plane anisotropy
field threshold.Comment: 17 pages, 4 figure
Magnetization dynamics of magnetic domain wall imprinted magnetic films
The influence of micromagnetic objects on the dynamic magnetic excitation in magnetic thin films is studied by imprinting periodic domain wall patterns through selective ion irradiation in exchange biased Ni81Fe 19/IrMn structures. For high domain wall densities an increased precessional frequency is achieved. The zero field resonance of the domain wall state hereby depends directly on the stripe period, showing a pronounced increase with decrease of domain wall spacing. With the abrupt annihilation of magnetic domain walls with an applied bias field a jump-like decrease in precessional frequency takes place. The experimental data and micromagnetic simulations prove that the characteristic collective dynamic mode for the domain wall configurations is attributed to strongly coupled tilted magnetization structure. This is evidenced by an overlapping Néel wall structure for the narrowly spaced imprinted antiparallel unidirectional anisotropy state. The controlled introduction of high density frozen-in micromagnetic objects is a novel way to control the dynamic magnetic properties of continuous magnetic thin films
XMMU J100750.5+125818: A strong lensing cluster at z=1.082
We report on the discovery of the X-ray luminous cluster XMMU
J100750.5+125818 at redshift 1.082 based on 19 spectroscopic members, which
displays several strong lensing features. SED modeling of the lensed arc
features from multicolor imaging with the VLT and the LBT reveals likely
redshifts ~2.7 for the most prominent of the lensed background galaxies. Mass
estimates are derived for different radii from the velocity dispersion of the
cluster members, M_200 ~ 1.8 10^{14} Msun, from the X-ray spectral parameters,
M_500 ~ 1.0 10^{14} Msun, and the largest lensing arc, M_SL ~ 2.3 10^{13} Msun.
The projected spatial distribution of cluster galaxies appears to be elongated,
and the brightest galaxy lies off center with respect to the X-ray emission
indicating a not yet relaxed structure. XMMU J100750.5+125818 offers excellent
diagnostics of the inner mass distribution of a distant cluster with a
combination of strong and weak lensing, optical and X-ray spectroscopy.Comment: A&A, accepted for publicatio
XMM-Newton detection of two clusters of galaxies with strong SPT Sunyaev-Zel'dovich effect signatures
We report on the discovery of two galaxy clusters, SPT-CL J2332-5358 and
SPT-CL J2342-5411, in X-rays. These clusters were also independently detected
through their Sunyaev-Zel'dovich effect by the South Pole Telescope, and
confirmed in the optical band by the Blanco Cosmology Survey. They are thus the
first clusters detected under survey conditions by all major cluster search
approaches. The X-ray detection is made within the frame of the XMM-BCS cluster
survey utilizing a novel XMM-Newton mosaic mode of observations. The present
study makes the first scientific use of this operation mode. We estimate the
X-ray spectroscopic temperature of SPT-CL J2332-5358 (at redshift z=0.32) to T
= 9.3 (+3.3/-1.9) keV, implying a high mass, M_{500} = 8.8 +/- 3.8 \times
10^{14} M_{sun}. For SPT-CL J2342-5411, at z=1.08, the available X-ray data
doesn't allow us to directly estimate the temperature with good confidence.
However, using our measured luminosity and scaling relations we estimate that T
= 4.5 +/- 1.3 keV and M_{500} = 1.9 +/- 0.8 \times 10^{14} M_{sun}. We find a
good agreement between the X-ray masses and those estimated from the
Sunyaev-Zel'dovich effect.Comment: Submitted to A&A, 8 pages, 5 figures, 1 tabl
Exploring the galaxy cluster-group transition regime at high redshifts: Physical properties of two newly detected z > 1 systems
Context: Multi-wavelength surveys for clusters of galaxies are opening a
window on the elusive high-redshift (z>1) cluster population. Well controlled
statistical samples of distant clusters will enable us to answer questions
about their cosmological context, early assembly phases and the thermodynamical
evolution of the intracluster medium. Aims: We report on the detection of two
z>1 systems, XMMU J0302.2-0001 and XMMU J1532.2-0836, as part of the XMM-Newton
Distant Cluster Project (XDCP) sample. We investigate the nature of the
sources, measure their spectroscopic redshift and determine their basic
physical parameters. Methods: The results of the present paper are based on the
analysis of XMM-Newton archival data, optical/near-infrared imaging and deep
optical follow-up spectroscopy of the clusters. Results: We confirm the X-ray
source XMMU J0302.2-0001 as a gravitationally bound, bona fide cluster of
galaxies at spectroscopic redshift z=1.185. We estimate its M500 mass to
(1.6+/-0.3) times 10^{14} Msun from its measured X-ray luminosity. This ranks
the cluster among intermediate mass system. In the case of XMMU J1532.2-0836 we
find the X-ray detection to be coincident with a dynamically bound system of
galaxies at z=1.358. Optical spectroscopy reveals the presence of a central
active galactic nucleus, which can be a dominant source of the detected X-ray
emission from this system. We provide upper limits of X-ray parameters for the
system and discuss cluster identification challenges in the high-redshift
low-mass cluster regime. A third, intermediate redshift (z=0.647) cluster, XMMU
J0302.1-0000, is serendipitously detected in the same field as XMMU
J0302.2-0001. We provide its analysis as well.Comment: Accepted to A&A, 13/04/2011. 15 pages, 18 figures, 5 tables, 2
appendice
Fe-implanted ZnO: Magnetic precipitates versus dilution
Nowadays ferromagnetism is often found in potential diluted magnetic
semiconductor systems. However, many authors argue that the observed
ferromagnetism stems from ferromagnetic precipitates or spinodal decomposition
rather than from carrier mediated magnetic impurities, as required for a
diluted magnetic semiconductor. In the present paper we answer this question
for Fe-implanted ZnO single crystals comprehensively. Different implantation
fluences and temperatures and post-implantation annealing temperatures have
been chosen in order to evaluate the structural and magnetic properties over a
wide range of parameters. Three different regimes with respect to the Fe
concentration and the process temperature are found: 1) Disperse Fe and
Fe at low Fe concentrations and low processing temperatures, 2)
FeZnO at very high processing temperatures and 3) an intermediate
regime with a co-existence of metallic Fe (Fe) and ionic Fe (Fe and
Fe). Ferromagnetism is only observed in the latter two cases, where
inverted ZnFeO and -Fe nanocrystals are the origin of the
observed ferromagnetic behavior, respectively. The ionic Fe in the last case
could contribute to a carrier mediated coupling. However, their separation is
too large to couple ferromagnetically due to the lack of p-type carrier. For
comparison investigations of Fe-implanted epitaxial ZnO thin films are
presented.Comment: 14 pages, 17 figure
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