1,035 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
Evidence for Rapid Redshift Evolution of Strong Cluster Cooling Flows
We present equivalent widths of the [OII] and Ha nebular emission lines for
77 brightest cluster galaxies (BCGs) selected from the 160 Square Degree
X-ray survey. We find no [OII] or Ha emission stronger than -15
angstroms or -5 angstroms, respectively, in any BCG. The corresponding emission
line luminosities lie below 6E40 erg/s, which is a factor of 30 below that of
NGC1275 in the Perseus cluster. A comparison to the detection frequency of
nebular emission in BCGs lying at redshifts above z = 0.35 drawn from the
Brightest Cluster Survey (Crawford et al. 1999) indicates that we should have
detected roughly one dozen emission-line galaxies, assuming the two surveys are
selecting similar clusters in the X-ray luminosity range 10E42 erg/s to 10E45
erg/s. The absence of luminous nebular emission (ie., Perseus-like systems) in
our sample is consistent with an increase in the number density of {\it strong}
cooling flow (cooling core) clusters between and today. The decline
in their numbers at higher redshift could be due to cluster mergers and AGN
heating.Comment: Accepted for publication in Ap
The ROSAT North Ecliptic Pole Survey: The Optical Identifications
The X-ray data around the North Ecliptic Pole (NEP) of the ROSAT All Sky
Survey have been used to construct a contiguous area survey consisting of a
sample of 445 individual X-ray sources above a flux of ~2x10^-14 erg cm^-2 s^-1
in the 0.5-2.0 keV energy band. The NEP survey is centered at RA (2000) = 18h
00m, DEC(2000) = +66deg 33arcmin and covers a region of 80.7 sq. deg at a
moderate Galactic latitude of b = 29.8deg. Hence, the NEP survey is as deep and
covers a comparable solid angle to the ROSAT serendipitous surveys, but is also
contiguous. We have identified 99.6% of the sources and determined redshifts
for the extragalactic objects. In this paper we present the optical
identifications of the NEP catalog of X-ray sources including basic X-ray data
and properties of the sources. We also describe with some detail the optical
identification procedure. The classification of the optical counterparts to the
NEP sources is very similar to that of previous surveys, in particular the
Einstein Extended Medium Sensitivity Survey (EMSS). The main constituents of
the catalog are active galactic nuclei (~49%), either type 1 or type 2
according to the broadness of their permitted emission lines. Stellar
counterparts are the second most common identification class (~34%). Clusters
and groups of galaxies comprise 14%, and BL Lacertae objects 2%. One non-AGN
galaxy, and one planetary nebula have also been found. The NEP catalog of X-ray
sources is a homogeneous sample of astronomical objects featuring complete
optical identification.Comment: Accepted for publication in the ApJS; 33 pages including 12
postscript figures and 3 tables; uses emulateapj.sty. On-line source catalog
at http://www.eso.org/~cmullis/research/nep-catalog.htm
Literacy practices of primary education children in Andalusia (Spain): a family-based perspective
Primary school children develop literacy practices in various domains and situations in everyday life.
