2,547 research outputs found
Discovery of a Large-scale Wall in the Direction of Abell 22
We report on the discovery of a large-scale wall in the direction of Abell
22. Using photometric and spectroscopic data from the Las Campanas Observatory
and Anglo-Australian Telescope Rich Cluster Survey, Abell 22 is found to
exhibit a highly unusual and striking redshift distribution. We show that Abell
22 exhibits a foreground wall-like structure by examining the galaxy
distributions in both redshift space and on the colour-magnitude plane. A
search for other galaxies and clusters in the nearby region using the 2dF
Galaxy Redshift Survey database suggests that the wall-like structure is a
significant large-scale, non-virialized filament which runs between two other
Abell clusters either side of Abell 22. The filament stretches over at least
>40 Mpc in length and 10 Mpc in width at the redshift of Abell 22.Comment: 6 pages, 4 figures, accepted for publication in MNRAS letter
The ROSAT Deep Cluster Survey: the X-ray Luminosity Function out to z=0.8
We present the X-ray Luminosity Function (XLF) of the ROSAT Deep Cluster
Survey (RDCS) sample over the redshift range 0.05-0.8. Our results are derived
from a complete flux-limited subsample of 70 galaxy clusters, representing the
brightest half of the total sample, which have been spectroscopically
identified down to the flux limit of 4*10^{-14} erg/cm^2/s (0.5-2.0 keV) and
have been selected via a serendipitous search in ROSAT-PSPC pointed
observations. The redshift baseline is large enough that evolutionary effects
can be studied within the sample. The local XLF (z < 0.25) is found to be in
excellent agreement with previous determinations using the ROSAT All-Sky Survey
data. The XLF at higher redshifts, when combined with the deepest number counts
constructed to date (f>2*10^{-14} arg/cm^2/s), reveal no significant evolution
at least out to z=0.8, over a luminosity range 2*10^{42}-3*10^{44} erg/s in the
[0.5-2 keV] band. These findings extend the study of cluster evolution to the
highest redshifts and the faintest fluxes probed so far in X-ray surveys. They
complement and do not necessarily conflict with those of the Einstein Extended
Medium Sensitivity Survey, leaving the possibility of negative evolution of the
brightest end of the XLF at high redshifts.Comment: 12 pages, 4 figures, LaTeX (aasms4.sty). To appear in ApJ Letter
Statistical Topological Insulators
We define a class of insulators with gapless surface states protected from
localization due to the statistical properties of a disordered ensemble, namely
due to the ensemble's invariance under a certain symmetry. We show that these
insulators are topological, and are protected by a invariant.
Finally, we prove that every topological insulator gives rise to an infinite
number of classes of statistical topological insulators in higher dimensions.
Our conclusions are confirmed by numerical simulations.Comment: 6 pages, 1 table, 5 figures, this is the final, published versio
Non-hydrostatic gas in the core of the relaxed galaxy cluster A1795
Chandra data on A1795 reveal a mild edge-shaped discontinuity in the gas
density and temperature in the southern sector of the cluster at r=60/h kpc.
The gas inside the edge is 1.3-1.5 times denser and cooler than outside, while
the pressure is continuous, indicating that this is a "cold front", the surface
of contact between two moving gases. The continuity of the pressure indicates
that the current relative velocity of the gases is near zero, making the edge
appear to be in hydrostatic equilibrium. However, a total mass profile derived
from the data in this sector under the equilibrium assumption, exhibits an
unphysical jump by a factor of 2, with the mass inside the edge being lower. We
propose that the cooler gas is "sloshing" in the cluster gravitational
potential well and is now near the point of maximum displacement, where it has
zero velocity but nonzero centripetal acceleration. The distribution of this
non-hydrostatic gas should reflect the reduced gravity force in the
accelerating reference frame, resulting in the apparent mass discontinuity.
