4,903 research outputs found
The U-band Galaxy Luminosity Function of Nearby Clusters
Despite the great potential of the U-band galaxy luminosity function (GLF) to
constrain the history of star formation in clusters, to clarify the question of
variations of the GLF across filter bands, to provide a baseline for
comparisons to high-redshift studies of the cluster GLF, and to estimate the
contribution of bound systems of galaxies to the extragalactic near-UV
background, determinations have so far been hampered by the generally low
efficiency of detectors in the U-band and by the difficulty of constructing
both deep and wide surveys. In this paper, we present U-band GLFs of three
nearby, rich clusters to a limit of M_U=-17.5 (M*_U+2). Our analysis is based
on a combination of separate spectroscopic and R-band and U-band photometric
surveys. For this purpose, we have developed a new maximum-likelihood algorithm
for calculating the luminosity function that is particularly useful for
reconstructing the galaxy distribution function in multi-dimensional spaces
(e.g., the number of galaxies as a simultaneous function of luminosity in
different filter bands, surface brightness, star formation rate, morphology,
etc.), because it requires no prior assumptions as to the shape of the
distribution function.
The composite luminosity function can be described by a Schechter function
with characteristic magnitude M*_U=-19.82+/-0.27 and faint end slope
alpha_U=-1.09+/-0.18. The total U-band GLF is slightly steeper than the R-band
GLF, indicating that cluster galaxies are bluer at fainter magnitudes.
Quiescent galaxies dominate the cumulative U-band flux for M_U<-14. The
contribution of galaxies in nearby clusters to the U-band extragalactic
background is <1% Gyr^-1 for clusters of masses ~3*10^14 to 2*10^15 M_solar.Comment: 44 pages, 11 figures, accepted for publication in Ap
Nuisance parameter free inference on cointegration parameters in the presence of a variance shift
Inference on co-integration parameters in heteroskedastic vector autoregressions
We consider estimation and hypothesis testing on the coefficients of the co-integrating relations and the adjustment coefficients in vector autoregressions driven by shocks which display both conditional and unconditional heteroskedasticity of a quite general and unknown form. We show that the conventional results in Johansen (1996) for the maximum likelihood estimators and associated likelihood ratio tests derived under homoskedasticity do not in general hold under heteroskedasticity. As a result, standard confidence intervals and hypothesis tests on these coefficients are potentially unreliable. Solutions based on Wald tests (using a “sandwich” estimator of the variance matrix) and on the use of the wild bootstrap are discussed. These do not require the practitioner to specify a parametric model for volatility. We establish the conditions under which these methods are asymptotically valid. A Monte Carlo simulation study demonstrates that significant improvements in finite sample size can be obtained by the bootstrap over the corresponding asymptotic tests in both heteroskedastic and homoskedastic environments. An application to the term structure of interest rates in the US illustrates the difference between standard and bootstrap inferences regarding hypotheses on the co-integrating vectors and adjustment coefficients
Measurements of Sunyaev-Zel'dovich Effect Scaling Relations for Clusters of Galaxies
We present new measurements of the Sunyaev-Zel'dovich (SZ) effect from
clusters of galaxies using the Sunyaev-Zel'dovich Infrared Experiment (SuZIE
II). We combine these new measurements with previous cluster observations with
the SuZIE instrument to form a sample of 15 clusters of galaxies. For this
sample we calculate the central Comptonization, y, and the integrated SZ flux
decrement, S, for each of our clusters. We find that the integrated SZ flux is
a more robust observable derived from our measurements than the central
Comptonization due to inadequacies in the spatial modelling of the
intra-cluster gas with a standard Beta model. This is highlighted by comparing
our central Comptonization results with values calculated from measurements
using the BIMA and OVRO interferometers. On average, the SuZIE calculated
central Comptonizations are approximately 60% higher in the cooling flow
clusters than the interferometric values, compared to only approximately 12%
higher in the non-cooling flow clusters. We believe this discrepancy to be in
large part due to the spatial modelling of the intra-cluster gas. From our
cluster sample we construct y-T and S-T scaling relations. The y-T scaling
relation is inconsistent with what we would expect for self-similar clusters;
however this result is questionable because of the large systematic uncertainty
in the central Comptonization. The S-T scaling relation has a slope and
redshift evolution consistent with what we expect for self-similar clusters
with a characteristic density that scales with the mean density of the
universe. We rule out zero redshift evolution of the S-T relation at 90%
confidence.Comment: Accepted to Astrophysical Journal. 52 pages, 14 tables, 7 figures
;replaced to match ApJ accepted versio
Images, structural properties and metal abundances of galaxy clusters observed with Chandra ACIS-I at 0.1<z<1.3
We have assembled a sample of 115 galaxy clusters at 0.1<z<1.3 with archived
Chandra ACIS-I observations. We present X-ray images of the clusters and make
available region files containing contours of the smoothed X-ray emission. The
structural properties of the clusters were investigated and we found a
significant absence of relaxed clusters (as determined by centroid shift
measurements) at z>0.5. The slope of the surface brightness profiles at large
radii were steeper on average by 15% than the slope obtained by fitting a
simple beta-model to the emission. This slope was also found to be correlated
with cluster temperature, with some indication that the correlation is weaker
for the clusters at z>0.5. We measured the mean metal abundance of the cluster
gas as a function of redshift and found significant evolution, with the
abundances dropping by 50% between z=0.1 and z~1. This evolution was still
present (although less significant) when the cluster cores were excluded from
the abundance measurements, indicating that the evolution is not solely due to
the disappearance of relaxed, cool core clusters (which are known to have
enhanced core metal abundances) from the population at z>0.5.Comment: 23 pages, 12 figures. Accepted for publication in ApJS. Updated to
match published version. Redshifts of two clusters (RXJ1701 and CL0848)
corrected and two observations of MACSJ0744.8 have been combined into one.