This study focused on the analysis of literacy practices of children aged 8â12 years from the perspec-
tive of their families. 1,843 families participated in the non-experimental explanatory study. The
children in these families speak Spanish as a first language and are schooled in this language. The
instrument used was a self-report questionnaire about childrenâs home-literacy practices. The data
obtained were analysed using categorical principal components analysis (CATPCA) and analysis of
variance (ANOVA). The results show the complex relationship between literacy practices developed
by children in the domains of home and school and the limited development of a literacy-promoting
âthird spaceâ. In conclusion, the families in our study had limited awareness of their role as literacy-
promoting agents and thought of literacy learning as restricted to formal or academic spaces
Redshift Evolution in the Iron Abundance of the Intracluster Medium
Clusters of galaxies provide a closed box within which one can determine the
chemical evolution of the gaseous baryons with cosmic time. We studied this
metallicity evolution in the hot X-ray emitting baryons through an analysis of
XMM-Newton observations of 29 galaxy clusters in the redshift range 0.3 < z <
1.3. Taken alone, this data set does not show evidence for significant
evolution. However, when we also include a comparable sample of 115 clusters
observed with Chandra (Maughan et al. 2008) and a lower redshift sample of 70
clusters observed with XMM at z < 0.3 (Snowden et al. 2008), there is
definitive evidence for a decrease in the metallicity. This decrease is
approximately a factor of two from z = 0 to z \approx 1, over which we find a
least-squares best-fit line Z(z) / Z_{\odot} = (0.46 \pm 0.05) - (0.38 \pm
0.03)z. The greatest uncertainty in the evolution comes from poorly constrained
metallicities in the highest redshift bin
Evolution of the Cluster X-ray Luminosity Function
We report measurements of the cluster X-ray luminosity function out to z=0.8
based on the final sample of 201 galaxy systems from the 160 Square Degree
ROSAT Cluster Survey. There is little evidence for any measurable change in
cluster abundance out to z~0.6 at luminosities less than a few times 10^44
ergs/s (0.5-2.0 keV). However, between 0.6 < z < 0.8 and at luminosities above
10^44 ergs/s, the observed volume densities are significantly lower than those
of the present-day population. We quantify this cluster deficit using
integrated number counts and a maximum-likelihood analysis of the observed
luminosity-redshift distribution fit with a model luminosity function. The
negative evolution signal is >3 sigma regardless of the adopted local
luminosity function or cosmological framework. Our results and those from
several other surveys independently confirm the presence of evolution. Whereas
the bulk of the cluster population does not evolve, the most luminous and
presumably most massive structures evolve appreciably between z=0.8 and the
present. Interpreted in the context of hierarchical structure formation, we are
probing sufficiently large mass aggregations at sufficiently early times in
cosmological history where the Universe has yet to assemble these clusters to
present-day volume densities.Comment: 15 pages, 10 figures, accepted for publication in Ap
The WARPS survey: III. The discovery of an X-ray luminous galaxy cluster at z=0.833 and the impact of X-ray substructure on cluster abundance measurements
The WARPS team reviews the properties and history of discovery of
ClJ0152.7-1357, an X-ray luminous, rich cluster of galaxies at z=0.833. At L_X
= 8 x 10^44 h^(-2) erg/s (0.5-2.0 keV) ClJ0152.7-1357 is the most X-ray
luminous cluster known at redshifts z>0.55. The high X-ray luminosity of the
system suggests that massive clusters may begin to form at redshifts
considerably greater than unity. This scenario is supported by the high degree
of optical and X-ray substructure in ClJ0152.7-1357, which is similarly complex
as that of other X-ray selected distant clusters and consistent with the
picture of cluster formation by mass infall along large-scale filaments. X-ray
emission from ClJ0152.7-1357 was detected already in 1980 with the EINSTEIN
IPC. However, because the complex morphology of the emission caused its
significance to be underestimated, the corresponding source was not included in
the EMSS cluster sample and hence not previously identified. Simulations of the
EMSS source detection and selection procedure suggest a general bias of the
EMSS against X-ray luminous clusters with pronounced substructure. If highly
unrelaxed, merging clusters are common at high redshift, they could create a
bias in some samples as the morphological complexity of mergers may cause them
to fall below the flux limit of surveys that assume a unimodal spatial source
geometry. Conversely, the enhanced X-ray luminosity of mergers might cause them
to, temporarily, rise above the flux limit. Either effect could lead to
erroneous conclusions about the evolution of the comoving cluster space
density. A high fraction of morphologically complex clusters at high redshift
would also call into question the validity of cosmological studies that assume
that the systems under investigation are virialized.Comment: 17 pages, 7 figures; revised to focus on possible detection biases
caused by substructure in clusters; accepted for publication in ApJ; uses
emulateapj.sty; eps files of figures 1 and 2 can be obtained from
ftp://hubble.ifa.hawaii.edu/pub/ebeling/warp
- âŚ