261 research outputs found
A Systematic Study of X-Ray Substructure of Galaxy Clusters Detected in the ROSAT All-Sky Survey
Results of a systematic study of substructure in X-ray surface brightness distributions of a combined sample of 470 REFLEX+BCS clusters of galaxies are presented. The fully automized morphology analysis is based on data of the 3rd processing of the ROSAT All-Sky survey (RASS-3). After correction for several systematic effects, percent of the REFLEX+BCS clusters are found to be substructured in metric apertures of 1 Mpc radius (). Future simulations will show statistically which mass spectrum of major and minor mergers contributes to this number. Another important result is the discovery of a substructure-density relation, analogous to the morphology-density relation for galaxies. Here, clusters with asymmetric or multi-modal X-ray surface brightness distributions are located preferentially in regions with higher cluster number densities. The substructure analyses techniques are used to compare the X-ray morphology of 53 clusters with radio halos and relics, and 22 cooling flow clusters with the REFLEX+BCS reference sample. After careful equalization of the different `sensitivities' of the subsamples to substructure detection it is found that the halo and relic sample tends to show more often multi-modal and elongated X-ray surface brightness distributions compared to the REFLEX+BCS reference sample. The cooling flow clusters show more often circular symmetric and unimodal distributions compared to the REFLEX+BCS and the halo/relic reference samples. Both findings further support the idea that radio halos and relics are triggered by merger events, and that pre-existing cooling flows might be disrupted by recent major mergers
Cosmic Mass Functions from Gaussian Stochastic Diffusion Processes
Gaussian stochastic diffusion processes are used to derive cosmic mass functions. To get analytic relations previous studies exploited the sharp -space filter assumption yielding zero drift terms in the corresponding Fokker-Planck (Kolmogorov's forward) equation and thus simplifying analytic treatments significantly (excursion set formalism). In the present paper methods are described to derive for given diffusion processes and Gaussian random fields the corresponding mass and filter functions by solving the Kolmogorov's forward and backward equations including nonzero drift terms. This formalism can also be used in cases with non-sharp -space filters and for diffusion processes exhibiting correlations between different mass scales
Physical Properties of Four SZE-Selected Galaxy Clusters in the Southern Cosmology Survey
We present the optical and X-ray properties of four clusters recently
discovered by the South Pole Telescope (SPT) using the Sunyaev-Zel'dovich
effect (SZE). The four clusters are located in one of the common survey areas
of the southern sky that is also being targeted by the Atacama Cosmology
Telescope (ACT) and imaged by the CTIO Blanco 4-m telescope. Based on publicly
available griz optical images and XMM-Newton and ROSAT X-ray observations we
analyse the physical properties of these clusters and obtain photometric
redshifts, luminosities, richness and mass estimates. Each cluster contains a
central elliptical whose luminosity is consistent with SDSS cluster studies.
Our mass estimates are well above the nominal detection limit of SPT and ACT;
the new SZE clusters are very likely massive systems with M>~5x10^14 M_sun.Comment: 5 pages, 2 figures. ApJL accepte
Virial mass in DGP brane cosmology
We study the virial mass discrepancy in the context of a DPG brane-world
scenario and show that such a framework can offer viable explanations to
account for the mass discrepancy problem. This is done by defining a
geometrical mass that we prove to be proportional to the virial
mass. Estimating using observational data, we show that it
behaves linearly with and has a value of the order of , pointing
to a possible resolution of the virial mass discrepancy. We also obtain the
radial velocity dispersion of galaxy clusters and show that it is compatible
with the radial velocity dispersion profile of such clusters. This velocity
dispersion profile can be used to differentiate various models predicting the
virial mass.Comment: 12 pages, 1 figure, to appear in CQ
Chandra detection of diffuse X-ray emission from the globular cluster Terzan 5
Terzan 5, a globular cluster (GC) prominent in mass and population of compact
objects, is searched for diffuse X-ray emission, as proposed by several models.