Assuming that the gas outside the edge is hydrostatic, the acceleration of the
moving gas can be estimated from the mass jump, a ~ 800 h km/s/(10^8 yr). The
gravitational potential energy of this gas that is available for dissipation is
about half of its current thermal energy. The length of the cool filament
extending from the cD galaxy (Fabian et al.) may give the amplitude of the gas
sloshing, 30-40/h kpc. Such gas bulk motion might be caused by a disturbance of
the central gravitational potential by past subcluster infall.Comment: Minor text clarifications to correspond to published version. 5
pages, 1 figure in color, uses emulateapj.sty. ApJ Letters in pres
The X-ray Fundamental Plane and Relation of Clusters of Galaxies
We analyze the relations among central gas density, core radius, and
temperature of X-ray clusters by plotting the observational data in the
three-dimensional (, , and ) space and find that
the data lie on a 'fundamental plane'. Its existence implies that the clusters
form a two-parameter family. The data on the plane still has a correlation and
form a band on the plane. The observed relation turns
out to be the cross section of the band perpendicular to the major axis, while
the major axis is found to describe the virial density. We discuss implications
of this two-parameter family nature of X-ray clusters.Comment: 7 pages, 2 figures. To be published in ApJ Letter
An infrared survey of brightest cluster galaxies: Paper I
We report on an imaging survey with the Spitzer Space Telescope of 62
brightest cluster galaxies with optical line emission. These galaxies are
located in the cores of X-ray luminous clusters selected from the ROSAT All-Sky
Survey. We find that about half of these sources have a sign of excess infrared
emission; 22 objects out of 62 are detected at 70 microns, 18 have 8 to 5.8
micron flux ratios above 1.0 and 28 have 24 to 8 micron flux ratios above 1.0.
Altogether 35 of 62 objects in our survey exhibit at least one of these signs
of infrared excess. Four galaxies with infrared excesses have a 4.5/3.6 micron
flux ratio indicating the presence of hot dust, and/or an unresolved nucleus at
8 microns. Three of these have high measured [OIII](5007A)/Hbeta flux ratios
suggesting that these four, Abell 1068, Abell 2146, and Zwicky 2089, and
R0821+07, host dusty active galactic nuclei (AGNs). 9 objects (including the
four hosting dusty AGNs) have infrared luminosities greater than 10^11 L_sol
and so can be classified as luminous infrared galaxies (LIRGs). Excluding the
four systems hosting dusty AGNs, the excess mid-infrared emission in the
remaining brightest cluster galaxies is likely related to star formation.Comment: accepted for publication in ApJ
Metallic phase of the quantum Hall effect in four-dimensional space
We study the phase diagram of the quantum Hall effect in four-dimensional
(4D) space. Unlike in 2D, in 4D there exists a metallic as well as an
insulating phase, depending on the disorder strength. The critical exponent
of the diverging localization length at the quantum Hall
insulator-to-metal transition differs from the semiclassical value of
4D Anderson transitions in the presence of time-reversal symmetry. Our
numerical analysis is based on a mapping of the 4D Hamiltonian onto a 1D
dynamical system, providing a route towards the experimental realization of the
4D quantum Hall effect.Comment: 4+epsilon pages, 3 figure
The Galaxy Cluster Luminosity-Temperature Relationship and Iron Abundances - A Measure of Formation History ?
Both the X-ray luminosity-temperature (L-T) relationship and the iron
abundance distribution of galaxy clusters show intrinsic dispersion. Using a
large set of galaxy clusters with measured iron abundances we find a
correlation between abundance and the relative deviation of a cluster from the
mean L-T relationship. We argue that these observations can be explained by
taking into account the range of cluster formation epochs expected within a
hierarchical universe. The known relationship of cooling flow mass deposition
rate to luminosity and temperature is also consistent with this explanation.
From the observed cluster population we estimate that the oldest clusters
formed at z>~2. We propose that the iron abundance of a galaxy cluster can
provide a parameterization of its age and dynamical history.Comment: 13 pages Latex, 2 figures, postscript. Accepted for publication in
ApJ Letter
- âŠ