94 research outputs found
AGN feedback with the Square Kilometer Array (SKA) and implications for cluster physics and cosmology
AGN feedback is regarded as an important non-gravitational process in galaxy
clusters, providing useful constraints on large-scale structure formation. It
modifies the structure and energetics of the intra-cluster medium (ICM) and
hence its understanding is crucially needed in order to use clusters as high
precision cosmological probes. In this context, particularly keeping in mind
the upcoming high quality radio data expected from radio surveys like SKA with
its higher sensitivity, high spatial and spectral resolutions, we review our
current understanding of AGN feedback, its cosmological implications and the
impact that SKA can have in revolutionizing our understanding of AGN feedback
in large-scale structures. Recent developments regarding the AGN outbursts and
its possible contribution to excess entropy in the hot atmospheres of groups
and clusters, its correlation with the feedback energy in ICM, quenching of
cooling flows and the possible connection between cool core clusters and radio
mini-halos, are discussed. We describe current major issues regarding modeling
of AGN feedback and its impact on the surrounding medium. With regard to the
future of AGN feedback studies, we examine the possible breakthroughs that can
be expected from SKA observations. In the context of cluster cosmology, for
example, we point out the importance of SKA observations for cluster mass
calibration by noting that most of clusters discovered by eROSITA X-ray
mission can be expected to be followed up through a 1000 hour SKA-1 mid
programme. Moreover, approximately radio mini halos and
radio halos at can be potentially detected by SKA1 and SKA2 and used as
tracers of galaxy clusters and determination of cluster selection function.Comment: 14 pages, 10 figures, Review article accepted in Journal of
Astrophysics and Astronomy (JOAA
The Signature of a Correlation between Cosmic Ray Sources and Large Scale Structure
We analyze the anisotropy signature expected if the high energy (above
eV) cosmic ray (CR) sources are extra-Galactic and trace the
distribution of luminous matter on large scales. We investigate the dependence
of the anisotropy on both the relative bias between the CR sources and the
galaxy distribution and on the (unknown) intrinsic CR source density. We find
that the expected anisotropy associated with the large scale structure (LSS)
should be detected once the number of CR events observed above is increased by a factor of . This would require
observation-years with existing experiments, but less then year with the
proposed Auger detectors. We find that the recently
reported concentration of the Haverah Park CR events towards the super-galactic
plane is not consistent with the known LSS. If real, the Haverah Park result
suggests that the CR sources are much more concentrated towards the
super-galactic plane than the known LSS. Our results are not sensitive to the
number density of CR sources. We show that once the number of detected events
is increased by a factor of , the number density would be strongly
constrained by considering the probability for having repeating sources.Comment: Submitted to the ApJ; 19 pages (LaTeX file, 7 PostScript figures,
aaspp4.sty style file, tar'ed, gzip'ed and uuencoded
Disorder-enhanced delocalization and local-moment quenching in a disordered antiferromagnet
The interplay of disorder and spin-fluctuation effects in a disordered
antiferromagnet is studied. In the weak-disorder regime (W \le U), while the
energy gap decreases rapidly with disorder, the sublattice magnetization,
including quantum corrections, is found to remain essentially unchanged in the
strong correlation limit. Magnon energies and Neel temperature are enhanced by
disorder in this limit. A single paradigm of disorder-enhanced delocalization
qualitatively accounts for all these weak disorder effects. Vertex corrections
and magnon damping, which appear only at order (W/U)^4, are also studied. With
increasing disorder a crossover is found at W \sim U, characterized by a rapid
decrease in sublattice magnetization due to quenching of local moments, and
formation of spin vacancies. The latter suggests a spin-dilution behavior,
which is indeed observed in softened magnon modes, lowering of Neel
temperature, and enhanced transverse spin fluctuations.Comment: 12 pages, includes 8 postscript figures. To appear in Physical Review
B. References adde
Exploring diffuse radio emission in galaxy clusters and groups with the uGMRT and the SKA
Diffuse radio emission has been detected in a considerable number of galaxy
clusters and groups, revealing the presence of pervasive cosmic magnetic
fields, and of relativistic particles in the large-scale structure (LSS) of the
Universe. Since cluster radio emission is faint and steep spectrum, its
observations are largely limited by the instrument sensitivity and frequency of
observation, leading to a dearth of information, more so for lower-mass
systems. The unprecedented sensitivity of recently commissioned low-frequency
radio telescope arrays, aided by the development of advanced calibration and
imaging techniques, have helped in achieving unparalleled image quality. At the
same time, the development of sophisticated numerical simulations and the
availability of supercomputing facilities have paved the way for
high-resolution numerical modeling of radio emission, and the structure of the
cosmic magnetic fields in LSS, leading to predictions matching the capabilities
of observational facilities. In view of these rapidly-evolving scenerio in
modeling and observations, in this review, we summarise the role of the new
telescope arrays and the development of advanced imaging techniques and discuss
the detections of various kinds of cluster radio sources. In particular, we
discuss observations of the cosmic web in the form of supercluster filaments,
studies of emission in poor clusters and groups of galaxies, and of ultra-steep
spectrum sources. We also review the current theoretical understanding of
various diffuse cluster radio sources and the associated magnetic field and
polarization. As the statistics of detections improve along with our
theoretical understanding, we update the source classification schemes based on
their intrinsic properties. We conclude by summarising the role of the upgraded
GMRT and our expectations from the upcoming Square Kilometre Array (SKA)
observatories.Comment: 32 pages, 10 figures, accepted for publication in the Journal of
