292,296 research outputs found
The XXL Survey VII: A supercluster of galaxies at z=0.43
The XXL Survey is the largest homogeneous and contiguous survey carried out
with XMM-Newton. Covering an area of 50 square degrees distributed over two
fields, it primarily investigates the large-scale structures of the Universe
using the distribution of galaxy clusters and active galactic nuclei as tracers
of the matter distribution. Given its depth and sky coverage, XXL is
particularly suited to systematically unveiling the clustering of X-ray
clusters and to identifying superstructures in a homogeneous X-ray sample down
to the typical mass scale of a local massive cluster. A friends-of-friends
algorithm in three-dimensional physical space was run to identify large-scale
structures. In this paper we report the discovery of the highest redshift
supercluster of galaxies found in the XXL Survey. We describe the X-ray
properties of the clusters members of the structure and the optical follow-up.
The newly discovered supercluster is composed of six clusters of galaxies at a
median redshift z around 0.43 and distributed across approximately 30 by 15 arc
minutes (10 by 5 Mpc on sky) on the sky. This structure is very compact with
all the clusters residing in one XMM pointing; for this reason this is the
first supercluster discovered with the XXL Survey. Spectroscopic follow-up with
WHT (William Herschel Telescope) and NTT (New Technology Telescope) confirmed a
median redshift of z = 0.43. An estimate of the X-ray mass and luminosity of
this supercluster and of its total gas mass put XLSSC-e at the average mass
range of superclusters; its appearance, with two members of equal size, is
quite unusual with respect to other superclusters and provides a unique view of
the formation process of a massive structure.Comment: A&A, accepted; special XXL issu
Combinatorial Properties of Finite Models
We study countable embedding-universal and homomorphism-universal structures
and unify results related to both of these notions. We show that many universal
and ultrahomogeneous structures allow a concise description (called here a
finite presentation). Extending classical work of Rado (for the random graph),
we find a finite presentation for each of the following classes: homogeneous
undirected graphs, homogeneous tournaments and homogeneous partially ordered
sets. We also give a finite presentation of the rational Urysohn metric space
and some homogeneous directed graphs.
We survey well known structures that are finitely presented. We focus on
structures endowed with natural partial orders and prove their universality.
These partial orders include partial orders on sets of words, partial orders
formed by geometric objects, grammars, polynomials and homomorphism orders for
various combinatorial objects.
We give a new combinatorial proof of the existence of embedding-universal
objects for homomorphism-defined classes of structures. This relates countable
embedding-universal structures to homomorphism dualities (finite
homomorphism-universal structures) and Urysohn metric spaces. Our explicit
construction also allows us to show several properties of these structures.Comment: PhD thesis, unofficial version (missing apple font
Homogeneous spaces with symmetries
We give a brief survey of main recent results about invariant pseudo-Riemannian metrics and classical affinor structures on homogeneous spaces with symmetries obtained by the participants of the joint BRFFI-RFFI project “Homogeneous spaces with symmetries”
Seeing patterns in noise: Gigaparsec-scale `structures' that do not violate homogeneity
Clowes et al. (2013) have recently reported the discovery of a Large Quasar
Group (LQG), dubbed the Huge-LQG, at redshift z~1.3 in the DR7 quasar catalogue
of the Sloan Digital Sky Survey. On the basis of its characteristic size ~500
Mpc and longest dimension >1 Gpc, it is claimed that this structure is
incompatible with large-scale homogeneity and the cosmological principle. If
true, this would represent a serious challenge to the standard cosmological
model. However, the homogeneity scale is an average property which is not
necessarily affected by the discovery of a single large structure. I clarify
this point and provide the first fractal dimension analysis of the DR7 quasar
catalogue to demonstrate that it is in fact homogeneous above scales of at most
130 Mpc/h, which is much less than the upper limit for \Lambda CDM. In
addition, I show that the algorithm used to identify the Huge-LQG regularly
finds even larger clusters of points, extending over Gpc scales, in explicitly
homogeneous simulations of a Poisson point process with the same density as the
quasar catalogue. This provides a simple null test to be applied to any cluster
thus found in a real catalogue, and suggests that the interpretation of LQGs as
`structures' is misleading.Comment: 9 pages, 6 figures. MNRAS published online. v2: minor typo corrected,
added one missing referenc
Extended shells around B[e] stars - implications for B[e] star evolution
Aims. The position of B[e] stars in the upper left part of the Hertzsprung-Russell diagram creates a quandary. Are these stars young stars evolving onto the main sequence or old stars that are evolving off of it? Spectral characteristics suggest that B[e] stars can be
placed into five subclasses and are not a homogeneous set. Such sub-classification is believed to coincide with varying origins and different evolutions. However, the evolutionary connection of B[e] stars – and notably sgB[e] – to other stars is unclear, particularly to evolved massive stars. We attempt to provide insight into the evolutionary past of B[e] stars.
Methods. We performed an Hα narrow-band CCD imaging survey of B[e] stars, in the northern hemisphere. Prior to the current work, no emission-line survey of B[e] stars had yet been made, while only two B[e] stars appeared to have a shell nebula as seen in the Digital Sky Survey. Of nebulae around B[e] stars, only the ring nebula around MWC137 has been previously observed extensively.
Results. In this presentation we report the findings from our narrow-band optical imaging survey of the environments of 25 B[e] stars. Of the objects surveyed, 7 show bipolar or uni- polar structures up to 15' across; 5 show faint, large, or filamentary shells; and 2 are compact planetary nebula-type systems. The most spectacular system observed is a large bipolar structure associated with MWC314.
Conclusions. The possible links between B[e] stars and other evolved stars, implied by our observations, are investigated
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