Conclusions unchanged. A version with images of all of the clusters is
available at http://hea-www.harvard.edu/~bmaughan/clusters.htm
Large Eddy Simulation/Conditional Moment Closure modeling of swirl-stabilized non-premixed flames with local extinction
This is the accepted manuscript. The final version is available from Elsevier at http://www.sciencedirect.com/science/article/pii/S1540748914000558.The Large Eddy Simulation (LES)/three-dimensional Conditional Moment Closure (3D-CMC) model
with detailed chemistry and finite-volume formulation is employed to simulate a swirl-stabilized nonpremixed
flame with local extinction. The results demonstrate generally good agreement with the
measurements concerning velocity, flame shape, and statistics of flame lift-off, but the penetration of fuel
jet into the recirculation zone is under-predicted possibly due to the over-predicted swirl velocities in the
chamber. Localized extinctions are seen in the LES, in agreement with experiment. The local extinction
event is shown by very low heat release rate and hydroxyl mass fraction and reduced temperature, and is
accompanied by relatively high scalar dissipation. In mixture fraction space, CMC cells with strong
turbulence-chemistry interaction and local extinction show relatively large fluctuations between fully
burning and intermediate distributions. The probability density functions of conditional reactedness,
which shows how far the conditionally-filtered scalars are from reference fully burning profiles, indicate
that for CMC cells with local extinction, some reactive scalars demonstrate pronounced bimodality while
for those cells with strong reactivity the PDFs are very narrow.HZ acknowledges the financial support from EPSRC through a Dorothy Hodgkin Postgraduate
Award
Dark Energy and the mass of galaxy clusters
Up to now, Dark Energy evidences are based on the dynamics of the universe on
very large scales, above 1 Gpc. Assuming it continues to behave like a
cosmological constant on much smaller scales, I discuss its effects
on the motion of non-relativistic test-particles in a weak gravitational field
and I propose a way to detect evidences of at the scale of
about 1 Mpc: the main ingredient is the measurement of galaxy cluster masses.Comment: 5 pages, no figures, references adde
Measurement of the electron-pressure profile of galaxy clusters in Wilkinson Microwave Anisotropy Probe (WMAP) 3-year data
Using WMAP 3-year data at the locations of close to X-ray selected
clusters we have detected the amplitude of the thermal Sunyaev-Zeldovich (TSZ)
effect at the 15 level, the highest statistical significance reported
so far. Owing to the large size of our cluster sample, we are able to detect
the corresponding CMB distortions out to large cluster-centric radii. The
region over which the TSZ signal is detected is, on average, four times larger
in radius than the X-ray emitting region, extending to Mpc.
We show that an isothermal model does not fit the electron pressure at
large radii; instead, the baryon profile is consistent with the
Navarro-Frenk-White profile, expected for dark matter in the concordance
CDM model. The X-ray temperature at the virial radius of the clusters
falls by a factor from the central value, depending on the cluster
concentration parameter. Our results suggest that cluster dynamics at large
radii is dominated by dark matter and is well described by Newtonian gravity.Comment: ApJ Lett, to be published on March 10th, 200
Merging clusters of galaxies observed with XMM-Newton
We present results from the XMM-Newton observations of our ongoing program on
merging clusters. To date three clusters have been observed, covering the
temporal sequence from early to late stage mergers: A1750, A2065 and A3921.
Using spatially-resolved spectroscopy of discrete regions, hardness ratio and
temperature maps, we show that all three clusters display a complex temperature
structure. In the case of A1750, a double cluster, we argue that the observed
temperature structure is not only related to the ongoing merger but also to
previous merger events. A2065 seems an excellent example of a `compact merger',
i.e. when the centres of the two clusters have just started to interact,
producing a shock in the ICM. Using comparisons with numerical simulations and
complementary optical data, the highly complex temperature structure evident in
A3921 is interpreted as an off-axis merger between two unequal mass components.
These results illustrate the complex physics of merger events. The relaxation
time can be larger than the typical time between merger events, so that the
present day morphology of clusters depends not only on on-going interaction but
also on the more ancient formation history.Comment: 10 pages, 3 figures. Use elsart.cls. Accepted for publication in
Advances in Space Research. A version with full resolution figures can be
found at http://www.star.bris.ac.uk/elena/cospar_3clusters.pd
Massive Coronae of Galaxies
There is reason to suspect that about half of the baryons are in
pressure-supported plasma in the halos of normal galaxies, drawn in by gravity
along with about half of the dark matter. To be consistent with the
observations this baryonic component, the corona, would have to be hotter than
the kinetic temperature of the dark matter in the halo so as to produce
acceptable central electron densities. We ascribe this hotter plasma
temperature to the addition of entropy prior to and during assembly of the
system, in an analogy to cluster formation. The plasma cooling time would be
longer than the gravitational collapse time but, in the inner parts, shorter
than the Hubble time, making the corona thermally unstable to the formation of
a cloudy structure that may be in line with what is indicated by quasar
absorption line systems. The corona of an isolated spiral galaxy would be a
source of soft X-ray and recombination radiation, adding to the more commonly
discussed effects of stars and supernovae. In this picture the mass in the
corona is much larger than the mass in condensed baryons in a spiral galaxy.
The corona thus would be a substantial reservoir of diffuse baryons that are
settling and adding to the mass in interstellar matter and stars, so that star
formation in isolated spirals will continue well beyond the present epoch.Comment: 14 pages, 5 figure
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