We analyzed the data of an archival Chandra observation of Terzan 5 to search
for extended diffuse X-ray emission outside the half-mass radius of the GC. We
removed detected point sources from the data to extract spectra from diffuse
regions around Terzan 5. The Galactic background emission was modeled by a
2-temperature thermal component, which is typical for Galactic diffuse
emission.
We detected significant diffuse excess emission above the particle background
level from the whole field-of-view. The surface brightness appears to be peaked
at the GC center and decreases smoothly outwards. After the subtraction of
particle and Galactic background, the excess spectrum of the diffuse emission
between the half-mass radius and 3' can be described by a power-law model with
photon index = 0.90.5 and a surface flux of F =
(1.170.16) 10 erg s cm sr in the 1--7 keV
band. We estimated the contribution from unresolved point sources to the
observed excess to be negligible. The observations suggest that a purely
thermal origin of the emission is less likely than a non-thermal scenario.
However, from simple modeling we cannot identify a clearly preferred scenario.Comment: 6 pages, 4 figures, accepted for publication by A&
What is a Cool-Core Cluster? A Detailed Analysis of the Cores of the X-ray Flux-Limited HIFLUGCS Cluster Sample
We use the largest complete sample of 64 galaxy clusters (HIghest X-ray FLUx
Galaxy Cluster Sample) with available high-quality X-ray data from Chandra, and
apply 16 cool-core diagnostics to them, some of them new. We also correlate
optical properties of brightest cluster galaxies (BCGs) with X-ray properties.
To segregate cool core and non-cool-core clusters, we find that central cooling
time, t_cool, is the best parameter for low redshift clusters with high quality
data, and that cuspiness is the best parameter for high redshift clusters. 72%
of clusters in our sample have a cool core (t_cool < 7.7 h_{71}^{-1/2} Gyr) and
44% have strong cool cores (t_cool <1.0 h_{71}^{-1/2} Gyr). For the first time
we show quantitatively that the discrepancy in classical and spectroscopic mass
deposition rates can not be explained with a recent formation of the cool
cores, demonstrating the need for a heating mechanism to explain the cooling
flow problem. [Abridged]Comment: 45 pages, 19 figures, 7 tables. Accepted for publication in A&A.
Contact Person: Rupal Mittal ([email protected]
Deep Photometry of GRB 041006 Afterglow: Hypernova Bump at Redshift z=0.716
We present deep optical photometry of the afterglow of gamma-ray burst (GRB)
041006 and its associated hypernova obtained over 65 days after detection (55
R-band epochs on 10 different nights). Our early data (t<4 days) joined with
published GCN data indicates a steepening decay, approaching F_nu ~t^{-0.6} at
early times (<<1 day) and F_nu ~t^{-1.3} at late times. The break at
t_b=0.16+-0.04 days is the earliest reported jet break among all GRB
afterglows. During our first night, we obtained 39 exposures spanning 2.15
hours from 0.62 to 0.71 days after the burst that reveal a smooth afterglow,
with an rms deviation of 0.024 mag from the local power-law fit, consistent
with photometric errors. After t~4 days, the decay slows considerably, and the
light curve remains approximately flat at R~24 mag for a month before decaying
by another magnitude to reach R~25 mag two months after the burst. This
``bump'' is well-fitted by a k-corrected light curve of SN1998bw, but only if
stretched by a factor of 1.38 in time. In comparison with the other GRB-related
SNe bumps, GRB 041006 stakes out new parameter space for GRB/SNe, with a very
bright and significantly stretched late-time SN light curve. Within a small
sample of fairly well observed GRB/SN bumps, we see a hint of a possible
correlation between their peak luminosity and their ``stretch factor'', broadly
similar to the well-studied Phillips relation for the type Ia supernovae.Comment: ApJ Letters, accepted. Additional material available at
ftp://cfa-ftp.harvard.edu/pub/kstanek/GRB041006
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