Astrophysics and Astronomy (JoAA) (to appear in the special issue on "Indian
participation in the SKA"
A Giant Metrewave Radio Telescope/Chandra view of IRAS 09104+4109: A type 2 QSO in a cooling flow
IRAS 09104+4109 is a rare example of a dust enshrouded type 2 QSO in the
centre of a cool-core galaxy cluster. Previous observations of this z=0.44
system showed that as well as powering the hyper-luminous infrared emission of
the cluster-central galaxy, the QSO is associated with a double-lobed radio
source. However, the steep radio spectral index and misalignment between the
jets and ionised optical emission suggested that the orientation of the QSO had
recently changed. We use a combination of new, multi-band Giant Metrewave Radio
Telescope observations and archival radio data to confirm that the jets are no
longer powered by the QSO, and estimate their age to be 120-160 Myr. This is in
agreement with the ~70-200 Myr age previously estimated for star-formation in
the galaxy. Previously unpublished Very Long Baseline Array data reveal a 200
pc scale double radio source in the galaxy core which is more closely aligned
with the current QSO axis and may represent a more recent period of jet
activity. These results suggest that the realignment of the QSO, the cessation
of jet activity, and the onset of rapid star-formation may have been caused by
a gas-rich galaxy merger. A Chandra X-ray observation confirms the presence of
cavities associated with the radio jets, and we estimate the energy required to
inflate them to be ~7.7x10^60 erg. The mechanical power of the jets is
sufficient to balance radiative cooling in the cluster, provided they are
efficiently coupled to the intra-cluster medium (ICM). We find no evidence of
direct radiative heating and conclude that the QSO either lacks the radiative
luminosity to heat the ICM, or that it requires longer than 100-200 Myr to
significantly impact its environment. [Abridged]Comment: 23 pages, 18 figures and 7 tables. Accepted for publication in MNRA
Empirical Relationship between Intra-Purine and Intra-Pyrimidine Differences in Conserved Gene Sequences
DNA sequences seen in the normal character-based representation appear to have a formidable mixing of the four nucleotides without any apparent order. Nucleotide frequencies and distributions in the sequences have been studied extensively, since the simple rule given by Chargaff almost a century ago that equates the total number of purines to the pyrimidines in a duplex DNA sequence. While it is difficult to trace any relationship between the bases from studies in the character representation of a DNA sequence, graphical representations may provide a clue. These novel representations of DNA sequences have been useful in providing an overview of base distribution and composition of the sequences and providing insights into many hidden structures. We report here our observation based on a graphical representation that the intra-purine and intra-pyrimidine differences in sequences of conserved genes generally follow a quadratic distribution relationship and show that this may have arisen from mutations in the sequences over evolutionary time scales. From this hitherto undescribed relationship for the gene sequences considered in this report we hypothesize that such relationships may be characteristic of these sequences and therefore could become a barrier to large scale sequence alterations that override such characteristics, perhaps through some monitoring process inbuilt in the DNA sequences. Such relationship also raises the possibility of intron sequences playing an important role in maintaining the characteristics and could be indicative of possible intron-late phenomena
Electronic Structure Calculation by First Principles for Strongly Correlated Electron Systems
Recent trends of ab initio studies and progress in methodologies for
electronic structure calculations of strongly correlated electron systems are
discussed. The interest for developing efficient methods is motivated by recent
discoveries and characterizations of strongly correlated electron materials and
by requirements for understanding mechanisms of intriguing phenomena beyond a
single-particle picture. A three-stage scheme is developed as renormalized
multi-scale solvers (RMS) utilizing the hierarchical electronic structure in
the energy space. It provides us with an ab initio downfolding of the global
band structure into low-energy effective models followed by low-energy solvers
for the models. The RMS method is illustrated with examples of several
materials. In particular, we overview cases such as dynamics of semiconductors,
transition metals and its compounds including iron-based superconductors and
perovskite oxides, as well as organic conductors of kappa-ET type.Comment: 44 pages including 38 figures, to appear in J. Phys. Soc. Jpn. as an
invited review pape
An all-sky study of compact, isolated high-velocity clouds
We combine the catalogs of compact high-velocity HI clouds extracted from the
LDS and HIPASS surveys and analyze the all-sky properties of the ensemble. Five
principal observables are defined for the CHVC population: (1) the spatial
deployment of the objects on the sky, (2) the kinematic distribution, (3) the
number distribution of observed HI column densities, (4) the number
distribution of angular sizes, and (5) the number distribution of HI linewidth.
Two classes of models are considered to reproduce the observed properties. The
agreement of models with the data is judged by extracting these same
observables from simulations, in a manner consistent with the sensitivities of
the observations and explicitly taking account of Galactic obscuration. We show
that models in which the CHVCs are the HI counterparts of dark-matter halos
evolving in the Local Group potential provide a good match to the observables.
The best-fitting populations have a maximum HI mass of 10^7 M_Sun a power-law
slope of the HI mass distribution in the range -1.7 to -1.8, and a Gaussian
dispersion for their spatial distributions of between 150 and 200 kpc centered
on both the Milky Way and M31. Given its greater mean distance, only a small
fraction of the M31 sub-population is predicted to have been detected in
present surveys. An empirical model for an extended Galactic halo distribution
for the CHVCs is also considered. While reproducing some aspects of the
population, this class of models does not account for some key systematic
features of the population.Comment: 39 pages, 29 (low res.) png figs, accepted for pub. in A&
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